This article throws light upon the twenty-two main physical properties of water. Some of the physical properties are: 1. Temperature 2. Water Velocity 3. Transparency 4. Dissolved Solids 5. Acidity 6. Alkalinity 7. D.O. (Dissolved Oxygen) 8. Biochemical Oxygen Demand 9. Chemical Oxygen Demand 10. Determination of Nitrite 11. Nitrate and Others.
Physical Property # 1. Temperature:
The limnological characteristics of the water body depend upon solar radiation hence temperature of water must be recorded depth — wise to understand the mechanism of the effect of heat in different layers of water. A good thermometer with 0° to 60°C range and having a least count of about 0.1°C should be selected for heat measurement.
Physical Property # 2. Water Velocity:
ADVERTISEMENTS:
As loading and self-purification of the water body depend upon the velocity of water current hence it should measure with great accuracy by flow meters available in the market.
The velocity can also be measured by fixing two poles at a distance of 10 meter in the direction of flow and measuring the time taken by float to travel the distance between two poles with the help of the following formula.
V = D/1.2 T
where V = Velocity in meter per sound
T = Time in second and
D = Distance between two poles (in meter)
Physical Property # 3. Transparency:
ADVERTISEMENTS:
An Italian Scientists A. Secchi discovered a very simple technique for measuring transparency of water bodies. The measurement of transparency is, in fact, a measure of penetration of solar radiation to produce activity in the aquatic system.
If water is dirty containing solid suspensions then light will not penetrate inside the water and will cause decay of the aquatic system. The depth to which light reaches 1% of surface value is called compensation depth and indicates lower limit of trophogenic zone.
A secchi disc is a metallic plate of 20 cm diameter with four (alternate black and white) quadrants on upper surface and a hook in the centre to tie a rope. The disc remains visible where light is about 15% of the radiation at the lower level.
Requirement & method:
ADVERTISEMENTS:
Secchi disc, rope and a measuring tape.
i. First of all lower the Secchi disc in water and note the depth (in cm) at which it disappears.
ii. Now raise the Secchi disc upward gently till it reappears and note the depth.
ADVERTISEMENTS:
iii. The average of the two values gives the idea of transparency as Secchi disc depth (Sdd).
Calculation:
Euphotic limit (cm) = 2.5 × Sdd
Vertical attenuation Coefficient = 1.9 / Sdd
Physical Property # 4. Dissolved Solids (DS):
Filter 500 ml sample in a Gooch crucible to free it from suspended matter. The filtrate is collected in a beaker and evaporated to about 50 ml volume. It should be noted that any deposit on the walls of the beaker due to evaporation of water should not touch the flame of the burner.
The 50 mL liquid is carefully transferred to a weighed platinum dish with the help of a policeman and with distilled water. Evaporate the solution to dryness on steam bath and dry the dish in an oven at about 100-110°C for about an hour. Cool it in a desiccators, and weigh.
Calculation:
Weight of solids × 106/500 = ppm. dissolved solids.
Result:
Disposal of industrial effluents and sewage contributes suspended solids to the water bodies. The ISI has specified a maximum limit of 30 mg/L for suspended solids discharged into rivers. Solids determination is particularly useful in the analysis of sewage and other waste waters.
It is as significant as BOD determination. It is used to evaluate the strength of domestic waste waters and to determine the efficiency of threated units.
Physical Property # 5. Acidity:
Acidity is a measure of the effects of combination of compounds and conditions in water. It is the power of water to neutralize hydroxyl ions and is expressed in terms of calcium carbonate.
Water attains acidity from industrial effluents, acid mine drainage, pickling liquors and from humic acid:
Measurement of acidity by titration method:
Principle:
Acidity of water can be determined by titration with sodium hydroxide solution. The amount of sodium hydroxide required for the sample (pH below 4.5) to reach pH 4.5 (methyl orange end point) is a measure of mineral acidity while the amount of sodium hydroxide to reach pH 8.3 (phenolphthalein end point) is a measure of total acidity.
Samples containing acidic water (pH below 4.5) correspond to both mineral and CO2 acidity.
Procedure:
Mineral acidity:
Take 50 mL or suitable dechlorinated aliquot of the sample in a 250 mL conical flask. Add 2 drops of methyl orange indicator and titrate with 0.02 N-NaOH solutions till faint orange colour.
Total acidity at room temperature:
Place suitable aliquot of the sample is 250 mL flask.
Add 2 drops of phenolphthalein indicator and titrate with 0.02 N-NaOH solutions to light pink colour.
Total acidity at boiling temperature:
To 50 mL of the sample; add 5 drops of phenolphthalein indicator. Heat to boil for 2 minutes. Titrate with 0.02 N-NaOH solutions to pink colour.
Calculation:
Acidity as CaCO3 = mL titrant (NaOH) × 1 × 1000/mL sample taken for titration
Result:
Methyl orange acidity value shows mineral acidity. In absence of mineral acidity, total acidity is only the CO2 acidity of the sample.
Physical Property # 6. Alkalinity:
It is the measure of carbonate and hydroxide ions in water. It is generally determined by titration method.
Titration method for the Determination of Alkalinity:
Principle:
Alkalinity is determined by titration with 0.02 H2SO4 using methyl orange and phenol phthalein as indicators.
Reactions – 2CaCO3 + H2SO4 → CaSO4 + Ca(HCO3)2
Ca(HCO3)2 + H2SO4 → CaSO4 + 2CO2 + 2H2O
Ca(OH)2 + H2SO4 → CaSO4 + 2H2O
Reagents —
(i) Sulphuric acid 0.02 N:
Dilute 20 mL of 1 n – H2SO4 to 1000 mL with distilled water.
(ii) Sodium carbonate solution:
Dissolve 13.25 g Na2CO3 in distilled water to 250 mL.
(iii) Phenolphthalein indicator solution:
Dissolve 500 mg phenolphthalein in 50 mL alcohol and 50 mL distilled water. Add 0.02 N-NaOH solutions till light pink colour appears.
Procedure:
Take 50 mL of the sample in a 250 mL conical flask. Add 2 drops of phenolphthalein indicator. Titrate the pink colour with 0.02 N —H2SO4 till it becomes colourless. If the sample contains wastewater, then remove the suspended matter by filtration or centrifugation and then determine alkalinity.
Calculation:
If H2SO4 used for titration is 0.02N, phenolphthalein alkalinity (as CaCO3) mg/L.
Alkalinity provides an idea of the salts present in water.
