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STANDARD METHODS: FOR THE EXAMINATION OF WATER AND WASTEWATER

18TH EDITION 1992

Prepared and published jointly by:
AMERICAN PUBLIC HEALTH ASSOCIATION
AMERICAN WATER WORKS ASSOCIATION
WATER ENVIRONMENT FEDERATION

Joint Editorial Board
Arnold E. Greenberg, APHA,
Chairman
Lenore S. Clesceri, WEF
Andrew D. Eaton, AWWA

Managing Editor
Mary Ann H. Franson

Publication Office
American Public Health Association
1015 Fifteenth Street, NW
Washington, DC 20005

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         Standard methods for the examination of water and wastewater.
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3500-Zn ZINC*

3500-Zn A. Introduction

1. Occurrence

Zinc is an essential and beneficial element in human growth. Concentrations above 5 mg/L can cause a bitter astringent taste and an opalescence in alkaline waters. The zinc concentration of U.S. drinking waters varies between 0.06 and 7.0 mg/L with a mean of 1.33 mg/L. Zinc most commonly enters the domestic water supply from deterioration of galvanized iron and dezincification of brass. In such cases lead and cadmium also may be present because they are impurities of the zinc used in galvanizing. Zinc in water also may result from industrial waste pollution.

2. Selection of Method

The atomic absorption spectrometric and inductively coupled plasma methods are preferred. Dithizone method I is intended for potable water whereas dithizone method II is intended for polluted water and the zincon method can be used for either.

3. Sampling and Storage

For methods B, C, E, and F, refer to section 3010B.2 for sample handling and storage. For method D, preferably analyze sample within 6 h after collection to avoid acid removal step in procedure. Alternatively, preserve samples with acid according to 3010B.2 and remove acid as described in D.4c.

*Approved by Standard Methods Committee. 1990.

3500-Zn B. Atomic Absorption Spectrometric Method

See flame atomic absorption spectrometric method, Sections 3111B and C.

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3500-Zn C. Inductively Coupled Plasma Method

See Section 3120.

3500-Zn D. Dithizone Method I

1. General Discussion

a. Principle: Nearly 20 metals can react with dithizone to produce colored coordination compounds. These dithizonates are extractable into organic solvents such as carbon tetrachloride (CCl4). Most interferences in the zinc-dithizone reaction can be overcome by adjusting the pH to 4.0 to 5.5 and by adding sufficient sodium thiosulfate. Zinc also forms a weak thiosulfate complex that tends to retard the slow and incomplete reaction between zinc and dithizone. For this reason, the determination is empirical and demands the use of an identical technique in standard and sample analysis. The duration and vigor of shaking, the volumes of sample, sodium thiosulfate, and dithizone, and the pH should be kept constant.

b. Interference: Interference from bismuth, cadmium, cobalt, copper, gold, lead, mercury, nickel, palladium, silver, and stannous tin in the small quantities found in potable waters is eliminated by complexing with sodium thiosulfate and by pH adjustment. Ferric iron, residual chlorine, and other oxidizing agents convert dithizone to a yellow-brown color. The zinc-dithizone reaction is extremely sensitive and unusual precautions must be taken to avoid contamination. High and erratic blanks often are traceable to glass containing zinc oxide, surface-contaminated glassware, rubber products, stopcock greases, reagent-grade chemicals, and distilled water. Because of the extreme sensitivity of the reaction, prepare and segregate glassware especially for this determination and extract reagents with dithizone solution to remove all traces of zinc and contaminating metals. Dithizone and dithizonates decompose rapidly in strong light. Perform analyses in subdued light and do not expose solutions to the light of the photometer longer than is necessary. Avoid direct sunlight.

c. Minimum detectable quantity: 1 µg Zn.

2. Apparatus

a. Colorimetric equipment: Use one of the following:

1) Spectrophotometer, for use at either 535 or 620 nm, providing a light path of 1 cm or longer.

