Chloride Test in Wastewater

Introduction :

Chloride, in the form of chloride (Cl ~) ion, is one of the major inorganic anions in water and wastewater. The salty taste produced by chloride concentrations is variable and dependent on the chemical composition of water. Some waters containing 250 mg C1~/L may have a detectable salty taste if the cation is sodium.

On the other hand, the typical salty taste may be absent in waters containing as much as 1000 mg/L when the predominant cations are calcium and magnesium.

The chloride concentration is higher in wastewater than in raw water because sodium chloride (NaCl) is a common article of diet and passes unchanged through the digestive system. Along the sea coast, chloride may be present in high concentrations because of leakage of salt water into the sewerage system. It also may be increased by industrial processes.

A high chloride content may harm metallic pipes and structures, as well as growing plants.

Selection of Method

Six methods are presented for the determination of chloride. Because the first two are similar in most respects, selection is largely a matter of personal preference. The argentometric method (B) is suitable for use in relatively clear waters when 0.15 to 10 mg Cl~ are present in the portion titrated. The end point of the mercuric nitrate method (C) is easier to detect. The potentiometric method (D) is suitable for colored or turbid samples in which color-indicated end points might be difficult to observe. The potentiometric method can be used without a pretreatment step for

samples containing ferric ions (if not present in an amount greater than the chloride concentration), chromic, phosphate, and ferrous and other heavy-metal ions. The ferricyanide method (E) is an automated technique. Flow injection analysis (G), an automated colorimetric technique, is useful for analyzing large numbers of samples. Preferably determine chloride by ion chromatography (Section 4110). Chloride also can be determined by the capillary ion electrophoresis method (Section 4140). Methods (C and G) in which mercury, a highly toxic reagent, is used require special disposal practices to avoid improper sewage discharges. Follow appropriate regulatory procedures (see Section 1090).

Sampling and Storage

Collect representative samples in clean, chemically resistant glass or plastic bottles. The maximum sample portion required is 100 mL. No special preservative is necessary if the sample is to be stored.

B. Argentometric method

1. General Discussion

a. Principle: In a neutral or slightly alkaline solution, potassium chromate can indicate the end point of the silver nitrate titration of chloride. Silver chloride is precipitated quantitatively before red silver chromate is formed.

b. Interference: Substances in amounts normally found in potable waters will not interfere. Bromide, iodide, and cyanide register as equivalent chloride concentrations. Sulfide, thiosulfate, and sulfite ions interfere but can be removed by treatment with hydrogen peroxide. Orthophosphate in excess of 25 mg/L interferes by precipitating as silver phosphate. Iron in excess of 10 mg/L interferes by masking the end point.

2. Apparatus

a. Erlenmeyer flask, 250-mL.

b. Buret, 50-mL.

3. Reagents

a. Potassium chromate indicator solution: Dissolve 50 g K2CrO4 in a little distilled water. Add AgNO3 solution until a definite red precipitate is formed. Let stand 12 h, filter, and dilute to 1 L with distilled water.

b. Standard silver nitrate titrant, 0.0141M (0.0141A1): Dissolve 2.395 g AgNO3 in distilled water and dilute to 1000 mL. Standardize against NaCl by the procedure described in 11 4b below; 1.00 mL = 500 fig Cl~. Store in a brown bottle.

c. Standard sodium chloride, 0.0141M (0.0141AT): Dissolve 824.0 mg NaCl (dried at 140°C) in distilled water and dilute to 1000 mL; 1.00 mL = 500 jjig Cr.

d. Special reagents for removal of interference:

1) Aluminum hydroxide suspension: Dissolve 125 g aluminum potassium sulfate or aluminum ammonium sulfate, AIK(SO4)2-12H2O or A1NH4(SO4)2-12H2O, in 1 L distilled water. Warm to 60°C and add 55 mL cone ammonium hydroxide (NH4OH) slowly with stirring. Let stand about 1 h, transfer to a large bottle, and wash precipitate by successive additions, with thorough mixing and decanting with distilled water, until free from chloride. When freshly prepared, the suspension occupies a volume of approximately 1 L.

2) Phenolphthalein indicator solution.

3) Sodium hydroxide, NaOH, IN.

4) Sulfuric acid, H2SO4, IN.

5) Hydrogen peroxide, H2O2, 30%.