Physical Property # 7. D.O. (Dissolved Oxygen):
(i) Since the DO content of a sample can change rapidly hence it must be fixed immediately after collecting the sample. This is done by means of an oxidation reduction reaction brought about by the addition of manganese (ii) Sulphate and potassium hydroxide (which is added along with Potassium Iodide)
4 Mn++ + O2 + 8 OH– + 2H2O = 4 Mn (OH)3
The result of fixing is that brown manganese (III) hydroxide is precipitated.
(ii) The manganese (III) is converted into its equivalent of iodine. For this the solution is just acidified with some sulphamic acid, as iodide is already present
(iii) A portion of the solution is titrated with sodium thiosulphate
The indicator of the reaction is starch, which forms a blue complex with iodine.
I2 + starch = I2 Starch (blue)
Requirements:
Reagents:
1. Manganese (II) sulphate solution:
Weigh 100 g of MnSO4. 4H2O and dissove in 200 ml of previously boiled distilled water.
2. Alkaline potassium iodide solution:
Take 200 ml of previously boiled and cooled distilled water and dissolve 100 g of KOH and 50 g of KI in it.
3. Sulphamic acid (NH2SO2OH):
This is a solid substance and is used in that form.
4. Starch solution:
Heat 100 ml of distilled water close to boiling point and dissolve 1 g of soluble starch powder in it. Keep it overnight. Decant the solution and store after adding a few drops of formaldehyde solution.
5. Sodium thiosulphate:
Weigh 6.21g of Na2S2O3 5H2O and dissolve it in previously boiled and cooled distilled water and make the volume up to 1 litre. Keep the solution in a brown or black bottle after adding a pellet of sodium hydroxide as stabilizer.
Apparatus:
A 250 ml DO bottle (glass bottle with Stopper), three syringes (2 ml), one plastic syringe 20ml, spoon, plastic volumetric flask (100 ml), conical flask 250ml.
Testing Method:
Testing of the sample must be done at the spot immediately after collection. The rest of the process can be done in the laboratory.
Rinse the DO bottle 2-3 times with sample water and then fill it with more sample water until it overflows. Put on the stopper after ensuring that no air bubbles are trapped in the bottle.
Draw manganese (II) sulphate solution into a 2 ml plastic syringe, slightly above the top mark. If there is an air bubble, invert and rotate the syringe to bring the bubble at the nozzle. Push the piston gently to expel the bubble. Inject 2 mg manganese (II) sulphate into the sample. Repeat the procedure with alkaline potassium iodide (KOH/KI) solution by using another 2 ml syringe.
Wait for a minute, a brown precipitate will be formed. Place the stopper firmly on the bottle. Shake the contents thoroughly by inverting and straightening the bottle about 15 times keep the bottle in a safe place for approximately 20 minutes (until the precipitate formed settles in a compact layer at the bottom of bottle).
Open the bottle and add a spoonful of sulphamic acid. Replace the stopper. Shake the bottle thoroughly in the same manner as before, until the brown precipitate dissolves.
Draw the solution of sodium thiosulphate upto the top mark in a 20 ml syringe. Remove the air bubbles and keep the syringe ready for titration. Take 100 ml of solution from the bottle in a 250 ml conical flask. Use the 100 ml volumetric flask to measure the required quantity of the solution.
Hold the conical flask in one hand and add thiosulphate solution drop by drop from the syringe with the other hand. Or one chemist may hold the flask and the other do the addition of sodium thiosulphate. Addition must be done slowly. Shake the flask gently while adding the drops. Take care that the contents do not spill out.
When the colour of the contents of the flask becomes pale yellow, stop adding thiosulphate. Set the 20 ml. syringe carefully aside without spilling its contents or disturbing the plunger in any way. Add 2 ml starch solution to the contents of the conical flask with the help of 2ml syringe. Shake it.
The colour of the contents changes to blue. Now take the 20 ml. syringe again. Continue adding sodium thiosulphate solution drop by drop until the blue colour just disappears. End point is blue to colourless. Ask the chemistry students to note the total volume (i.e. amount before adding starch plus amount after adding starch) of the sodium thiosulphate solution consumed from the syringe.
Volume of sodium thiosulphate consumed, say x ml, is equivalent to the milligram concentration of dissolved oxygen in 0.5 litre. Therefore, 2x ml of sodium thiosulphate is equivalent to DO in mg per litre.
Physical Property # 8. Biochemical Oxygen Demand (B.O.D.):
The B.O.D. is the amount of oxygen required by bacteria while stabilizing decomposable organic matter under aerobic conditions. The decomposition of organic impurities in the presence of bacteria results in utilization of a part of the dissolved oxygen by the bacteria during their respiratory and metabolic activities. This depletion of oxygen is considered as a measure of the strength of water.
All organic constituents of sewage degrade under aerobic conditions.
Requirement and method:
BOD bottles, Pipette, beaker, BOD incubator, allylthio urea solution (0.05%), Phosphate buffer solution (pH 7.2), Sulphuric acid (IN), Sodium hydroxide (IN) etc.
i. Adjust the pH of sample to about 7 by adding IN alkali or acid.
ii. Add 1 ml of alkylthiourea in each bottle first & then fill the sample in 6 BOD bottles without bubbling.
iii. Determine dissolved oxygen in three bottles of oxygen meter.
iv. Incubate the remaining the remaining three bottles in BOD incubator at 20°C for 5 days.
v. After 5 days determine the oxygen concentration in the three bottles.
Observation:
Calculation:
BOD5 at 20°C in mg 1-1 = β1 – β2
where β1 = Mean of initial D.O. in sample (mg1-1)
β2= Mean of D.O. in Sample after 5 days (mg1-1)
Result:
BOD5 days at 20°C = (β1 – β2) mg 1-1.
Physical Property # 9. Chemical Oxygen Demand (COD):
Chemical oxygen demand is used for measuring the pollutional strength of waste water. Most of the organic compounds can be oxidized to carbon dioxide and water by the action of strong oxidizing agents of the biological assimilability of the substances.
Determination of COD:
In the determination of COD, the organic matter is first oxidized with a standard solution of KM11O4 and then excess of oxygen reacts with Potassium iodide to liberate iodine in amount equal to that of excess of oxygen which is calculated by titration with sodium thiosulphate solution using starch as indicator.
Sodium thiosulphate (0.1 M), Potassium Permaganate (0.1 N), Sulphuric acid (2M), Potassium iodide Solution (10%), Starch solution (1%), water bath, beaker, burette and Pipette etc.
i. Take three flask of 100 ml capacity and add 50 ml of sample in each flask.
ii. Add 5 ml of KMnO4 solution in all the flasks and heat them on water bath for one hour at boiling temperature.
iii. Cool all the three flasks & add to each 5 ml solution + 10 ml of H2SO4.
iv. Now titrate with standard this solution until pale yellow colour comes. At this point 2ml starch is added & the solution turns blue.
v. Perform same experiments with blanks.