2) Filter photometer, providing a light path of 2 cm or longer and equipped with either a green filter having maximum transmittance near 535 nm or a red filter having maximum transmittance near 620 nm.

3) Nessler tubes, matched.

b. Separatory funnels, capacity 125 to 150 mL. Squibb form, preferably with inert TFE stopcocks.

c. Glassware: Rinse all glassware with 1 + 1 HNO3 and water.

d. pH meter.

3. Reagents

a. Zinc-free water: Use redistilled or deionized distilled water for rinsing apparatus and preparing solutions and dilutions.

b. Stock zinc solution: Dissolve 100.0 mg 30-mesh zinc metal in a slight excess of 1 + 1 HCl; about 1 mL is required. Dilute to 1000 mL with water: 1.00 mL = 100 µg Zn.

c. Standard zinc solution: Dilute 10.00 mL stock zinc solution to 1000 mL with water; 1.00 mL = 1.00 µg Zn.

d. Hydrochloric acid, HCl, 0.02N: Dilute 1.0 mL cone HCl to 600 mL with water. If high blanks are traced to this reagent, dilute cone HCl with an equal volume of distilled water and redistill in an all-borosilicate-glass still.

e. Sodium acetate, 2M: Dissolve 68 g NaC2H3O2·3H2O and dilute to 250 mL with water.

f. Acetic acid, 1 + 7.

g. Acetate buffer solution: Mix equal volumes of 2M sodium acetate solution and 1 + 7 acetic acid solution. Extract with 10-mL portions of dithizone solution I until the last extract remains green; then extract with CCl4 to remove excess dithizone.

h. Sodium thiosulfate solution: Dissolve 25 g Na2S2O3·5H2O in 100 mL water. Purify by dithizone extraction as in ¶ 3g above.

i. Stock dithizone solution (CCl4): See Section 1070D.2b3). (CAUTION: CCl4 is toxic. Avoid inhalation, ingestion, and contact with the skin.)

j. Dithizone solution I: Dilute 40 mL stock dithizone solution (CCl4) to 100 mL with CCl4. Prepare daily.

k. Dithizone solution II: Dilute 10 mL dithizone solution I to 100 mL with CCl4. Prepare daily.

l. Carbon tetrachloride, CCl4, ACS grade.

m. Sodium citrate solution: Dissolve 10 g Na3C6H5O7·2H2O in 90 mL water. Purify by dithizone extraction as in ¶ 3g preceding. Use this reagent in final cleansing of glassware.

4. Procedure

a. Preparation of colorimetric standards: To a series of thoroughly cleansed (see ¶ 2c above) 125-mL Squibb separatory funnels, add 0, 1.00, 2.00, 3.00, 4.00, and 5.00 mL standard zinc solution to provide standards containing 0, 1.00, 2.00, 3.00, 4.00, and 500 µg Zn, respectively. Bring each volume up to 10.0 mL by adding water. To each funnel add 5.0 mL acetate buffer and 1.0 mL Na2S2O3 solution, and mix. The pH should be between 4 and 5.5. To each funnel and 10.0 mL dithizone solution II, stopper, and shake vigorously for 4.0 min. Let layers separate, dry inside of stem below stopcock of funnel with strips of filter paper, and run lower (CCl4) layer into a clean, dry absorption cell.

b. Photometric measurement: Measure either the red color of zinc dithizonate at 535 nm or the green color of unreacted dithizone at 620 nm.

Set photometer at 100% transmittance with the blank if the 535-nm wavelength is selected. If 620 nm is used, set blank at 10.0% transmittance. Plot a calibration curve. Run a new calibration curve with each set of samples.

