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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

All rights reserved. No part of this publication may be reproduced, graphically or electronically, including entering in storage or retrieval systems, without the prior written permission of the publishers.

30M7/92

The Library of Congress has catalogued this work as follows:
American Public Health Association.
          Standard methods for the examination of water and wastewater.
ISBN 0-87553-207-1

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3500-Pb LEAD^*

3500-Pb A. Introduction

1. Occurrence

Lead is a serious cumulative body poison. Natural waters seldom contain more than 5 μg/L, although much higher values have been reported. Lead in a water supply may come from industrial, mine, and smelter discharges or from the dissolution of old lead plumbing. Tap waters that are soft, acid, and not suitably treated may contain lead resulting from an attack on lead service pipes or solder pipe joints.

2. Selection of Method

The atomic absorption spectrometric method has a relatively high detection limit in the flame mode and requires an extraction procedure for the low concentrations common in potable water: the electrothermal atomic absorption method is much more sensitive for low concentrations and does not require extraction. The inductively coupled plasma method has a sensitivity similar to that of the flame atomic absorption method. The dithizone method is sensitive and specific as a colorimetric procedure.

*Approved by Standard Methods Committee. 1990.

3500-Pb B. Atomic Absorption Spectrometric Method

See flame atomic absorption spectrometric method, Sections 3111B and C, and electrothermal atomic absorption spectrometric method, Section 3113.

3500-Pb C. Inductively Coupled Plasma Method

See Section 3120.

3500-Pb D. Dithizone Method

1. General Discussion

a. Principle: An acidified sample containing microgram quantities of lead is mixed with ammoniacal citrate-cyanide reducing solution and extracted with dithizone in chloroform (CHCl₃) to form a cherry-red lead dithizonate. The color of the mixed color solution is measured photometrically.^1,2 Sample volume taken for analysis may be 2 L when digestion is used.

b. Interference: In a weakly ammoniacal cyanide solution (pH 8.5 to 9.5) dithizone forms colored complexes with bismuth, stannous tin, and monovalent thallium. In strongly ammoniacal citrate-cyanide solution (pH 10 to 11.5) the dithizonates of these ions are unstable and are extracted only partially.³ This method uses a high pH, mixed color, single dithizone extraction. Interference from stannous tin and monovalent thallium is reduced further when these ions are oxidized during preliminary digestion. A modification of the method allows detection and elimination of bismuth interference. Excessive quantities of bismuth, thallium, and tin may be removed.⁴

Dithizone in CHCl₃ absorbs at 510 nm; control its interference by using nearly equal concentrations of excess dithizone in samples, standards, and blank.

The method is without interference for the determination of 0.0 to 30.0 μg Pb in the presence of 20 μg T1⁺, 100 μg Sn²⁺, 200 μg In³⁺, and 1000 μg each of Ba²⁺, Cd²⁺, Co²⁺, Cu²⁺, Mg²⁺, Mn²⁺, Hg²⁺, Sr²⁺, Zn²⁺, Al³⁺, Sb³⁺, As³⁺, Cr³⁺, Fe³⁺, V³⁺, PO³_3—, and SO₄^2—. Gram quantities of alkali metals do not interfere. A modification is provided to avoid interference from excessive quantities of bismuth or tin.

c. Preliminary sample treatment: At time of collection acidify with conc HNO₃ to pH <2 but avoid excess HNO₃. Add 5 mL 0.1N iodine solution to avoid losses of volatile organo-lead compounds during handling and digesting of samples. Prepare a blank of lead-free distilled water and carry through the procedure.

d. Digestion of samples: Unless digestion is shown to be unnecessary, digest all samples for dissolved or total lead as described in 3030G or H.

e. Minimum detectable concentration: 1.0 μg Pb/10 mL dithizone solution.

2. Apparatus

a. Spectrophotometer for use at 510 nm, providing a light path of 1 cm or longer.

b. pH meter.

c. Separatory funnels: 250-mL Squibb type. Clean all glassware, including sample bottles, with 1 + 1 HNO₃. Rinse thoroughly with distilled or deionized water.

d. Automatic dispensing burets: Use for all reagents to minimize indeterminate contamination errors.