Observations:
Calculation:
COD of sample in mg/I.
= 8 × C × (B-S)/A – mg 1-1 CI × 0.23
where C = concentration of titrant (m mol 1-1).
B = Volume of titrant used for blank (in ml).
S = Volume of titrant used for sample (in ml).
A = Volume of water sample (in ml).
Result:
The COD of given sample =….. mg/l.
Physical Property # 10. Determination of Nitrite:
In solution nitrite gives nitrous acid which on reaction with sulphanilamide and N-1—naphthyl ethylene diamine dihydro chloride, flasks, Pipette, beaker etc.
i. Take 45 ml of Sample in 50 ml flask.
ii. Add 1 ml of Sulphanil amide and after 3 minutes.
Add 1 ml of aromatic amine reagent and mix thoroughly.
iii. Make up the flask upto 50 ml by adding distilled water.
iv. Now determine the concentration of nitrite with the help of Beer—Lambert law.
Result:
The concentration of nitrite in given sample = mg 1-1
Physical Property # 11. Nitrate:
The reaction between nitrate and 1, 2, 4, and disulphonic acid gives 6-nitro-l, 2, 4 Phenol disulphonic acid which after some time yields yellow colour.
The concentration is determined with the help of colorimeter:
Requirement & method:
Colorimeter, Phenoldisulphonic acid, NH4OH solution or 12N KOH solution, standard potassium nitrate solution, pipette, beaker etc.
i. Take 25 ml of sample in a 50 ml beaker and evaporate to dryness on water bath.
ii. Add 0.5 ml phenol disulphonic acid and dissolve all residues.
iii. Add 5 ml of distilled water & 1.5 ml concentrated NH4OH (or 12N KOH)
iv. A yellow colour comes out and the concentration is determined by colorimeter.
Physical Property # 12. Total Phosphorus:
The total Phosphorus consists of soluble phosphate, poly phosphate and soluble and insoluble organic phosphorus.
All types of phosphorus is converted to ortho phosphate by digestion and oxidation & then can be determined by colorimeter.
Requirement & method:
Ammonium molybdate, stannous chloride solution, Perchloric acid (70%), Sodium hydroxide (IN), Phenolphthalein indicator, Pipette, beaker etc.
i. Take 25 ml of sample and evaporate to dryness. Cool and add 1 ml of Perchloric acid.
ii. Heat till the residue becomes colourless & then add 10 ml of distilled water and a drop of Phenolphthalein.
iii. Titrate with NaOH solution until pink colour develops at the end point. Now make up the volume upto 25 ml and add it 1 ml of ammonium molybdate solution and 3 drops of stannous chloride solution until the blue colour develops. Now determine the concentration of total P by colorimeter.
Physical Property # 13. Ammonia:
Ammonia in water comes from decaying organic matter. Sewage is rich in nitrogenous matter. Disposal of such wastes increases the ammonia content of water. Ammonia in high concentration is harmful to fish and other aquatic animals, and also to man
Nessler’s reagent (K2HgI4) forms a brown precipitate with ammonia in the presence of sodium hydroxide.
Requirements:
Reagents:
1. Nessler’s reagent. It is readily available in market.
2. Sodium hydroxide solution, 2N. Dissolve 8.0 g of NaOH in distilled water and make up the volume to 100 ml.
Apparatus:
Test-tube and dropper bottle.
Testing Method:
Take about a quarter of a test tube full of the water sample. Add about 5 drops of Nessler’s reagent and 10 drops of sodium hydroxide solution. Observe the colour of the contents. If they turn brown or form a brown precipitate, presence of ammonia is inferred.
Physical Property # 14. Determination of Hazardous Inorganic Ion:
(a) Cyanide:
Amount of cyanide in water can be calculated as: Take 250-500 mL sample in a distillation flask in a fume chamber and add 50 mL of 18 N NH2SO4 and 20 mL of 50% MgCI2.6H2O (in order to remove SCN), Reflux the contents for about one hour. Collect the HCN gas in 500 mL of N NaOH in a gas washer and rinse the connecting tube to gas washer with distilled water.
The cyanide content is then measured by titration method. In this method 0.5 mL of 0.02% p-dimethylamino-benzal rhodamine solution in acetone is added to an aliquot of the distillate and then titrated against 0.02 N AgNO3 solutions till the colour changes from yellow to salmon blue. It is compared against a blank.
(b) Sulphate:
100 mL of the sample water is taken in a conical flask and 10 mL of benzidine hydrochloride solution (solution of benzidine in dilute HCl containing 4.0 g. of the diamine base per litre of the solution) are added. The precipitate formed is filtered and washed free of acid with minimum amount of distilled water.
The precipitate and filter paper are taken in a conical flask and 50 mL of distilled water is added to it. Now a few drops of phenolphthalein are also added. The conical flask is well shaken to dissolve the precipitate and the solution so obtained is titrated against N/7 NaOH until pink colour is obtained at the end point. Sulphate (as Na2SO4) ppm = No. of mL of N/7 NaOH × 100.
Na2SO4 ppm can be converted into CaCO3 ppm by multiplying with a factor of 0.705.
(c) Sulphide:
For the estimation of sulphide, take 500 mL of the sample in one litre flask and add 1 mL of 2N zinc acetate (200g/L) and 2mL of 1N NaOH. Then add 20 mL of 18 N H2SO4 and rapidly distil the solution into 200 mL of 2.2% zinc acetate solution. Under these conditions ZnS precipitate dissolves.
The receiving flask is removed and 10 mL of 0.05 N I2 solution is added to it after acidifying the solution with H3PO4 (124 mL/500 mL distilled water). After about 20-30 minutes, back titrate the excess of iodine with 0.05 N hypo solution using starch as an indicator.
Physical Property # 15. Hardness:
Water is termed ‘hard’ when it produces little or no lather with soap (not synthetic detergents). Chemically soaps are sodium salts of organic acids which are soluble in water and they form good lather. The presence of some chemical substances, mainly calcium & magnesium carbonates, sulphates, bicarbonates or chlorides makes soap insoluble.
Certain other ions also contribute to hardness of water, but their role is not as significant as that of calcium and magnesium.
Hard water is unsuitable for laundry purposes, as a large portion of soap is wasted. In equipment where water is boiled, the hardness causing substances get deposited on the walls of the equipment. Such deposits may or may not be advantageous. They are advantageous as they prevent corrosion of metal. The disadvantage is that they form a heat insulating layer, thus increasing the energy consumption in boilers.
