Use 1 liter of a sample containing 0.1 to 1.0 part per million of
copper, and proportionate amounts for other concentrations. Evaporate to
about 75 cc., and wash into a 100 cc. platinum dish. Add 2 cc. of dilute
sulfuric acid for clear and soft waters; add more acid to very alkaline
waters to offset the alkalinity; add 5 cc. of acid to waters carrying
much organic matter or clay to insure the formation of a soluble copper
salt. Then place the dish as the anode in a direct current circuit,
suspend a spiral wire cathode in the solution so that it is parallel to
and about half an inch from the bottom of the dish, and close the
circuit.
Electrolyze for about four hours with occasional stirring, or over
night, if convenient. The current may be supplied by two gravity cells
in series, yielding a current through the solution of about 0.02 ampere.
Lift out the cathode without previously opening the circuit, and immerse
the spiral in a small amount of dilute nitric acid previously heated to
boiling. Wash off the wire and evaporate the nitric acid solution to
dryness on the water bath. If the presence of silver is suspected add a
few drops of hydrochloric acid before evaporation. Dissolve the residue
in water and wash it into a 50 cc. Nessler tube. Dilute to 50 cc. and
add 10 cc. of the potassium sulfide solution. The color of the copper
sulfide develops at once and is fairly permanent, lasting at least
several hours. Add 10 cc. of the potassium sulfide solution to a similar
tube containing 50 cc. of distilled water, and then add to it standard
copper solution in 0.2 cc. portions until the colors of the two tubes
match. If 1 liter of the sample is used copper in parts per million is
equal to the number of cubic centimeters of standard copper solution
required to match the color of the sample multiplied by 0.2.