4. Procedure

a. Sample preparation: Use a 100-mL sample or a suitable portion diluted to 100 mL. If the sample is highly colored, add 3 mL A1(OH)3 suspension, mix, let settle, and filter. If sulfide, sulfite, or thiosulfate is present, add 1 mL H2O2 and stir for 1 min.

b. Titration: Directly titrate samples in the pH range 7 to 10. Adjust sample pH to 7 to 10 with H2SO4 or NaOH if it is not in this range. For adjustment, preferably use a pH meter with a nonchloride- type reference electrode. (If only a chloride-type electrode is available, determine amount of acid or alkali needed for adjustment and discard this sample portion. Treat a separate portion with required acid or alkali and continue analysis.) Add 1.0 mL K2CrO4 indicator solution. Titrate with standard AgNO3 titrant to a pinkish yellow end point. Be consistent in end-point recognition. Standardize AgNO3 titrant and establish reagent blank value by the titration method outlined above. A blank of 0.2 to 0.3 mL is usual.

5. Calculation

rag C1~/L =(A - B) X TV X 35450/mL sample

where:

A — mL titration for sample,

B = mL titration for blank, and

N = normality of AgNO3.

mg NaCl/L = (mg Cr/L) X 1.65

6. Precision and Bias

A synthetic sample containing 241 mg C1~/L, 108 mg Ca/L, 82 mg Mg/L; 3.1 mg K/L, 19.9 mg Na/L, 1.1 mg NO3--N/L, 0.25 mg NCV- N/L, 259 mg SO4 2~/L, and 42.5 mg total alkalinity/ L (contributed by NaHCO3) in distilled water was analyzed in 41 laboratories by the argentometric method, with a relative standard deviation of 4.2% and a relative error of 1.7%.

7. Bibliography

HAZEN, A. 1889. On the determination of chlorine in water. Amer. Chem. J. 11:409.

KOLTHOFF, I.M. & V.A. STENOER. 1947. Volumetric Analysis. 2nd ed.

Vol. 2. Interscience Publishers, New York, N.Y., pp. 242-245, 256-258.

PAUSTIAN, P. 1987. A novel method to calculate the Mohr chloride titration.

In Advances in Water Analysis and Treatment, Proc. 14^th Annu. AWWA Water Quality Technology Conf., November 16-20, 1986, Portland, Ore., p. 673. American Water Works Assoc., Denver,Colo.

C. Mercuric Nitrate Method

1. General Discussion

a. Principle'. Chloride can be titrated with mercuric nitrate, Hg(NO3)2, because of the formation of soluble, slightly dissociated mercuric chloride. In the pH range 2.3 to 2.8, diphenylcarbazone indicates the titration end point by formation of a purple complex with the excess mercuric ions. Xylene cyanol FF serves as a pH indicator and end-point enhancer. Increasing the strength of the titrant and modifying the indicator mixtures extend the range of measurable chloride concentrations.

b. Interference: Bromide and iodide are titrated with Hg(NO3)2 in the same manner as chloride. Chromate, ferric, and sulfite ions interfere when present in excess of 10 mg/L.

2. Apparatus

a. Erlenmeyer flask, 250-mL.

b. Microburet, 5-mL with 0.01-mL graduation intervals.

3. Reagents

a. Standard sodium chloride, 0.0141M (0.01417V): See Method B, H 3c above.

b. Nitric acid, HNO3, 0.1 AT.

c. Sodium hydroxide, NaOH, 0.1 TV.

d. Reagents for chloride concentrations below 100 mg/L:

1) Indicator-acidifier reagent: The HNO3 concentration of this reagent is an important factor in the success of the determination and can be varied as indicated in a) or b) to suit the alkalinity range of the sample. Reagent a) contains sufficient HNO3 to neutralize a total alkalinity of 150 mg as CaCO3/L to the proper pH in a 100-mL sample. Adjust amount of HNO3 to accommodate samples of alkalinity different from 150 mg/L.

a) Dissolve, in the order named, 250 mg s-diphenylcarbazone, 4.0 mL cone HNO3, and 30 mg xylene cyanol FF in 100 mL 95% ethyl alcohol or isopropyl alcohol. Store in a dark bottle in a refrigerator. This reagent is not stable indefinitely. Deterioration causes a slow end point and high results.

b) Because pH control is critical, adjust pH of highly alkaline or acid samples to 2.5 ± 0.1 with 0.1/V HNO3 or NaOH, not with sodium carbonate (Na2CO3). Use a pH meter with a nonchloride type of reference electrode for pH adjustment. If only the usual chloride-type reference electrode is available for pH adjustment, determine amount of acid or alkali required to obtain a pH of 2.5 ±0.1 and discard this sample portion. Treat a separate sample portion with the determined amount of acid or alkali and continue analysis. Under these circumstances, omit HNO3 from indicator reagent.