Hardness of water can be demonstrated from its low foam forming capacity with soap but its exact amount is represented by the concentration of calcium and magnesium ions. These ions can be analysed by volumetric analysis.
The concentration of calcium and magnesium ions is determined by titration with EDTA reagent. The titration is based on the principle that Ca and Mg ions form weak complex with Eriochrome Black-T dye and a more stable and soluble complex with EDTA.
When the dye is added to hard water at pH 10 ± 0.1, it forms a wine red coloured complex. When EDTA is added to this complex, it breaks and forms a more stable complex. The dye is released and thus turns the solution from wine red to blue at the end of the reaction.
Requirements:
Reagents:
1. EDTA solution, 0.01M:
Weigh 3.723 gm. of disodium salt of EDTA. Dissolve it in distilled water and make up the solution to one litre.
2. Buffer solution:
(a) Dissolve 16.9 g ammonium chloride in 143 ml of concentrated ammonium hydroxide, (b) Dissolve 1.179 g of disodium salt of EDTA and 0.780 g of magnesium sulphate heptahydrate (Mg so4 7H2O) in 50 ml distilled water. Mix both, (a) and (b) solutions and dilute to 250 ml with distilled water.
3. Eriochrome Black – T indicator:
Take 0.40 g of Eriochrome Black – T and 100 g of sodium chloride, mix both and grind.
Apparatus:
Conical flask (250 ml), beaker, plastic syringe 20 ml, plastic syringe 2 ml, and measuring cylinder (50 ml).
Testing Method:
Take 50 ml of the sample in a 250 ml conical flask with the help of a measuring cylinder. Draw buffer solution up to the 1 ml mark in a 2 ml plastic syringe and transfer this into the conical flask; add 100-200 mg for one-fifth of a spoon or a pinch) of Eriochrome Black-T indicator to it. The solution will turn wine red in colour.
Now take up the EDTA solution in a 2 ml. syringe. Add this solution slowly into the conical flask. While doing so gently rotate the flask, taking care not to spill the contents. When the colour starts becoming violet add the EDTA solution drop by drop. Stop adding when the colour of the contents of the flask just becomes blue.
Note the volume of EDTA solution consumed in titration and calculate the hardness of the sample by the formula given below:
Hardness (mg/1 CaCO3) = ml. of EDTA used × 1000/ml. of sample
Now perform the blank experiment (B) by taking 100 ml distilled water, 75 ml acid and 25 ml potassium dichromate solution. Reflux for 2 hours and titrate the excess dichromate with the ferrous ammonium sulphate.
Calculation:
C.O.D. mg/litre = (A – B) C × 8 × 1000/ml. of sample
where A = ml. of ferrous ammonium sulphate used with sample,
B = ml of ferrous ammonium sulphate used with distilled water,
C = Normality of ferrous ammoniumn sulphate.
Physical Property # 16. Free Carbondioxde:
The CO2 which is found in well water and surface water to a great extent can cause corrosion. The CO2 present in water in excess of carbonates and bicarbonates is known as free CO2.
It can be determined by titration with NaOH using phenol phthalein as indicator.
Reagents:
(1) 0.0454 N Standard Na2CO3.
(2) 0.0227 N Standard NaOH.
(3) Phenolphthalein indicator.
Procedure:
Take 100 ml of sample and keep it cool with ice until taken for estimation. Add 2-3 drops of phenolphthalein indicator. If the sample does not become red, it indicates the presence of free CO2. Titrate immediately with NaOH solution stirring with glass rod until a pink colour (pH 8.3) appears for about 1 minute.
Calculation:
CO2 mg/litre = m. of NaOH × Normality of NaOH × 44 × 1000/ml of sample taken
Physical Property # 17. Dissolve CO2:
When water containing CO2 is treated with Na2CO3, the following reaction takes place.
Thus a standard solution of N/10 Na2CO3 is first prepared by dissolving 5.3 g of anhydrous Na2CO3 in one litre of distilled water. 250 mL of sample water is taken in a 400 mL beaker and it is carefully titrated against N/10 Na2CO3 solution using phenolphthalein as an indicator. The end point of the change is detected by sudden appearance of permanent pink colour.
Thus
Number of ML of N/10 Na2CO3 × 0.513 = CO2 grains per gallon.
Physical Property # 18. Free Available and Combined Available Chlorine:
The chlorine present in water as Cl2, HOCl, OCl-H2OCl+, Cl3-is called as free available chlorine. When chlorine in water combines with ammonia to form chloramines and other chloroderivatives then the mixture of chloramines and other derivatives is called available chlorine. Both free and combined chlorine may be present simultaneously in the chlorinated water.
Estimation of free chlorine in water:
It is based on the oxidation of KI by free chlorine. When water sample is treated with an excess of KI solution, the free chlorine, present in water sample oxidizes KI and liberates I2 in equivalent amounts. The iodine so formed is dissolved in excess of KI giving a deep violet complex KI3.
The amount of liberated iodine is then treated against a standard solution of sodium thiosulphate using starch as an indicator.
I2 + 2Na2S2O3 → 2NaI + Na2S4O6
I2 + Starch → Complex having deep blue colour
Reagents:
(a) Sodium thiosulphate solution (0.1 N):
It can be prepared by dissolving 24.82 g sodium thiosulphate in boiled and cooled distilled water in 1000 mL measuring flask. Standardize it against standard potassium dichromate solution (0.1 N). It can be preserved by adding a few mL of chloroform.
(b) Glacial acetic acid.
(c) Potassium iodide crystals.
(d) Starch.
(e) Sodium thiosulphate solution (0.0025N).
Standardization of sodium thiosulphate:
(a) Dissolve 1.226 g. potassium dichromate in distilled water in 250 mL volumetric flask.
(b) Dissolve 3g potassium iodide and 2g sodium bicarbonate in 100 mL boiled and cooled distilled water in a 500 mL conical flask.
(c) Add 6 ml conc. HCl followed by 25.0 mL standard potassium dichromate solution (0.1N). Keep it for 5 minutes in dark.
(d) Dilute it with 300 mL water. Titrate the liberated iodine with hypo solution. When the colour becomes yellowish green, add 2 mL starch solution. Titrate it until the colour changes to light green.
(e) Determine the normality of sodium thiosulphate solution.
Procedure:
Take 500 mL of the sample in a conical flask. Add 1 g of KI and 5 mL acetic acid. Titrate with 0.0025N hypo solution until the colour of iodine is discharged. Add 1 mL starch indicator and titrate until the blue colour disappears.