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c. Treatment of samples: If the zinc content is not within the working range, dilute sample with water or concentrate it in a silica dish. If the sample has been preserved with acid, evaporate a portion to dryness in a silica dish to remove excess acid. Do not neutralize with hydroxides because these usually contain excessive amounts of zinc. Using a pH meter, adjust sample to pH 2 to 3 with HCl. Transfer 10.0 mL to a separatory funnel. Complete analysis as in ¶ 4a. beginning with “To each funnel add 5.0 mL acetate buffer.”

d. Visual comparison: If a photometric instrument is not available, run samples and standards at the same time and transfer to matched test tubes or nessler tubes. The range of colors obtained with various amounts of zinc are roughly:

Zinc µg Color
0 (blank) green
1 blue
2 blue-violet
3 violet
4 red-violet
5 red-violet

5. Calculation

Image

6. Precision and Bias

A synthetic sample containing 650 µg Zn/L, 500 µg Al/L, 50 µg Cd/L, 110 µg Cr/L, 470 µg Cu/L, 300 µg Fe/L, 70 µg Pb/L, 120 µg Mn/L, and 150 µg Ag/L in distilled water was analyzed in 46 laboratories by the dithizone method with a relative standard deviation of 18.2% and a relative error of 25.9%.

7. Bibliography

HIBBARD, P.L. 1937. A dithizone method for measurement of small amounts of zinc. Ind. Eng. Chem., Anal. Ed. 9:127.

SANDELL, E.B. 1937. Determination of copper, zinc, and lead in silicate rocks. Ind. Eng. Chem., Anal. Ed. 9:464.

HIBBARD, P.L. 1938. Estimation of copper, zinc, and cobalt (with nickel) in soil extracts. Ind. Eng. Chem., Anal. Ed. 10:615.

WICHMAN, H.J. 1939. Isolation and determination of traces of metals: The dithizone system. Ind. Eng. Chem., Anal. Ed. 11:66.

COWLING, H. & E.J. MILLER. 1941. Determination of small amounts of zinc in plant materials: A photometric dithizone method. Ind. Eng. Chem., Anal. Ed. 13:145.

ALEXANDER, O.R. & L.V. TAYLOR. 1944. Improved dithizone procedure for determination of zinc in foods. J. Assoc. Offic. Agr. Chem. 27:325.

SERFASS, E.J. et al. 1947. Chem. Anal. 35:55.

SERFASS, E.J. 1947. Research Report Serial No. 3. American Electroplaters Soc., Newark, N.J., p. 22.

SERFASS, E.J. et al. 1949. Determination of impurities in electroplating solutions, Plating 36:254, 818.

SNELL, F.D. & C.T. SNELL. 1949. Colorimetric Methods of Analysis, 3rd ed. D. Van Nostrand Co., Princeton, N.J., Vol. 2, pp. 1-7, 412-419.

BUTTS, P.G., A.R. GAHLER & M.G. MELLON. 1950. Colorimetric determination of metals in sewage and industrial wastes. Sewage Ind. Wastes 22:1543.

BARNES, H. 1951. The determination of zinc by dithizone. Analyst 76:220.

COOPER, S.S. & M.L. SULLIVAN. 1951. Spectrophotometric studies of dithizone and some dithizonates. Anal. Chem. 23:613.

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

SANDELL., E.B. 1959. Colorimetric Determination of Traces of Metals. 3rd ed. Interscience Publishers. New York, N.Y.

3500-Zn E. Dithizone Method II

1. Principle

Zinc is separated from other metals by extraction with dithizone and is determined by measuring the color of the zincdithizone complex in carbon tetrachloride (CCl4). Specificity in the separation is achieved by extracting from a nearly neutral solution containing bis (2-hydroxyethl) dithiocarbamyl ion and cyanide ion. which prevent moderate concentrations of cadmium, copper, lead, and nickel from reacting with dithizone. If excessive amounts of these metals are present, follow the special procedure given in ¶ 4b2) below.

See Section 3500-Zn.D for general precautions.

2. Apparatus

a. Colorimetric equipment: Once of the following is required:

1) Spectrophotometer, for use at 535 nm. providing a light path of 1 cm or longer.

