Cyanide in Waste Water Test

A. I would recommend that they have the POTW analyze the samples for nitrates as well. It has been shown that the presence of nitrates and nitrites can cause a false high total cyanide reading in the SM 4500 analysis method, along with the presence of sulfides and ascorbic acid. Further, it is highly possible that nitrogen in the samples will form higher levels of nitrate compound as the samples sit for longer periods of time.

The addition of sulfamic acid ⇦ this on eBay or Amazon [affil links] to the samples, which is described in EPA method 335.2, will usually do the trick. Your client should explain this to the POTW, and ask that they do a companion test using 335.2 as a gauge. This might allow the POTW to better understand the issue of false positives on cyanide.

A. The high total cyanide (but not amenable cyanide) may be due to iron cyanide complexes hat are not easily treated by alkaline chlorination. A simple AA test for total iron will tell if this is a problem. Since each iron ion is complexed by six cyanide ions, a little iron can cause a lot of cyanide to pass through your treatment system.

If it turns out that this is the problem, there are a few simple ways to deal with it, both at the source (drag-in) and in the treatment system.

A. The answer that you are looking for concerning spurious results from total cyanide comes down to multiple chemical interferences. Depending where you take the sample the number of interferences can increase. If there is a nitrogen source which may or may not include nitrate and a carbon source you are likely to get spurious and non-reproducible results.

Method 335.2 is absolutely subject to error if the POTW is collecting samples from the sewer. Think about it, the sample is digested in 18 Molar or so sulfuric acid for an hour and the analysis collects the off gassing. Odds are that you are creating cyanide in from the soup that's in sewerage. I've been through this with the local POTW, EPA, and the POTW's certified laboratory and shown that method 335.2 analytical results are pure junk when there are multiple contaminates present.

As one of the many examples, I have I sent out a sample from my acid gas scrubber that was use to capture NOx from a nitric acid copper etch process. We used sodium hydroxide in the scrubber. The only chemicals present were nitrates, carbonates and some copper and sodium hydroxide. There was no possibility of cyanide in the scrubber solution. We sent a sample of the scrubber solution out to a state certified lab for the cyanide analysis using Method 335.2 and noted that there was nitrate present in the mix so the lab could correct for it. The cyanides results were sky high. There's no correction for multiple interferences and the only God knows what interferences you will find in any particular sewerage sample. The POTW continued to find cyanide long after we stopped using it entirely. I've included the title note from a paper that EPA published concerning this issue. EPA notes in the first paragraph of the paper, which I included below, the statement concerning Method 335.4 "However, when multiple interferences are present, the total cyanide methods produce questionable analytical results". Note that Method 335.4 and 335.2 are identical in sample preparation, the acid digestion.

If you want a good cyanide sample for analysis take the sample at your treatment tank where there will be minimal chemical inferences -- not from a sewer line.

Ed. note: Thank you, David!
The EPA conference where that paper was presented is public domain but nearly 900 pages. We've extracted the 33 pages of that paper for our readers.

Thiosulphate? I always used to use ascorbic acid. Have they changed the method?

False negatives are much more common than false positives, in my experience.

Dave Wichern
Consultant - The Bronx, New York

A. I've seen this phenomenon many times. I believe it to be due to the chlorine destroying some of the many poorly understood interferences with this analysis, some of which there is no way of compensating for.

You might try doing "Weak and Dissociable Cyanide", where the distillation is done under much milder coniditions and the aforementioned interferences have much less effect.

A. It is very possible to generate CN during the chlorination process of the CATC test. A possible scenario is the presence of SCN and NH4. When you chlorinate the sample, and spin it the chlorine combines with ammonia ⇦ this on eBay or Amazon [affil links] to create chloramine. Chloramine then reacts with SCN and generates CN. This is but one reaction that generates cyanide during the chlorination step. Other compounds, such as organonitrogens (proteins, etc) can do this as well. OIA1677 was created specifically because the CATC method does not work. Besides the formation of CN during the chlorination half of the CATC there are multiple interferences during the total distillation process as well. These potential reactions can create and/or destroy cyanide all at the same time. Basically, when running the CATC test you can never have any confidence that the numbers obtained are real. Why even do this test when an interference-free method that takes minutes instead of hours is approved, available, and obtains better detection limits and precision?

A. SCN- reacts with chloramine T to produce cyanogen chloride, not cyanide. I suppose that some could dissolve in the sparger solution to give a positive test.

One might wait a few minutes, in the colorimetry, after the addition of the phosphate buffer, to see if the CNCl hydrolyzes to cyanate. This reaction takes place much faster at lower pH values.

To the best of my knowledge, CNCl cannot go back to cyanide.

A. Make up a set of known concentrations and measure them.

Then take your waste water sample and spike it with known concentrations and measure it.

This should lead you down the road to some useful information