3500-Cu COPPER*

3500-Cu A. Introduction

1. Occurrence and Significance

Copper (Cu) is the first element in Group IB in the periodic

table; it has an atomic number of 29, an atomic weight of 63.54,

and valences of 1 and 2. The average abundance of Cu in the

earth’s crust is 68 ppm; in soils it is 9 to 33 ppm; in streams it

is 4 to 12 g/L; and in groundwater it is 0.1 mg/L. Copper

occurs in its native state, but is also found in many minerals, the

most important of which are those containing sulfide compounds

(e.g., chalcopyrite), but also those with oxides and carbonates.

Copper is widely used in electrical wiring, roofing, various

alloys, pigments, cooking utensils, piping, and in the chemical

industry. Copper salts are used in water supply systems to

control biological growths in reservoirs and distribution pipes

and to catalyze the oxidation of manganese. Copper forms a

number of complexes in natural waters with inorganic and organic ligands. Among the common aqueous species are Cu2,

Cu(OH)2, and CuHCO3

. Corrosion of copper-containing alloys

in pipe fittings may introduce measurable amounts of copper into

the water in a pipe system.

Copper is considered an essential trace element for plants and

animals. Some compounds are toxic by ingestion or inhalation.

The United Nations Food and Agriculture Organization recommended maximum level for irrigation waters is 200 g/L. Under

the lead-copper rule, the U.S. EPA drinking water 90th percentile action level is 1.3 mg/L.

2. Selection of Method

The atomic absorption spectrometric methods (3111B and C),

the inductively coupled plasma methods (3120 and 3125), and

the neocuproine method (B) are recommended because of their

freedom from interferences. The electrothermal atomic absorption method (3113B) also may be used with success with an

appropriate matrix modifier. The bathocuproine method (C) may

be used for potable waters.

3. Sampling and Storage

Copper ion tends to be adsorbed on the surface of sample containers. Therefore, analyze samples as soon as possible after collection. If storage is necessary, use 0.5 mL 1 1 HCl/100 mL sample,

or acidify to pH 2 with HNO3, to prevent this adsorption.

3500-Cu B. Neocuproine Method

1. General Discussion

a. Principle: Cuprous ion (Cu) in neutral or slightly acidic

solution reacts with 2,9-dimethyl-1,10-phenanthroline (neocuproine) to form a complex in which 2 moles of neocuproine are

bound by 1 mole of Cuion. The complex can be extracted by

a number of organic solvents, including a chloroform-methanol

(CHCl3-CH3OH) mixture, to give a yellow solution with a molar

absorptivity of about 8000 at 457 nm. The reaction is virtually

specific for copper; the color follows Beer’s law up to a concentration of 0.2 mg Cu/25 mL solvent; full color development

is obtained when the pH of the aqueous solution is between 3 and

9; the color is stable in CHCl3-CH3OH for several days.

The sample is treated with hydroxylamine-hydrochloride to

reduce cupric ions to cuprous ions. Sodium citrate is used to

complex metallic ions that might precipitate when the pH is

raised. The pH is adjusted to 4 to 6 with NH4OH, a solution of

neocuproine in methanol is added, and the resultant complex is

extracted into CHCl3. After dilution of the CHCl3 to an exact

volume with CH3OH, the absorbance of the solution is measured

at 457 nm.

b. Interference: Large amounts of chromium and tin may

interfere. Avoid interference from chromium by adding sulfurous acid to reduce chromate and complex chromic ion. In the

presence of much tin or excessive amounts of other oxidizing

ions, use up to 20 mL additional hydroxylamine-hydrochloride

solution.

Cyanide, sulfide, and organic matter interfere but can be

removed by a digestion procedure (see Section 3030).

c. Minimum detectable concentration: The minimum detectable concentration, corresponding to 0.01 absorbance or 98%

transmittance, is 3 g Cu when a 1-cm cell is used and 0.6 g

Cu when a 5-cm cell is used.

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 use at 457 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 narrow-band violet filter having maximum

transmittance in the range 450 to 460 nm.

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

TFE stopcock and stopper.

* Approved by Standard Methods Committee, 1999. Editorial revisions, 2011.

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

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3. Reagents

a. Redistilled water, copper-free: Because most ordinary distilled water contains detectable amounts of copper, use redistilled water, prepared by distilling singly distilled water in a

resistant-glass still, or distilled water passed through an ionexchange unit, to prepare all reagents and dilutions.

b. Stock copper solution: To 200.0 mg polished electrolytic

copper wire or foil in a 250-mL conical flask, add 10 mL water

and 5 mL conc HNO3. After the reaction has slowed, warm

gently to complete dissolution of the copper and boil to expel oxides

of nitrogen, using precautions to avoid loss of copper. Cool, add

about 50 mL water, transfer quantitatively to a 1-L volumetric flask,

and dilute to the mark with water; 1 mL  200 g Cu.

