That wonderful chemical compound that has solved so many problems in obtaining sound plated deposits and even allowed us to plate alloys such as brass and bronze has turned into a pariah! Mention cyanide to the average person on the street and he will immediately conjure up a vision of skull and crossbones. Mention cyanide to an EPA representative and he may begin reciting the strict regulations imposed on its discharge.

However, not long after he begins, some confusion is likely to arise in making distinctions among the many forms of cyanide; total, free, amenable to chlorination, simple, complex, and, for good measure, cyanate.

What are all these "cyanides" and how are they determined? How can we reduce cyanide in our discharge to as low a level as possible? I n the next few months, well endeavor to answer those questions.

Total Cyanide

If we had an analytical procedure that could detect all the cyanide groups present in a waste or wastewater sample, we could refer to the readout or result as "total cyanide." The truth is, such an analytical procedure is beyond present-day chemistry, so we try to come as close as possible.

Typical effluent from cyanide-using plating shops contains the "simple" and "complex" types. Simple cyan ides are those that are combined with a metal or alkali for example, sodium, cyanide or copper cyanide. Complex cyanides come in numerous forms but generally have more than one cation (positively charged element) combined with the cyanide. For instance, K3[Cu(CN)4], K4[Fe(CN)6] or Na2[Ni(CN)4] are complex cyanides, whereas KCN, Cd(CN)2 and CuCN are simple cyanides.

To determine how much total cyanide is in a sample, we need to chemically convert all the simple and complex cyanides to hydrogen cyanide gas, which can then be analyzed by titration or a colorimetric procedure. This conversion is necessary because complexed cyanides are extremely stable (meaning that other chemicals wont react with them under most conditions) and therefore can not be analyzed readily.

The currently accepted methodologies (EPA Method 9010 and Standard Methods for the Examination of Water and Wastewater Method 412B) employ sulfuric acid to convert the simple cyanides to gas. A de-complexing agent (magnesium chlorides is added to aid in the destruction of complexed cyanide.

The whole reaction takes place in an apparatus like that in Fig. 1. The sample is placed in the flask and the apparatus is connected as shown. Suction is applied to create an air stream through the flask, a condenser (which returns water vapor to the reaction flask), and a gas absorber flask (which contains sodium hydroxide solution). The gas absorber flask "traps" the cyanide gas, converting it to "simple" sodium cyanide. The reaction is timed, at boiling, for 1 hr. Afterwards, the gas-absorbing solution is analyzed by one of two major methods: titration with silver nitrate and P-dimethylamino-benzalrhodanine indicator or color development with chloramine-T and pyridine barbituric acid measured with a spectrophotometer at 578 nm.

Although the procedure appears simple, it is subject to a wide array of interference and the de-complexing agent is not 100 percent efficient. The best laboratories can analyze to a precision of +10 percent.

In 1983, EPA commissioned a study of the various analytical methods for cyanide. It was found that cobalt cyanide was only 32 percent decomposed and some organic cyanide compounds were not decomposed at all by the test procedure at a level of 2 ppm total cyanide.

The most commonly found interferences for the total cyanide test are sulfides, thiocyanates, aldehydes, and oxidizers (e.g., residual chlorine). Sulfides and thiocyanates yield erroneously high test results, whereas aldehydes and residual chlorine produce erroneously low figures.

The three analytical "bibles" for cyanide determination are: EPAs Methods for Chemical Analysis of Water and Wastes, available from EPA Environmental Monitoring & Support Lab.,26 W. St. Clair St., Cincinnati, OH 45268; Standard Methods for the Examination of Water and Wastewater, published by the American Public Health Assoc., 1015 15th St. NW, Washington, DC 20005; and ASTM D2036, Cyanides in Water, available from ASTM, 1916 Race St., Philadelphia, PA 19103. All provide detailed instructions for performing the test but the recommended procedures for removing interferences are different. Because they differ and can therefore affect the test results, we recommend that whatever procedure the Control Authority adopts should also be used by the discharger or its laboratory.

Test Kits

A recent study showed that, in general, test kits will yield erroneously low results unless a reflux distillation per any of the above "bibles" is performed first. Once the reflux distillation has been carried out, it is easy to perform the rest of the analysis by titration (for test results above 1 mg/L) .

We have found that the prescribed calorimetric procedure is too complicated for the average plating shop lab. A calorimetric test kit can be used and will obtain reasonable results after distillation. However, for samples with 0.1 mg/L or more total cyanide, we recommend using the titration procedure but substituting an electronic burette (in place of a glass burette) capable of delivering 0.05 mL accurately. Electronic burettes are available from most scientific supply houses for about $300.

Next month, well talk about cyanide amenable to chlorination.

References

1. U.S. EPA, Report EPA 600/4-83-054.

2. "The Performance of Analytical Test Kits on Metal Finishing Wastewater Samples," NAMF, Chicago, IL.