2) Filter photometer, providing a light path of 1 cm or longer and equipped with a greenish yellow filter with maximum transmittance near 535 nm.

b. Separatory funnels, 125-mL, Squibb form, with ground-glass stoppers.

3. Reagents

a. Zinc-free water: See Section 3500-Zn.D.3a.

b. Stock zinc solution: Dissolve 1000 mg (1.000 g) zinc metal in 10 mL 1 + 1 HNO3. Dilute and boil to expel oxides of nitrogen. Dilute to 1000 mL; 1.00 mL = 1.00 mg Zn.

c. Methyl red indicator: Dissolve 0.1 g methyl red sodium salt and dilute to 100 mL with water.

d. Sodium citrate solution: Dissolve 10 g Na3C6H5O7·2H2O in 90 mL water. Shake with 10 mL dithizone solution to remove zinc, then filter.

e. Ammonium hydroxide, NH4OH, cone: Place 660 mL water in a 1-L polyethylene bottle and chill by immersion in an ice

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bath. Pass ammonia gas from a cylinder through a glass-wool trap into chilled bottle until volume of liquid has increased to 900 mL. Alternatively, place 900 mL cone reagent-grade NH4OH in a 1500-mL distillation flask and distill into a chilled 1-L polyethylene bottle initially containing 250 mL water. Continue distilling until volume of liquid in bottle has increased to 900 mL, keeping condenser tip below surface of liquid.

f. Potassium cyanide solution: Dissolve 5 g KCN in 95 mL water. (CAUTION: Potassium cyanide is a deadly poison. Avoid skin contract or inhalation of vapors. Do not pipet by mouth or bring in contact with acids.)

g. Acetic acid, cone.

h. Carbon tetrachloride, CCl4: See Section 3500-Zn. D.3l.

i. Bis (2-hydroxyethyl) dithiocarbamate solution: Dissolve 4.0 g diethanolamine and 1 mL CS2 in 40 mL methyl alcohol. Prepare every 3 or 4 d.

j. Dithizone solution: Dilute 50 mL stock dithizone solution (CCl4), prepared in accordance with Section 1070D. 2b3), to 250 mL with CCl4. Prepare fresh daily.

k. Sodium sulfide solution I: Dissolve 3.0 g Na2S·9H2O or 1.65 g Na2S·3H2O in 100 mL water.

l. Sodium sulfide solution II: Prepare just before use by diluting 4 mL Na2S solution 1 to 100 mL.

m. Nitric acid, HNO3, 6N.

n. Hydrogen sulfide, H2S.

4. Procedure

a. Preparation of calibration curve:

1) Prepare, just before use, a zinc solution containing 2.0 µg Zn/mL by diluting 5 mL standard zinc solution to 250 mL, then diluting 10 mL of the latter solution to 100 mL with water. Pipet 5.00, 10.00, 15.00, and 20.00 mL. containing 10 to 40 µg Zn. into separate 125-mL separatory funnels and adjust volume to about 20 mL. Set up another funnel containing 20 mL water as a blank.

2) Add 2 drops methyl red indicator and 2.0 mL sodium citrate solution to each funnel. If the indicator is not yellow, add cone NH4OH a drop at a time until it just turns yellow. Add 1.0 mL KCN solution and acetic acid, a drop at a time, until the indicator just turns peach color.

3) Extract methyl red by shaking with 5 mL CCl4 Discard yellow CCl4 layer. Add 1 mL dithiocarbamate solution. Extract with 10 mL dithizone solution, shaking for 1 min.

Draw CCl4 layer into another separatory funnel and repeat the extraction with successive 5-mL portions of dithizone solution until the last one shows no change from the green dithizone color. Discard aqueous layer.

4) Shake combined dithizone extracts with a 10-mL portion of Na2S solution II, separate layers and repeat the washing with further 10-mL portions of Na2S solution until the unreacted dithizone solution has been removed completely, as shown by the color of the aqueous layer, which remains colorless or very pale yellow; usually three washings are sufficient.