c. Standard copper solution: Dilute 50.00 mL stock copper

solution to 500 mL with water; 1.00 mL  20.0 g Cu.

d. Sulfuric acid, H2SO4, conc.

e. Hydroxylamine-hydrochloride solution: Dissolve 50 g

NH2OH  HCl in 450 mL water.

f. Sodium citrate solution: Dissolve 150 g Na3C6H5O7  2H2O

in 400 mL water. Add 5 mL NH2OH  HCl solution and 10 mL

neocuproine reagent. Extract with 50 mL CHCl3 to remove copper

impurities and discard CHCl3 layer.

g. Ammonium hydroxide, NH4OH, 5N: Dilute 330 mL conc

NH4OH (28-29%) to 1000 mL with water. Store in a polyethylene bottle.

h. Congo red paper, or other pH test paper showing a color

change in the pH range of 4 to 6.

i. Neocuproine reagent: Dissolve 100 mg 2,9-dimethyl1,10-phenanthroline hemihydrate* in 100 mL methanol. This

solution is stable under ordinary storage conditions for a

month or more.

j. Chloroform, CHCl3: Avoid or redistill material that comes

in containers with metal-lined caps.

k. Methanol, CH3OH, reagent grade.

l. Nitric acid, HNO3, conc.

m. Hydrochloric acid, HCl, conc.

4. Procedure

a. Preparation of calibration curve: Pipet 50 mL water into a

125-mL separatory funnel for use as a reagent blank. Prepare

standards by pipetting 1.00 to 10.00 mL (20.0 to 200 g Cu)

standard copper solution into a series of 125-mL separatory

funnels, and dilute to 50 mL with water. Add 1 mL conc H2SO4

and use the extraction procedure given in ¶ b below.

Construct a calibration curve by plotting absorbance versus

micrograms of copper.

To prepare a calibration curve for smaller amounts of copper,

dilute 10.0 mL standard copper solution to 100 mL. Carry 1.00-

to 10.00-mL volumes of this diluted standard through the previously described procedure, but use 5-cm cells to measure

absorbance.

b. Treatment of sample: Transfer 100 mL sample to a 250-mL

beaker, add 1 mL conc H2SO4 and 5 mL conc HNO3. Add a few

boiling chips and cautiously evaporate to dense white SO3 fumes

on a hot plate. If solution remains colored, cool, add another 5

mL conc HNO3, and again evaporate to dense white fumes.

Repeat, if necessary, until solution becomes colorless.

Cool, add about 80 mL water, and bring to a boil. Cool and

filter into a 100-mL volumetric flask. Make up to 100 mL with

water using mostly beaker and filter washings.

Pipet 50.0 mL or other suitable portion containing 4 to 200 g

Cu, from the solution obtained from preliminary treatment, into

a 125-mL separatory funnel. Dilute, if necessary, to 50 mL with

water. Add 5 mL NH2OH  HCl solution and 10 mL sodium

citrate solution, and mix thoroughly. Adjust pH to approximately

4 by adding 1-mL increments of NH4OH until Congo red paper

is just definitely red (or other suitable pH test paper indicates a

value between 4 and 6).

Add 10 mL neocuproine reagent and 10 mL CHCl3. Stopper

and shake vigorously for 30 s or more to extract the copperneocuproine complex into the CHCl3. Let mixture separate into

two layers and withdraw lower CHCl3 layer into a 25-mL

volumetric flask, taking care not to transfer any of the aqueous

layer. Repeat extraction of the water layer with an additional 10

mL CHCl3 and combine extracts. Dilute combined extracts to 25

mL with CH3OH, stopper, and mix thoroughly.

Transfer an appropriate portion of extract to a suitable absorption cell (1 cm for 40 to 200 g Cu; 5 cm for lesser amounts) and

measure absorbance at 457 nm or with a 450- to 460-nm filter.

Use a sample blank prepared by carrying 50 mL water through

the complete digestion and analytical procedure.

Determine micrograms copper in final solution by reference to

the appropriate calibration curve.

5. Calculation

mg Cu/L  g Cu (in 25 mL final volume)

mL portion taken for extraction

6. Bibliography

SMITH, G.F. & W.H. MCCURDY. 1952. 2,9-Dimethyl-1,10-phenanthroline: New specific in spectrophotometric determination of copper.

Anal. Chem. 24:371.

LUKE, C.L. & M.E. CAMPBELL. 1953. Determination of impurities in

germanium and silicon. Anal. Chem. 25:1586.

GAHLER, A.R. 1954. Colorimetric determination of copper with neocuproine. Anal. Chem. 26:577.

FULTON, J.W. & J. HASTINGS. 1956. Photometric determinations of copper in aluminum and lead-tin solder with neocuproine. Anal. Chem.

28:174.

FRANK, A.J., A.B. GOULSTON & A.A. DEACUTIS. 1957. Spectrophotomet-

* GFS Chemicals, Inc., Columbus, OH, or equivalent. ric determination of copper in titanium. Anal. Chem. 29:750.