2) Standard mercuric nitrate titrant, 0.007 05M (0.0141/V): Dissolve 2.3 g Hg(NO3)2 or 2.5 g Hg(NO3)2-H2O in 100 mL distilled water containing 0.25 mL cone HNO3. Dilute to just under 1 L. Make a preliminary standardization by following the procedure described in 11 4a. Use replicates containing 5.00 mL standard NaCl solution and 10 mg sodium bicarbonate (NaHCO3) diluted to 100 mL with distilled water. Adjust titrant to 0.0141/V and make a final standardization; 1.00 mL = 500 |_ig Cl ~. Store away from light in a dark bottle.

e. Reagent for chloride concentrations greater than 100 mg/L:

1) Mixed indicator reagent: Dissolve 0.50 g diphenylcarbazone powder and 0.05 g bromphenol blue powder in 75 mL 95% ethyl or isopropyl alcohol and dilute to 100 mL with the same alcohol.

2) Strong standard mercuric nitrate titrant, 0.0705M (0.141/V) Dissolve 25 g Hg(NO3)2-H2O in 900 mL distilled water containing 5.0 mL cone HNO3. Dilute to just under 1 L and standardize by following the procedure described in f 4b. Use replicates containing 25.00 mL standard NaCl solution and 25 mL distilled water. Adjust titrant to 0.141/V and make a final standardization; l.OOmL = S.OOmgCr.

4. Procedure

a. Titration of chloride concentrations less than 100 mg/L: Use a 100-mL sample or smaller portion so that the chloride content is less than 10 mg.

 Add 1.0 mL indicator-acidifier reagent. (The color of the solution should be green-blue at this point. A light green indicates pH less than 2.0; a pure blue indicates pH more than 3.8.) For most potable waters, the pH after this addition will be 2.5 ±0.1. For highly alkaline or acid waters, adjust pH to about 8 before adding indicator-acidifier reagent.

Titrate with 0.0141 N Hg(NO3)2 titrant to a definite purple end point. The solution turns from green-blue to blue a few drops before the end point.

Determine blank by titrating 100 mL distilled water containing 10 mg NaHCO3.

b. Titration of chloride concentrations greater than JOO mg/L: Use a sample portion (5 to 50 mL) requiring less than 5 mL titrant to reach the end point. Measure into a 150-mL beaker. Add approximately 0.5 mL mixed indicator reagent and mix well. The color should be purple. Add 0.1 AT HNO3 dropwise until the color just turns yellow. Titrate with strong Hg(NO3)2 titrant to first permanent dark purple. Titrate a distilled water blank using the same procedure.

5. Calculation

(A - B) X N X 35450

mg C1-/L = - '-

mL sample

where:

A = mL titration for sample,

B = mL titration for blank, and

N = normality of Hg(NO3)2.

mg NaCl/L = (mg Cr/L) X 1.65

6. Precision and Bias

A synthetic sample containing 241 mg C1~/L, 108 mg Ca/L, 82 mg Mg/L, 3.1 mg K/L, 19.9 mg Na/L, 1.1 mg NO3~-N/L, 0.25 mg NO2~-N/L, 259 mg SO4 2"/L, and 42.5 mg total alkalinity/ L (contributed by NaHCO3) in distilled water was analyzed in 10 laboratories by the mercurimetric method, with a relative standard deviation of 3.3% and a relative error of 2.9%.

7. Bibliography

KOLTHOFF, I.M. & V.A. STENGER. 1947. Volumetric Analysis, 2nd ed. Vol. 2. Interscience Publishers, New York, N.Y., pp. 334-335.

DOMASK, W.C. & K.A. KOBE. 1952. Mercurimetric determination of chlorides

and water-soluble chlorohydrins. Anal. Chem. 24:989.

GOLDMAN, E. 1959. New indicator for the mercurimetric chloride determination

in potable water. Anal. Chem. 31:1127.

D. Potentiometric method

1. General Discussion

a. Principle: Chloride is determined by potentiometric titration with silver nitrate solution with a glass and silver-silver chloride electrode system. During titration an electronic voltmeter is used to detect the change in potential between the two electrodes. The end point of the titration is that instrument reading at which the greatest change in voltage has occurred for a small and constant increment of silver nitrate added.

b. Interference: Iodide and bromide also are titrated as chloride. Ferricyanide causes high results and must be removed. Chromate and dichromate interfere and should be reduced to the chromic state or removed. Ferric iron interferes if present in an amount substantially higher than the amount of chloride. Chromic ion, ferrous ion, and phosphate do not interfere.

 Grossly contaminated samples usually require pretreatment. Where contamination is minor, some contaminants can be destroyed simply by adding nitric acid.

2. Apparatus

a. Glass and silver-silver chloride electrodes: Prepare in the laboratory or purchase a silver electrode coated with AgCl for use with specified instruments. Instructions on use and care of electrodes are supplied by the manufacturer.

b. Electronic voltmeter, to measure potential difference between electrodes: A pH meter may be converted to this use by substituting the appropriate electrode.

c. Mechanical stirrer, with plastic-coated or glass impeller.