Remove water adhering to stem of funnel with a cotton swab and drain pink CCl4 solution into a dry 50-mL volumetric flask. Use a few milliliters of CCl4 to rinse the last droplets from funnel and dilute to mark with CCl4.

5) Determine absorbance of the zinc dithizonate solutions at 535 nm, using CCl4 as a reference. Plot an absorbance-concentration curve after subtracting the blank absorbance. The calibration curve is linear if monochromatic light is used.

6) Clean separatory funnels by shaking several minutes successively with HNO3, distilled water, and finally a mixture of 5 mL sodium citrate and 5 mL dithizone, to minimize large or erratic blanks that result from adsorption of zinc on the glass surface. If possible, reserve separatory funnels exclusively for the zinc determination.

b. Treatment of sample:

1) Digest sample as directed under Preliminary Digestion for Metals, Section 3030D. Transfer a portion containing 10 to 40 µg Zn to a clean 125-mL separatory funnel and adjust volume to about 20 mL. Determine zinc in this solution as described in ¶ 4a.

If more than 30 mL dithizone solution is needed to extract zinc completely, the portion taken contains too much zinc or the quantity of other metals that react with dithizone exceeds the amount that can be withheld by the complexing agent. If this occurs, follow the procedure in ¶ 4b2) below.

2) Separation of excessive amounts of cadmium, copper, and lead—When the quantity of these metals, separately or jointly, exceeds 2 mg in the portion taken, adjust volume to about 20 mL in a 100-mL beaker. Adjust acidity to 0.4 to 0.5N* by adding dilute HNO3 or NH4OH as necessary. Pass H2S into cold solution for 5 min. Filter off the precipitated sulfides through a sintered glass filter and wash precipitate with two small portions of hot water. Boil filtrate 3 to 4 min to remove H2S, cool, transfer to a separatory funnel, and determine zinc as described in ¶ 4b1) et seq.

5. Calculation

Image

6. Bibliography

See 3500-Zn.D.7.

3500-Zn F. Zincon Method

1. General Discussion

a. Principle: Zinc forms a blue complex with 2-carboxy-2'-hydroxy-5'-sulfoformazyl benzene (zincon) in a solution buffered to pH 9.0. Other heavy metals likewise form colored complexes with zincon. Cyanide is added to complex zinc and heavy metals. Cyclohexanone is added to free zinc selectively from its cyanide complex so that it can be complexed with zincon to form a blue

* The normalities of the solutions obtained in the preliminary treatment are approximately 3N for the HNO3-H2SO4 digestion and approximately 0.8N for the HNO3-HClO4 digestion. color. Sodium ascorbate reduces manganese interference. The developed color is stable except in the presence of copper.

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b. Interferences: The following ions interfere in concentrations exceeding those listed:

Ion mg/L. Ion mg/L
Cd2+ 1 Cr3+ 10
Al3+ 5 Ni2+ 20
Mn2+ 5 Cu2+ 30
Fe3+ 7 Co2+ 30
Fe2+ 9 CrO42- 50

c. Minimum detectable concentration: 0.02 mg Zn/L.

2. Apparatus

Colorimetric equipment: Once of the following is required:

a. Spectrophotometer, for measurements at 620 nm, providing a light path of 1 cm or longer.

b. Filter photometer, providing a light path of 1 cm or longer and equipped with a red filter having maximum transmittance near 620 nm. Deviation from Beer's Law occurs when the filter band pass exceeds 20 nm.