3. Reagents

Prepare all reagents in lead-free distilled water.

a. Stock lead solution: Dissolve 0.1599 g lead nitrate. Pb(NO₃)₂ (minimum purity 99.5%), in approximately 200 mL water. Add 10 mL conc HNO₃ and dilute to 1000 mL with water. Alternatively, dissolve 0.1000 g pure Pb metal in 20 mL 1 + 1 HNO₃ and dilute to 1000 mL with water; 1.00 mL = 100 μg Pb.

b. Working lead solution: Dilute 2.0 mL stock solution to 100 mL with water; 1 mL = 2.00 μg Pb.

c. Nitric acid, HNO₃, 1 + 4: Dilute 200 mL conc HNO₃ to 1 L with water.

d. Ammonium hydroxide, NH₄OH, 1 + 9: Dilute 10 mL conc NH₄OH to 100 mL with water.

e. Citrate-cyanide reducing solution: Dissolve 400 g dibasic ammonium citrate, (NH₄)₂HC₆H₅O₇, 20g anhydrous sodium sulfite, Na₂SO₃, 10 g hydroxylamine hydrochloride, NH₂OH·HCl, and 40 g potassium cyanide, KCN (CAUTION: Poison) in water and dilute to 1 L. Mix this solution with 2 L conc NH₄OH. Do not pipet by mouth.

f. Stock dithizone solution: See 1070D. 2b1), stock dithizone solution I.

g. Dithizone working solution: Dilute 100 mL stock dithizone solution to 250 mL with CHCl₃; 1 mL = 40 μg dithizone.

h. Special dithizone solution: Dissolve 250 mg dithizone in 250 mL CHCl₃. This solution may be prepared without purification because all extracts using it are discarded.

i. Sodium sulfite solution: Dissolve 5 g anhydrous Na₂SO₃ in 100 mL water.

j. Iodine solution: Dissolve 40 g KI in 25 mL water, add 12.7 g resublimed iodine, and dilute to 1000 mL.

4. Procedure

a. With sample digestion: To a digested sample containing not more than 1 mL conc acid add 20 mL 1 + 4 HNO₃ and filter through lead-free filter paper* and filter funnel directly into a 250-mL separatory funnel. Rinse digestion beaker with 50 mL water and add to filter. Add 50 mL ammoniacal citrate-cyanide solution, mix, and cool to room temperature. Add 10 mL dithizone working solution, shake stoppered funnel vigorously for 30 s, and let layers separate. Insert lead-free cotton in stem of separatory funnel and draw off lower layer. Discard 1 to 2 mL CHCl₃ layer, then fill absorption cell. Measure absorbance of extract at 510 nm, using dithizone working solution, ¶ 3g, to zero spectrophotometer.

b. Without sample digestion: To 100 mL acidified sample (pH2) in a 250-mL separatory funnel add 20 mL 1 + 4 HNO₃ and 50 mL citrate-cyanide reducing solution; mix. Add 10 mL dithizone working solution and proceed as in ¶ 4a.

c. Calibration curve: Plot concentration of at least five standards and a blank against absorbance. Determine concentration of lead in extract from curve. All concentrations are μg Pb/10 mL final extract.

d. Removal of excess interferences: The dithizonates of bismuth, tin, and thallium differ from lead dithizonate in maximum absorbance. Detect their presence by measuring sample absorbance at 510 nm and at 465 nm. Calculate corrected absorbance of sample at each wavelength by subtracting absorbance of blank at same wavelength. Calculate ratio of corrected absorbance at 510 nm to corrected absorbance at 465 nm. The ratio of corrected absorbances for lead dithizonate is 2.08 and for bismuth dithizonate is 1.07. If the ratio for the sample indicates interference, i.e., is markedly less than 2.08, proceed as follows with a new 100-mL sample: If the sample has not been digested, add 5 mL Na₂SO₃ solution to reduce iodine preservative. Adjust sample to pH 2.5 using a pH meter and 1+ 4 HNO₃ or 1 + 9 NH₄OH as required. Transfer sample to 250-mL separatory funnel, extract with a minimum of three 10-mL portions special dithizone solution, or until the CHCl₃ layer is distinctly green. Extract with 20-mL portions CHCl₃ to remove dithizone (absence of green). Add 20 mL 1 + 4 HNO₃, 50 mL citrate-cyanide reducing solution, and 10 mL dithizone working solution. Extract as in ¶ 4a and measure absorbance.

5. Calculation

6. Precision and Bias

Single-operator precision in recovering 0.0104 mg Pb/L from Mississippi River water was 6.8% relative standard deviation and – 1.4% relative error. At the level of 0.026 mg Pb/L, recovery was made with 4.8% relative standard deviation and 15% relative error.

7. References

1. SNYDER. L.J. 1947. Improved dithizone method for determination of lead—mixed color method at high pH. Anal. Chem. 19:684.
2. SANDELL, E.B. 1959. Colorimetric Determination of Traces of Metals, 3rd ed. Interscience, New York, N.Y.
3. WICHMANN, H.J. 1939. Isolation and determination of trace metals—the dithizone system. Ind. Eng. Chem., Anal. Ed. 11:66.
4. AMERICAN SOCIETY FOR TESTING AND MATERIALS. 1977. Annual Book of ASTM Standards. Part 26, Method D3112-77, American Soc. Testing & Materials, Philadelphia, Pa.

*Whatman No. 541 or equivalent.