3500-Zn ZINC*

3500-Zn A. Introduction

1. Occurrence and Significance

Zinc (Zn) is the first element in Group IIB in the periodic

table; it has an atomic number of 30, an atomic weight of 65.38,

and a valence of 2. The average abundance of Zn in the earth’s

crust is 76 ppm; in soils it is 25 to 68 ppm; in streams it is 20

g/L, and in groundwaters it is 0.1 mg/L. The solubility of

zinc is controlled in natural waters by adsorption on mineral

surfaces, carbonate equilibrium, and organic complexes. Zinc is

used in a number of alloys such as brass and bronze, and in

batteries, fungicides, and pigments. Zinc is an essential growth

element for plants and animals but at elevated levels it is toxic to

some species of aquatic life. The United Nations Food and

Agriculture Organization recommended level for zinc in irrigation waters is 2 mg/L. The U.S. EPA secondary drinking water

standard MCL is 5 mg/L. Concentrations above 5 mg/L can

cause a bitter astringent taste and an opalescence in alkaline

waters. 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 methods (3111B and C)

and inductively coupled plasma methods (3120 and 3125) are

preferred. The zincon method (B), suitable for analysis of both

potable and polluted waters, may be used if instrumentation for

the preferred methods is not available.

3. Sampling and Storage

See Section 3010B.2 for sample handling and storage.

3500-Zn B. 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 color. Sodium ascorbate reduces manganese interference.

The developed color is stable except in the presence of copper

(see table below).

b. Interferences: The following ions interfere at 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 CrO4

2 50

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

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

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

2. Apparatus

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

1) Spectrophotometer, for measurements at 620 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 red filter having maximum transmittance

near 620 nm. Deviation from Beer’s Law occurs when the filter

band pass exceeds 20 nm.

b. Graduated cylinders, 50-mL, with ground-glass stoppers,

Class B or better.

c. Erlenmeyer flasks, 50-mL.

d. Filtration apparatus: 0.45-m filters and filter holders.

3. Reagents

a. Metal-free water: See Section 3111B.3c. Use water for

rinsing apparatus and preparing solutions and dilutions.

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. Standard zinc solution: Dilute 10.00 mL stock zinc solution

to 1000 mL; 1.00 mL  10.00 g 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.

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

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

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i. Hydrochloric acid, HCl, conc and 1N.

j. Sodium hydroxide, NaOH, 6N and 1N.

4. Procedure

a. Preparation of colorimetric standards: Accurately deliver

0, 0.5, 1.0, 3.0, 5.0, 10.0, and 14.0 mL standard zinc solution to

a series of 50-mL graduated mixing cylinders. 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. (Lower-range standards may be

prepared to extend the quantitation range. Longer optical path

cells can be used. Verify linearity of response in this lower

concentration range.) 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. Reserve remaining solution to zero the

instrument. Add 1.0 mL cyclohexanone to the erlenmeyer flask.

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 readily acid-extractable total zinc, add 1 mL conc HCl to 50 mL sample and mix

thoroughly. Filter and adjust to pH 7. To determine dissolved

zinc, filter sample through a 0.45-m membrane filter. Adjust to

pH 7 with 1N NaOH or 1N HCl if necessary after filtering.

c. Sample 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 Zn/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 g Zn/L, 500 g Al/L, 50

g Cd/L, 110 g Cr/L, 470 g Cu/L, 300 g Fe/L, 70 g Pb/L,

120 g Mn/L, and 150 g 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.

PANDE, S.P. 1980. Study on the determination of zinc in drinking water.

J. IWWA XII (3):275.

ZINC (3500-Zn)/Zincon Method

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