3. Reagents

a. Zinc-free water: See Section 3500-Zn. D.3a.

b. Stock zinc solution: See Section 3500-Zn. E.3b; 1.00 mL = 1.00 mg Zn.

c. Standard zinc solution: Dilute 10.00 mL stock zinc solution to 1000 mL; 1.00 mL = 10.00 µg Zn.

d. Sodium ascorbate, fine granular powder, USP.

e. Potassium cyanide solution: Dissolve 1.00 g KCN in approximately 50 mL water and dilute to 100 mL. CAUTION: Potassium cyanide is a deadly poison. Avoid skin contact or inhalation of vapors. Do not pipet by mouth or bring in contact with acids.

f. Buffer solution, pH 9.0: Dissolve 8.4 g NaOH pellets in about 500 mL water. Add 31.0 g H3BO3 and swirl or stir to dissolve. Dilute to 1000 mL with water and mix thoroughly.

g. Zincon reagent: Dissolve 100 mg zincon (2-carboxy-2'-hydroxy-5'-sulfoformazyl benzene) in 100 mL methanol. Because zincon dissolves slowly, stir and/or let stand overnight.

h. Cyclohexanone, purified.

i. Hydrochloric acid, HCl, cone and 6N

j. Sodium hydroxide, NaOH, 6N.

4. Procedure

a. Preparation of colorimetric standards: Add 0, 0.5, 1.0, 3.0, 5.0, 10.0, and 14.0 mL standard zinc solution to a series of clean 50-mL graduated mixing cylinders or erlenmeyer flasks. Dilute each to 20.0 mL to yield solutions containing 0, 0.25, 0.5, 1.5, 2.5, 5.0, and 7.0 mg Zn/L, respectively. Add the following to each solution in sequence, mixing thoroughly after each addition: 0.5 g sodium ascorbate, 5.0 mL buffer solution, 2.0 mL KCN solution, and 3.0 mL zincon solution. Pipet 20.0 mL of the solution into a clean 50-mL erlenmeyer flask and add 1.0 mL cyclohexanone. Swirl for 10 s and note time. Transfer portions of both solutions to clean sample cells. Use solution without cyclohexanone to zero colorimeter. Read and record absorbance for solution with cyclohexanone after 1 min. The calibration curve does not pass through zero because of the color enhancement effect of cyclohexanone on zincon.

b. Treatment of samples: To determine dissolved zinc, filter sample through a 0.45-µm membrane filter. Adjust to pH 7 with 6N NaOH or 6N HCl if necessary after filtering. For total zinc add 1 mL cone HCl to 50 mL sample and mix thoroughly. Filter and adjust to pH 7. Before analysis cool samples to less than 30°C if necessary. Analyze 20.0 mL of prepared sample as described in ¶ 4a above, beginning with “Add the following to each solution …” If the zinc concentration exceeds 7 mg/L prepare a sample dilution and analyze a 20.0-mL portion.

5. Calculation

Read zinc concentration (in milligrams per liter) directly from the calibration curve.

6. Precision and Bias

A synthetic sample containing 650 µg Zn/L, 500 µg Al/L, 50 µg Cd/L, 110 µg Cr/L, 470 µg Cu/L, 300 µg Fe/L, 70 µg Pb/L, 120 µg Mn/L, and 150 µg Ag/L in doubly demineralized water was analyzed in a single laboratory. A series of 10 replicates gave a relative standard deviation of 0.96% and a relative error of 0.15%.

A wastewater sample from an industry in Standard Industrial Classification (SIC) No. 3333, primary smelting and refining of zinc, was analyzed by 10 different persons. The mean zinc concentration was 3.36 mg Zn/L and the relative standard deviation was 1.7%. The relative error compared to results from an atomic absorption analysis of the same sample was – 1.0%.

7. Bibliography

PLATTE, J.A. & V.M. MARCY. 1959. Photometric determination of zinc with zincon. Anal. Chem. 31:1226.

RUSH, R.M. & J.H. YOE. 1954. Colorimetric determination of zinc and copper with 2-carboxy-2'hydroxy-5'-sulfoformazyl-benzene. Anal. Chem. 26:1345.

MILLER, D.G. 1979. Colorimetric determination of zinc with zincon and cyclohexanone. J. Water Pollut. Control Fed. 51:2402.

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