Modified method when manganese, nitrite and ferric iron (2 mg/L) are present:
The pH is adjusted from 4.5 to 8.0 by adding acetic acid. Add lg KI and titrate with 0.0025N hypo solution.
Calculation:
Result:
Excess of chlorine in water can accelerate corrosion, deteriorate lumber, may cause bad tastes in canned foods or frozen products. Therefore residual chlorine in pure water should not exceed 2 mg/L.
Estimation of dissolved chlorides:
A sample of hard water containing chlorides of calcium and magnesium, gives an instant white precipitate of AgCl with AgNO3 solution. The process continues as long as Cl– ions are there.
The completion of the chloride precipitate can be checked by adding a few drops of potassium chromate solution as indicator which suddenly gives a red precipitate when chloride removal is complete. This appearance of red precipitate of silver chromate only after complete removal of the dissolved chloride is due to wide difference in the solubility product values of AgCl (1.82 × 10-10) and AgCrO4(1.1 × 10-12).
The sample of water after neutralization with N/50 H2SO4 using methyl orange as indicator is taken. A few drops of potassium chromate indicator are added to it and then the sample is titrated against N/50 AgNO3 solution till the colour changes from white yellow to reddish brown.
The number of c.c. of AgNO3 solution added is calculated as parts per 106 parts of sodium chloride (as CaCO3), as
x × N × 106/1000 × 50 × 100
where x is the number of c.c. of N/50 AgNO3 and N is the equivalent weight of CaCO3.
Physical Property # 19. Chlorine Demand:
Chlorine is the primary requirement of polluted water for the removal of odour & bacterial growth. The amount of chlorine necessary to add to a given volume of water is called chlorine demand. Chlorine demand may be defined as the quantity of chlorine required to produce a residual chlorine content between a trace and 0.1 ppm after 10 minute contact.
The measurement of chlorine demand helps in estimating the quantity of chlorine necessary to add to a given volume of water. Take a reagent bottle containing 250 mL of the sample and add to it standard chlorine water containing 0.5 mg of chlorine from a burette.
Stir the mixture well and withdraw 0.25 mL of the solution with a pipette on a spot plate and immediately test it for free chlorine with a drop of orthotoluidine reagent. Continue adding the standard chlorine water in 0.5 mL doses, mixing, and testing as before until there is a slight excess of chlorine which is indicated by light yellow colour. The quantity of chlorine required is regarded as the immediate chlorine demand.
Now again take a similar sample and add to it standard chlorine water in an amount equivalent to the immediate chlorine demand plus 0.5 mL more. Agitate the mixture well and yellow it to stand for exactly 10 minutes. After completion of ten minutes, immediately test a portion for residual chlorine as described above.
Calculation:
mL of standard chlorine water x grams chlorine per litre in standard chlorine water × 1000/250 = ppm. Chlorine demand.
Reagents (i) Standard chlorine solution:
Weigh 350 of 30% bleaching powder and transfer its paste to a 1000 mL volumetric flask. Make up the solution to the mark with distilled water. The solution contains about 100 mg/L chlorine. Standardize it against 0.025 N sodium thiosulphate solution using starch as an indicator.
Calculate chlorine as follows:
Method:
Measure 200 mL portions of well mixed sample into each of ten buttles. Add increasing amounts of chlorine solution in increments of 0.1 mg/L. After contact period (10 minutes) determine free available chlorine and combined available chlorine by starch iodide method.
Calculation:
mg/L chlorine demand = mg/L chlorine added-mg/L residual chlorine.
Determination of residual chlorine:
Iodometric and orthotoludine methods are used for the determination of residual chlorine in potable water, moderately polluted water, cooling water and water treatment effluents, Iodometric method is considered to be the standard one and is best suited to samples containing chlorine more than 1 mg/L only.
Physical Property # 20. Detection of Metals:
The metals can be detected easily by N.M.R. (Nuclear Magnetic Resonance) or polarography.
(a) Determination of calcium:
EDTA titrimetric method and gravimetric method are used for the determination of calcium.
Principle:
Calcium reacts with EDTA in the presence of a selective indicator at high pH (12 to 13) and magnesium is allowed to precipitate as its hydroxide.
Ca2+ + 2 EDTA → Ca (EDTA)2
Mg2+ + 2NaOH → Mg (OH)2 + 2Na+
Reagents:
Sodium hydroxide (IN):
40 g NaOH is dissolved in water in 1000 mL measuring flask.
Indicators:
(i) Murexide (Ammonium pupurate indicator):
It is prepared by dissolving 150 mg of the indicator in 100 g. absolute ethylene 1g dye with 100 g sodium sulphate. End point is from wine red to blue.
(ii) Patton and Reeder’s indicator:
(2-Hydroxy-l- (2-hydroxy-4- sulphonaphthyl azo)-3- naphthoic acid), the indicator is prepared by grinding lg dye with 100 g sodium sulphate. End point is from wine red to blue.
Standard EDTA titrant (0.02N):
Procedure:
Take 50 mL of the sample in a conical flask and add 50 mL distilled water to it. Raise the pH to 12-13 by adding 2 mL sodium hydroxide. Add 1-2 drops of the indicator and titrate with EDTA to the proper end point.
Interference- Excess alkalinity (30 mg/L) may cause indistinct end points with hard water. The concentration of the following ions should not exceed the limit.
Fe2+ = 20 mg/L; Cu2+ = 2 mg/L; Pb2+ = 5 mg/L; Al3+ = 5 mg/L;
Fe3+ = 20 mg/L; Zn2+ =5mg/L; Sn2+ = 5mg/L; Mn2+ =10 mg/L;
The gravimetric method may be employed when the concentration of the above ions is very high and when orthophosphates are present which precipitate calcium at high pH.
Calculation:
When the EDTA titrant is 0.02N, then
mg /L calcium as Ca = mL EDTA titrant × 1 × 1000/mL sample taken for titration × 0.40
mg/L calcium as CaCO3 = mL EDTA titrant × 1 × 100/mL sample taken for titration
Result:
Calcium is an essential element for man (□ 2 g daily) and for plant growth. It is also desirable in water for irrigation. However, high calcium contents in water are undesirable for washing, bathing laundering. It tends to create scales and incrustations on utensils.
(b) Determination of magnesium:
Magnesium may be determined by gravimetric or photometric method.
It may also be calculated as the difference between total hardness and calcium hardness as follows:
Magnesium (as CaCO3) mg/L = mg/L Total hardness as CaCO3.
