3500-Pb LEAD*

3500-Pb A. Introduction

1. Occurrence and Significance

Lead (Pb) is the fifth element in Group IVA in the periodic

table; it has an atomic number of 82, an atomic weight of 207.19,

and valences of 2 and 4. The average abundance of Pb in the

earth’s crust is 13 ppm; in soils it ranges from 2.6 to 25 ppm; in

streams it is 3 g/L, and in groundwaters it is generally 0.1

mg/L. Lead is obtained chiefly from galena (PbS). It is used in

batteries, ammunition, solder, piping, pigments, insecticides, and

alloys. Lead also was used in gasoline for many years as an

anti-knock agent in the form of tetraethyl lead.

The common aqueous species are Pb2and hydroxide and

carbonate complexes. Lead in a water supply may come from

industrial, mine, and smelter discharges or from the dissolution of plumbing and plumbing fixtures. Tap waters that are inherently noncorrosive or not suitably treated may contain lead resulting

from an attack on lead service pipes, lead interior plumbing, brass

fixtures and fittings, or solder pipe joints.

Lead is nonessential for plants and animals. It is toxic by

ingestion and is a cumulative poison. The Food and Drug Administration regulates lead content in food and in house paints.

Under the lead-copper rule, the U.S. EPA drinking water 90th

percentile action level is 15 g/L.

2. Selection of Method

The atomic absorption spectrometric method (3111B) has a

relatively high detection limit in the flame mode and requires

an extraction procedure (3111C) for the low concentrations

common in potable water. The electrothermal atomic absorption (AA) method (3113B) is more sensitive for low concentrations and does not require extraction. The inductively coupled plasma/mass spectrometric method (3125) is even more

sensitive than the electrothermal AA method. The inductively

coupled plasma method (3120) has a sensitivity similar to that

of the flame atomic absorption method. Anodic stripping

voltammetry (3130B) can achieve superior detection levels,

but is susceptible to interferences from copper, silver, gold,

and organic compounds. The dithizone method (B) is sensitive and specific as a colorimetric procedure.

3500-Pb B. 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 g Pb in the presence of 20 g Tl, 100 g Sn2,

200 g In3, and 1000 g each of Ba2, Cd2, Co2, Cu2,

Mg2, Mn2, Hg2, Sr2, Zn2, Al3, Sb3, As3, Cr3, Fe3,

V3, PO4

3, and SO42. 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 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 3030H or K.

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

f. Quality control (QC): The QC practices considered to be an

integral part of each method can be found in Section 3020.

2. Apparatus

a. Spectrophotometer for use at 510 nm, providing a light

path of 1 cm or longer.

* Approved by Standard Methods Committee, 1997. Editorial revisions, 2011.

Joint Task Group: 20th Edition—See Section 3500-Al.

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b. pH meter.

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

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

3. Reagents

Prepare all reagents in lead-free 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 g Pb.

b. Working lead solution: Dilute 2.0 mL stock solution to 100

mL with water; 1 mL  2.00 g 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, 20 g 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. CAUTION:

Do not pipet by mouth. Prepare solution in a fume hood.

f. Stock dithizone solution: The dithizone concentration in the

stock dithizone solutions is based on having a 100% pure dithizone reagent. Some commercial grades of dithizone are contaminated with the oxidation product diphenylthiocarbodiazone or

with metals. Purify dithizone as directed below. For dithizone

solutions not stronger than 0.001% (w/v), calculate the exact

concentration by dividing the absorbance of the solution in a

1.00-cm cell at 606 nm by 40.6  103

, the molar absorptivity.

In a fume hood, dissolve 100 mg dithizone in 50 mL CHCl3 in

a 150-mL beaker and filter through a 7-cm-diam paper.* Receive

filtrate in a 500-mL separatory funnel or in a 125-mL erlenmeyer

flask under slight vacuum; use a filtering device designed to

handle the CHCl3 vapor. Wash beaker with two 5-mL portions

CHCl3, and filter. Wash the paper with three 5-mL portions

CHCl3, adding final portion dropwise to edge of paper. If filtrate

is in flask, transfer with CHCl3 to a 500-mL separatory funnel.

Add 100 mL 1 99 NH4OH to separatory funnel and shake

moderately for 1 min; excessive agitation produces slowly

breaking emulsions. Let layers separate, swirling funnel gently

to submerge CHCl3 droplets held on surface of aqueous layer.

Transfer CHCl3 layer to 250-mL separatory funnel, retaining the

orange-red aqueous layer in the 500-mL funnel. Repeat extraction, receiving CHCl3 layer in another 250-mL separatory funnel

and transferring aqueous layer, using 1 99 NH4OH, to the

500-mL funnel holding the first extract. Repeat extraction, transferring the aqueous layer to 500-mL funnel. Discard CHCl3

layer.

To combined extracts in the 500-mL separatory funnel add 1

1 HCl in 2-mL portions, mixing after each addition, until

dithizone precipitates and solution is no longer orange-red. Extract precipitated dithizone with three 25-mL portions CHCl3.

Dilute combined extracts to 1000 mL with CHCl3; 1.00 mL 

100 g dithizone.

g. Dithizone working solution: Dilute 100 mL stock dithizone

solution to 250 mL with CHCl3; 1 mL  40 g 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: CAUTION: Perform the following

procedure (excluding use of spectrophotometer) in a fume

hood. 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, 3500-Pb.B.3g, to zero spectrophotometer.

b. Without sample digestion: To 100 mL acidified sample (pH

2) 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 ¶ a above.

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 g 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.

* Whatman No. 42, or equivalent. † Whatman No. 541, or equivalent.

LEAD (3500-Pb)/Dithizone Method

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5. Calculation

mg Pb/L  g Pb (in 10 mL, from calibration curve)

mL sample

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.

LEAD (3500-Pb)/Dithizone Method

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