COPPER (3500-Cu)/Neocuproine Method

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3500-Cu C. Bathocuproine Method

1. General Discussion

a. Principle: Cuprous ion forms a water-soluble orange-colored chelate with bathocuproine disulfonate (2,9-dimethyl-4,7-

diphenyl-1,10-phenanthrolinedisulfonic acid, disodium salt).

While the color forms over the pH range 3.5 to 11.0, the

recommended pH range is between 4 and 5. The sample is

buffered at a pH of about 4.3 and reduced with hydroxylamine

hydrochloride. The absorbance is measured at 484 nm. The

method can be applied to copper concentrations up to at least

5 mg/L with a sensitivity of 20 g/L.

b. Interference: The following substances can be tolerated

with an error of less than 2%:

Substance

Concentration

mg/L

Cations

Aluminum 100

Beryllium 10

Cadmium 100

Calcium 1000

Chromium (III) 10

Cobalt (II) 5

Iron (II) 100

Iron (III) 100

Lithium 500

Magnesium 100

Manganese (II) 500

Nickel (II) 500

Sodium 1000

Strontium 200

Thorium (IV) 100

Zinc 200

Anions

Chlorate 1000

Chloride 1000

Fluoride 500

Nitrate 200

Nitrite 200

Orthophosphate 1000

Perchlorate 1000

Sulfate 1000

Compounds

Residual chlorine 1

Linear alkylate sulfonate (LAS) 40

Cyanide, thiocyanate, persulfate, and EDTA also can interfere.

c. Minimum detectable concentration: 20 g/L with a 5-cm

cell.

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, with a light

path of 1 to 5 cm (unless nessler tubes are used):

1) Spectrophotometer, for use at 484 nm.

2) Filter photometer, equipped with a blue-green filter exhibiting maximum light transmission near 484 nm.

3) Nessler tubes, matched, 100-mL, tall form.

b. Acid-washed glassware: Rinse all glassware with conc HCl

and then with copper-free water.

3. Reagents

a. Copper-free water: See 3500-Cu.B.3a.

b. Stock copper solution: Prepare as directed in 3500-

Cu.B.3b, but use 20.00 mg copper wire or foil; 1.00 mL  20.00

g Cu.

c. Standard copper solution: Dilute 250 mL stock copper

solution to 1000 mL with water; 1.00 mL  5.00 g Cu. Prepare

daily.

d. Hydrochloric acid, HCl, 1 1.

e. Hydroxylamine hydrochloride solution: See 3500-Cu.B.3e.

f. Sodium citrate solution: Dissolve 300 g Na3C6H5O7  2H2O in

water and make up to 1000 mL.

g. Disodium bathocuproine disulfonate solution: Dissolve

1.000 g C12H4N2(CH3)2(C6H4)2 (SO3Na)2 in water and make up

to 1000 mL.

4. Procedure

Pipet 50.0 mL sample, or a suitable portion diluted to 50.0 mL,

into a 250-mL erlenmeyer flask. In separate 250-mL erlenmeyer

flasks, prepare a 50.0-mL water blank and a series of 50.0-mL

copper standards containing 5.0, 10.0, 15.0, 20.0, and 25.0 g

Cu. To sample, blank, and standards add, mixing after each

addition, 1.00 mL 1 1 HCl, 5.00 mL NH2OH  HCl solution,

5.00 mL sodium citrate solution, and 5.00 mL disodium bathocuproine disulfonate solution. Transfer to cells and read sample

absorbance against the blank at 484 nm. Plot absorbance against

micrograms Cu in standards for the calibration curve. Estimate

concentration from the calibration curve.

5. Calculation

mg Cu/L  g Cu (in 66 mL final volume)

mL sample

6. Precision and Bias

A synthetic sample containing 1000 g Cu/L, 500 g Al/L, 50

g Cd/L, 110 g Cr/L, 300 g Fe/L, 70 g Pb/L, 50 g Mn/L,

150 g Ag/L, and 650 g Zn/L was analyzed in 33 laboratories

by the bathocuproine method, with a relative standard deviation

of 4.1% and a relative error of 0.3%.

7. Bibliography

SMITH, G.F. & D.H. WILKINS. 1953. New colorimetric reagent specific

for copper. Anal. Chem. 25:510.

BORCHARDT, L.G. & J.P. BUTLER. 1957. Determination of trace amounts

of copper. Anal. Chem. 29:414.

ZAK, B. 1958. Simple procedure for the single sample determination of

serum copper and iron. Clinica Chim. Acta 3:328.

BLAIR, D. & H. DIEHL. 1961. Bathophenanthrolinedisulfonic acid and

bathocuproinedisulfonic acid, water soluble reagents for iron and

copper. Talanta 7:163.

COPPER (3500-Cu)/Bathocuproine Method

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