— mg/L calcium hardness as CaCO3
Gravimetric method:
Magnesium is precipitated as magnesium ammonium phosphate. The precipitate is ignited and weighed as magnesium pyrophosphate.
Reaction:
Mg2+ + Na2HPO4 + NH4OH → MgNH4PO4 + 2Na+ + H2O
2MgNH4PO4 → Mg2P2O7 + 2NH3 + H2O
Regents:
(a) Hydrochloric acid, conc. and 1+1
(b) Nitric acid, conc.
(c) Ammonia solution conc. and 1+19
(d) Ammonium oxalate solution
(e) Diammonium hydrogen phosphate solution, 30%
Procedure:
Take suitable volume of sample containing 10 to 50 mg magnesium in a dish. Acidify with concentrated HC1 and evoporate to dryness.
Add 2 mL 1+1 HCl to the residue and dilute to 75 mL with distilled water. Boil and filter. Wash the filter with 1+1 HCl and hot water.
Determination of magnesium as magnesium pyrophosphate:
Dry the precipitate in a weighed crucible at 500°C until it becomes white. Ignite and weigh.
Calculate magnesium content as follows:
Magnesium (as Mg) mg/L = mg magnesium pyrophosphate × 218.5/mL sample taken for estimation
Magnesium (as CaCO3) mg/L = mg magnesium pyrophosphate × 218.5/mL sample taken for determination) × 4.16
(c) Determination of iron:
Total iron (both ferrous and ferric) in water can be analysed by titrimetric, colorimetric or photometric method.
Colorimetric method – Thiocyanate method:
Principle:
Ferric iron combines with thiocyanate ions to form red coloured ferric thiocyanate which is measured calorimetrically.
Reaction:
Fe3+ + 3 CNS– = Fe (CNS)3
Reagents:
(a) Hydrochloric acid, conc. and 1+1.
(b) Potassium permanganate solution, 0.2 N.
(c) Iron stock solution: It can he prepared by dissolving 0.7022g of ferrous ammonium sulphate crystals in 20 mL. conc. H2SO4 which is diluted by 50 mL distilled water. Warm it and add KMnO4 drop-wise until pink colour persists. Make upto 1000 mL in volumetric flask with distilled water.
1.0 mL = 0.01 mg Fe
(d) Iron standard solution: dilute 50 mL stock iron solution to 500 mL with distilled water.
(e) Thiocyanate solution: 10%
Procedure:
Take 100 mL of the sample and add 5 mL dilute HCl. Heat and reduce the volume to 40 mL. Cool and add potassium permagnate solution until a pink colour appears. Transfer it to Nessler tube and make the volume to 50 mL. Pipette 1 to 6 mL iron standard solution into 50 mL Nessler tubes. Add 1 mL dil. HC1 and 2 drops of KMnO4. Shake and make upto the mark with water.
Add 1 mL thiocyanate solution to the standard and sample solutions. Compare the colour of the sample with that of the standard. If the sample contains higher concentration of iron, organic matter and silt then evaporate 20 mL of the sample containing 10 mL conc. HCl. Ignite the residue until a grayish white ash is obtained.
Add 5 mL dil. HCl to the ash in a 250 mL volumetric flask and make up the volume to mark with water. Pipette suitable aliquots from this solution and proceed as above method.
Interference:
Zinc, copper, cobalt, chromium, mercury, phosphate and molybdat ions interfere.
Calculation:
Mg/L iron as Fe = matching standard × 1000 × 0.01/mL sample taken for determination
If the analysis of ferric is needed, subtract the ferrous iron value from total iron value.
(d) Determination of manganese by persulphate method:
The method is suitable for raw water, potable water, waste or treated water.
Principle:
The manganese salts are oxidized to permanganate by persulphate in acidicmedium. The resulting colour of the permanganate solution it measured either visually or photometrically.
2MnSO4 + 5(NH4)2S2O8 + 10HNO38H2O → 2HMnO4 +10NH4NO3 + 12H2SO4
Reagents:
(a) Manganese standard solution:
It can be prepared by dissolving 2 g manganese metal in 100 mL sulphuric acid (7%) and make up to 1000 mL in volumetric flask. Take 25 mL of this solution into a 1000 mL volumetric flask and make upto the mark with distilled water.
1.0mL = 50µ Mn
(b) Ammonium per sulphate (NH4)2S2O8,
(c) Special reagent:
Add 400 mL conc. HNO3 to 200 mL distilled water (DW) in a 2000 mL beaker. Dissolve 75g. mercuric sulphate in it. Add 200 mL of 85% phosphoric acid and 35 mg silver nitrate. Dilute the solution to 1000 mL.
Procedure:
To a suitable aliquot of the sample containing organic matter, turbidity and suspended matter add 5 mL conc. HCl. Evaporate to 25 mL. Digest at the boiling point and evaporate to dryness. Add 5 mL conc. H2SO4 and 10 mL conc. HNO3. Mix and evaporate until white dense fumes of SO3 appear.
Again add 10 mL conc HNO3 and evaporate to white dense fumes. Cool and add 40 mL water. Warm and filter through sintered crucible. Make up the filtrate to 100 mL with distilled water and take appropriate volume in a 250 mL beaker.
Add 5 mL special reagent and dilute to 100 mL. Heat and reduce the contents to 40 mL. Add 1 g ammonium persulphate and boil for one minute. Cool and transfer to 50 mL Nessler tube. Compare visually the standard and sample.
(E) Determination of Ag (silver):
Phenanthroline method:
This method is based on the extraction of a blue complex of silver by nitrobenzene in neutral solution (pH=7).
Take 1-5 mL of the sample in a 100 mL separatory funnel and add sufficient amount of 0.1 M EDTA to complex the interfering metals. Then add 1 mL of 20% ammonium acetate solution, 5 mL of 0.001 M 1, 10 phenanthroline, 1 mL of 0.1 M EDTA and 1 mL of 1 M NaNO3. Shake the contents well and then add 20 mL of C6H5NO2. Again shake the contents and allow to stand for half an hour.
Separate the organic extract and add 25 mL of 10-4 M bromopyrogallol red solution and wait for half an hour after shaking the contents. The blue complex thus extracted is then measured spectrophotometrically at 590 nm absorbance against a reagent blank.
(F) Arsenic:
Arsenic may be precipitated as inorganic arsenic or total Arsenic.
The method for the determination of Arsenic is as follows:
Atomic absorption method is used for the determination of both types of arsenic. In this method arsenic is reduced to As (III) state and then converted to AsH3 (arsine) which is directly aspirated into argon-hydrogen flame of an atomic absorption spectrophotometer and measured at 193.7 nm. Arsenic can thus be estimated down to 2.5 ppb.