3. Reagents

a. Standard sodium chloride solution, 0.0141M (0.0141A/): See K 4500-a~.B.3c.

b. Nitric acid, HNO3, cone.

c. Standard silver nitrate titrant, 0.0141A/ (0.0141A7): See 11 4500-C1-.B.36.

d. Pretreatment reagents:

1) Sulfuric acid, H2SO4, 1 + 1.

2) Hydrogen peroxide, H2O2, 30%.

3) Sodium hydroxide, NaOH, IN.

4. Procedure

a. Standardization: The various instruments that can be used in this determination differ in operating details; follow the manufacturer's instructions. Make necessary mechanical adjustments. Then, after allowing sufficient time for warm up (10 min), balance internal electrical components to give an instrument setting of 0 mV or, if a pH meter is used, a pH reading of 7.0.

1) Place 10.0 mL standard NaCl solution in a 250-mL beaker, dilute to about 100 mL, and add 2.0 mL cone HNO3. Immerse stirrer and electrodes.

2) Set instrument to desired range of millivolts or pH units. Start stirrer.

3) Add standard AgNO3 titrant, recording scale reading after each addition. At the start, large increments of AgNO3 may be added; then, as the end point is approached, add smaller and equal increments (0.1 or 0.2 mL) at longer intervals, so that the exact

end point can be determined. Determine volume of AgNO3 used at the point at which there is the greatest change in instrument reading per unit addition of AgNO3.

4) Plot a differential titration curve if the exact end point cannot be determined by inspecting the data. Plot change in instrument reading for equal increments of AgNO3 against volume of AgNO3 added, using average of buret readings before and after each addition. The procedure is illustrated in Figure 4500-C1~:1.

b. Sample analysis:

1) Pipet 100.0 mL sample, or a portion containing not more than 10 mg Cl~, into a 250-mL beaker. In the absence of interfering substances, proceed with H 3) below.

2) In the presence of organic compounds, sulfite, or other interferences (such as large amounts of ferric iron, cyanide, or sulfide) acidify sample with H2SO4, using litmus paper. Boil for 5 min to remove volatile compounds. Add more H2SO4, if necessary, to keep solution acidic. Add 3 mL H2C>2 and boil for 15 min, adding chloride-free distilled water to keep the volume above 50 mL. Dilute to 100 mL, add NaOH solution dropwise until alkaline to litmus, then 10 drops in excess. Boil for 5 min, filter into a 250-mL beaker, and wash precipitate and paper several times with hot distilled water.

3) Add cone HNO3 dropwise until acidic to litmus paper, then 2.0 mL in excess. Cool and dilute to 100 mL if necessary. Immerse stirrer and electrodes and start stirrer. Make any necessary adjustments according to the manufacturer's instructions and set selector switch to appropriate setting for measuring the difference of potential between electrodes.

4) Complete determination by titrating according to 11 4a4). If an end-point reading has been established from previous determinations for similar samples and conditions, use this predetermined end point. For the most accurate work, make a blank titration by carrying chloride-free distilled water through the procedure.

5. Calculation

mg

(A - B) X N X 35450

mL sample

where:

A = mL AgNO3,

B = mL blank, and

N = normality of titrant.

6. Precision and Bias

In the absence of interfering substances, the precision and bias are estimated to be about 0.12 mg for 5 ing Cl~, or 2.5% of the amount present. When pretreatment is required to remove interfering substances, the precision and bias are reduced to about 0.25 mg for 5 mg Cl~, or 5% of amount present.

7. Bibliography

KOLTHOFF, I.M. & N.H. FURMAN. 1931. Potentiometric Titrations, 2nd ed. John Wiley & Sons, New York, N.Y.

REFFENBURG, H.B. 1935. Colorimetric determination of small quantities of chlorides in water. Ind. Eng. Chetn., Anal. Ed. 7:14.

CALDWELL, J.R. & H.V. MEYER. 1935. Chloride determination. Ind. Eng. Chem., Anal. Ed. 7:38.

SERFASS, E.J. & R.F. MURACA. 1954. Procedures for Analyzing Metal- Finishing Wastes. Ohio River Valley Water Sanitation Commission, Cincinnati, Ohio, p. 80.

FURMAN, N.H., ed. 1962. Standard Methods of Chemical Analysis, 6^th ed. D. Van Nostrand Co., Princeton, N.J., Vol. I.

WALTON, H.F. 1964. Principles and Methods of Chemical Analysis. Prentice-

Hall, Inc., Englewood Cliffs, N.J.

WILLARD, H.H., L.L. MERRITT & J.A. DEAN. 1965. Instrumental Methods of Analysis, 4th ed. D. Van Nostrand Co., Princeton, N.J.