Procedure:
(a) Inorganic arsenic:
Take 25 mL sample in a 50 mL volumetric flask and add 20 mL of conc HCl and 5 mL of 18 N H2SO4.
(b) Total arsenic:
Take 50 mL of the sample in 150 mL beaker and add 10 mL of conc HNO3 and 12 mL of 18 N H2SO4. Evaporate to SO3 fumes. In order to avoid loss of As and to keep oxidizing conditions in the beaker, add small amounts of conc HNO3 from time to time whenever the red brown fumes of NO2 disappear.
Cool the contents and then add about 25 mL of double distilled or demineralized water, 1 mL of perchloric acid and again evaporate to SO3 fumes. Cool and add 40 mL of conc HCl and dilute the contents to about 100 mL with deionized water.
Now transfer 25 mL of solution (either (a) or (b) into a reaction vessel and add 1.5 mL of 15% KI solution and 0.5 mL SnCl2 reagent (prepared by dissolving 100 g of SnCl2 in 100 mL of conc (HCl). Allow the reduction to take place for about 15 minutes.
As (V) thus gets reduced to As (III). Now fill the medicine dropper with 1.5 mL zinc-slurry made by dissolving 50 g zinc in 100 mL deionized distilled water. The slurry is kept in the form of suspension by making use of mechanical stirrer.
Now place the stopper containing the medicine dropper into the side neck of the reaction vessel. The zinc slurry is then introduced into the sample by squeezing the bulb. The AsH3 thus formed gives a peak which can be measured at 193.7 nm.
(G) Beryllium:
Generally Aluminium method is used for determination of Be. Procedure for the method is as follows.
Be2+ ions produce a red lake with aluminon (aurinitricarboxylic acid) in acetate buffer (pH 4-6) and this red lake is measured at 515 nm.
Procedure:
Dissolve 500 g. of ammonium acetate in one litre of distilled water in a 2 litre beaker and add 80 mL of glacial acetic acid with constant shaking to dissolve all the ammonium acetate. Filter, if necessary.
Now dissolve 1 gm of ammonium salt of aurine tricarboxylic acid (aluminon) in 50 mL of distilled water and add it to the ammonium acetate buffer solution prepared above. Now prepare a solution of 3 g of benzoic acid in 20 mL of CH3COOH and also add this solution to the buffer solution with stirring.
Now transfer the contents to a dark glass bottle of about 4 L capacity, dilute to 2 L by adding distilled water and add 1% gelatin solution and shake the contents well.
Now pipette out 50 mL sample solution in 100 mL volumetric flask and add 2 mL of 2.5% solution of EDTA at about 6 pH. Finally dilute the contents to about 75 mL. To this solution add 15 mL thoroughly. Wait for 15-20 minutes. The red coloured lake thus formed is measured spectrophotometrically at 515 nm against a reagent blank.
(H) Cadmium:
It is very toxic and poisonous metal. Even traces of it can cause severe disease like Tai-Tai. So it becomes necessary to remove it from water.
The methods that can be used for the removal of thus metal are:
The presence of cadmium in water sample may be detected by p-nitrobenzene diazo amino benzene (cadion) or by dinitrodiphenyl carbazide.
Dissolve 0.02 g of cadion in 100 mL of 0.002 2 N alcoholic KOH solution. Place a drop of this solution upon a strip of filter paper resting upon a thick piece of blotting paper. Add a drop of sample solution, made slightly acidic with CH3COOH. Now add a drop of 10% KOH solution. A bright pink colour surrounded by a violet ring indicates cadmium.
If Cr, Co, Cu, Fe, Mg or Ni ions are present, add a few drops of saturated solution of Rochelle salt, sodium potassium tartrate. Silver, if present, may be precipitated by adding 1 % KI solution and then filtered. If Hg is present, it may be precipitated with H2S, and then filtered. Cyanides are boiled off from the acid solution of the sample.
In the case of dinitrodiphenyl carbazide reagent, place a drop of acid, neutral or ammonical solution of sample on a spot plate and add a drop each of 8% NaOH solution and 10% NaCN solution. Now add a drop of 0.1% alcoholic solution of the reagent and 2 drops of 40% HCHO solution.
A blue green colouration or precipitate indicates the presence of cadmium. The reagent is red in alkaline solution, but turns violet in presence of HCHO. The limit of sensitivity is 0.0001 mg of Cd in a concentration of 20 ppm.
Polarographic method:
Take 100 mL of the sample in an Erlenmeyer flask; add 0.1 mL conc. H2SO4 and evaporates to dense white fumes. Add conc. HNO3 to the fuming liquid carefully drop-by-drop till the solution becomes clear as well as colourless. The process of addition of HNO3 is repeated with process of fuming to remove excess of HNO3 as well as chlorides, which may otherwise interfere.
Now neutralize it with NH4OH. The solution is about 0.18 M (NH4)2SO4. Excess of NH4OH is removed by boiling. The solution is cooled and filtered and also diluted to 10 mL by adding distilled water.
This solution is now taken in an appropriate polarographic cell, and about 10 mg of gelatin is added to it in order to suppress maxima. Connect the cell to the polarograph and pass nitrogen through the solution for about 5 minutes to expel O2.
Run the polarogram from 0.0 to -1.6 volt. Now add sufficient NH4OH to make the solution basic (0.4 M in NH3). Again run the polarogram. Again repeat the process after adding 300 mg of EDTA. The half wave potential values (E1/2) are the relative wave heights under these conditions. The E1/2 values are – 0.57 to – 0.59, -0.67 to -0.74 V, nil and 0.0036, 0.0036 (µA/µg), 0.000 respectively.
(I) Chromium:
Take 50-100 mL of water sample containing chromium in a conical flask and add 5mL conc. HNO3 + 2mL H2O2 (30%) and evaporate on a steam bath. Now add a mixture of 5 mL HNO3 and 10 mL conc. H2SO4 and again evaporate to dense white fumes of SO3. The process of evaporation is repeated two or three times and finally the contents are cooled and diluted to 50 mL with distilled water.
The solution is heated and filtered through a sintered glass crucible. The filtrate, along with washing is transferred to a 100 mL volumetric flask, volume is made upto the mark with deionized distilled water. The residue on the sintered glass crucible consists of PbSO4, in case Pb in present in the sample. The residue may be treated for the analysis of lead.
(K) Copper:
Take 100 mL sample in a 250 mL beaker and add 1 mL of conc. H2SO4 + 5 mL of conc. HNO3 and evaporate to dense white fumes of SO3. Add 5 mL conc. HN03 and repeat heating to fumes until the solution becomes colourless. Now cool the solution and dilute it with 70-80 mL of distilled water and again boil. Cool and filter the solution in a 100 mL volumetric flask and make the volume upto the mark (100 mL) by adding distilled water.
Now transfer 50 mL of this solution into a 150 mL separatory funnel and dilute it with 50 mL of distilled water. Then add 5 mL of 1% hydroxyl amine hydrochloride (NH2OH.HCl), 10 mL of 40% sodium citrate and 10 mL of neocuproine reagent (100 mg/100 mL alcohol, CH3OH), keeping the pH of the solution between 4-6. Shake the contents for about half an hour.
Extract the solution with CHCl3 and repeat the extraction of the aqueous layer with 10 mL CHCl3 and transfer the CHCl3 extract in the volumetric flask containing CHCl3 extract. Dilute the combined extracts upto 25 mL with methyl alcohol (CH3OH) and mix well. Now measure the absorbance of the coloured solution at 475 m against a reagent blank carried through the same procedure.
(K) Iron:
Phenanthroline Method:
Take a 50 mL sample in 150 mL Erlenmeyor flask and add 2 mL of HCl+1 mL of NH2OH. HCl (10%). Heat the contents to boiling. Cool and transfer the solution to a 100 mL volumetric flask. Add 10 mL of acetate buffer (ammonium acetate + glacial acetic acid) and 2mL of phenanthroline reagent (100 mg/100 mL distilled water).
……………………… Shake the contents well and dilute upto the mark with distilled water. Wait for 10-15 minutes for the development of red coloured complex. Measure the absorbance at 510 nm against a reagent blank, as usual.
(L) Lead:
Lead is a serious cumulative body poison. Natural water generally contain up to 200 ppb of Pb. A human body contains about 120 mg Pb, 96% in the bones.
Polarographic Method:
Take a 20-50 mL sample in a flask and add 10 mL of KMnO4 (5%) solution and 3-4 mL of conc. H2SO4 and reflux the mixture for atleast 3 hours. Now cool and add 40% hydroxyl amine hydrochloride (NH2OH.HCl) solution in order to reduce KMnO4. Filter and dilute the filtrate to about 100 mL by adding distilled water.
Now take 20 mL aliquot portion of the solution in a separatory funnel and add 10 mL of 40% sod. Citrate solution, and then add dilute ammonium hydroxide (NH4OH) solution to adjust the pH between 8.0 to 8.5. Extract it with 0.01% diphenylthiocarbonzone or dithizone in CCl4 solution.
Back extract with 1% HNO3 and transfer to a 25 mL volumetric flask. Add 2.0 – 3.0 mL of 1 MKCl solution and add HNO3 to adjust pH to about 6. Make up the volume of the solution upto the mark (25 mL). Now measure lead polarographically at half wave potential E1/2 = -0.4 volts.
(M) Selemium:
Diaminobenzidine method is used in which selenium is first oxidized with acid KMnO4 to SeO42- and then reduced to SeO42- in warm 4N HCl. Then it forms a chelate with diaminobenzidine at 1.5 pH. The chelate is extracted into toluene and the yellow extract is measured spectrophotometrically at 420 nm.
(N) Mercury:
Mercury toxicity is a world-wide problem as mercury and its salts are industrial health hazards. More than 100 mg may cause headache, abdominal pain, hemolysis and tumors. Hg compounds affected central nervous system.
Dithizone Method:
Take 100 mL sample in a 500 mL distillation flask and add 5-10 mL of 5% KMnO4with constant shaking. Reflux this solution for about 3-4 hours with ice cold water circulation in the condenser of the distillation flask. Cool and remove excess of KMnO4 by adding some 30% H2O2. The excess of H2O2 is then removed by boiling.
Now cool and solution and add sufficient H2SO4 to make the solution IN in H2SO4. Extract the solution with dithizone solution (20 mg/100 mL CCl4). Make up the volume to about 25 mL with distilled water in a volumetric flask and measure the absorbance at 490 nm as usual against reagent blank. 2-10 µg Hg can be estimated by this method.
Physical Property # 21. Sulphur Dioxide:
A typical bubbler system is illustrated in Figure 8 in the combined sampling unit for smoke and SO2. In this case the absorber solution is dilute hydrogen adjusted to a pH of 4.5. This retains the SO2 by converting it to sulfuric acid, i.e.
SO2 + H2O2 → H2SO4
The quantity of pollutant collected can be determined by titration, with the end point at the original pH, using dilute base (e.g. 0.004 N sodium borate). Alternatively one can calculate the result directly from the pH change.
Obviously this method will not be specific for SO2. Interference can occur due to other gases, such as NO2 or NH3. The use of a pH of 4.5 is specifically designed to counter one potential interferent, CO2, which under these conditions is not absorbed. If the absorber solution is analysed for sulphure ion rather than acidity the method can be made quite specific, but at the expense of increased analytical requirements of time and/or cost.
The West-Gaeke method:
This involves converting the SO2 to the sulphite ion using a solution of potassium tetrachloromercurate, and then colorimetrically determining the concentration using a solution of pararosaniline and formaldehyde as follows:
There are a number of potential interferences in this reaction, but most of these are eliminated by the addition of appropriate reagents. The nitrite ion formed from NO2 is destroyed by reaction with sulphamic acid.
HOSO2NH2 + NO2– + H+ → N2 + H2SO4 + H2O
The use of EDTA in the absorber solution and phosphoric acid in the pararosaniline reagent solution serve to complex any metals present. Ozone is also a potential interferent, but a time delay of 20 minutes or more before analysis ensures that this breaks down in the absorber solution.
This is more specific than using hydrogen peroxide, but it is rarely used today because of concerns about the use of mercury required.
Physical Property # 22. O3 and Total Oxidants:
Most of the bubbler methods for ozone are based on the oxidizing properties of the gas, and hence the methods are indicative of total “oxidants” in the air sample.
The most commonly used procedure involves the reaction with neutral- buffered potassium iodide solution (NBKI). The reaction with ozone is approximated by:
O3 + 3KI + H2O → KI3 + O2 + 2KOH
The liberated iodine is measured at 352 nm.
The most significant interferences in the method are from SO2 and NO2, both of which will also liberate iodine. The former can be removed by a prefilter treated with CrO3. Interference due to the latter can be allowed for if NOx is also measured at the same time.
The only major limitation with the method is that sampling must be restricted to periods of 30 minutes or less, because of the deterioration of the iodine complex with time.