| When a sample containing 3 parts per million | | | | are absent from the water a distillation (or heated |
| thiocyanate and 5 parts per million nitrate nitrogen | | | | digestion) method for cyanide may be used. |
| was distilled and analyzed by GD-Amperometry 10.9 | | | | The point is that the methods believed to be |
| parts per billion cyanide was detected. When 15 parts | | | | accurate for cyanide are not, and cannot achieve |
| per million thiocyanate + 25 parts per million nitrate | | | | even 80% recovery for total cyanide in most |
| was distilled the cyanide detects jump to ~ 50 - 60 | | | | matrices. We have, on the other hand, a method |
| parts per billion. | | | | that even when not performing at optimum |
| A real wastewater sample containing 0.1 mg/l | | | | conditions gets mid to upper 80% recoveries for |
| thiocyanate and 63.5 ppm NO3 detected cyanide at | | | | ferric complexes within sample matrices where |
| 100 parts per billion using a distillation-colorimetric | | | | cyanide could either be not detected or severely |
| method. This same method was used on a synthetic | | | | positively biased by distillation. Is there room to |
| matrix containing 0.1 ppm thiocyanate and 25 ppm | | | | improve, yes, but let us consider that even with the |
| nitrate resulting in a false 60 parts per billion cyanide | | | | deficiencies pointed out, distillation free total cyanide |
| detect. | | | | is far superior to any other method for the |
| As can be seen by the data, distillation produces false | | | | determination of total cyanide. But even when |
| positives in samples containing thiocyanate and | | | | considering this, it also must be established the intent |
| nitrate. Since thiocyanate and nitrate are almost | | | | of the cyanide measurements. When the US first |
| always present in wastewater this interference | | | | began regulating cyanide in the late 1970's there were |
| demonstrates that distillation methods are unsuitable | | | | no methods other than distillation capable of running |
| for wastewater matrices. Note that nitrate does not | | | | different cyanide forms. |
| only react with thiocyanate during distillation, almost | | | | So, even though the toxic species of cyanide is free |
| any organic compounds can be oxidized during | | | | cyanide, the regulations were all written for total. |
| distillation to form cyanide. When checking for | | | | Ferric and strong metal cyanide complexes may be |
| cyanide interferences from thiocyanate by distillation | | | | present as water emerges from a pipe, but in an |
| (or heated) methods it is necessary to spike cyanide | | | | open water body these complexes are likely to |
| into a sample containing thiocyanate. For example, a | | | | settle out and any cyanide releases from them will be |
| sample containing 20 parts per million thiocyanate (no | | | | measurable as free cyanide (the toxic form). The |
| nitrate) and 200 parts per billion cyanide detected 174 | | | | advantage of analyzing for free cyanide is that |
| parts per billion cyanide (a negative bias). This means | | | | digestion is not required, drastically minimizing potential |
| that thiocyanate causes a positive bias when nitrate | | | | interferences, simplifying the method, and lowering |
| is present, and a negative bias when nitrate is absent. | | | | day to day Method Detection Limits (MDL). Free |
| There are significant negative interferences that can | | | | cyanide is also the most accurate measure of |
| occur if samples are distilled. Oxidized forms of sulfur, | | | | cyanide toxicity. Please note that in this example free |
| such as sulfite, and thiosulfate destroy cyanide during | | | | cyanide is defined as the amount of hydrogen |
| distillation causing results to be reported low. For | | | | cyanide liberated from a buffer that is at the pH of |
| instance, a sample containing 20 ppm SO3 and 200 | | | | the receiving stream. If an effluent contains trace |
| parts per billion cyanide detected 80 parts per billion | | | | metals such as copper, zinc, silver, mercury, and |
| cyanide after distillation, and a sample containing 20 | | | | nickel of sufficient quantity an available cyanide |
| ppm thiosulfate and 200 parts per billion cyanide | | | | method such as ASTM D6888-04 or OIA 1677 should |
| detected 124 parts per billion cyanide after distillation. | | | | be used and are sufficient to measure low levels of |
| These interferences are significant because | | | | toxic cyanide potential without a necessary digestion |
| thiosulfate and sulfite are often used to dechlorinate | | | | distillation. |
| samples or to dechlorinate disinfected wastewater. | | | | Total cyanide should only be regulated at the point of |
| Native sulfur causes a negative bias and must be | | | | discharge and in waters where strong metal |
| filtered prior to analysis. Metallic sulfides cause a | | | | complexes are known, or suspected, to be present. |
| negative bias and must be filtered prior to analysis. | | | | Total cyanide methods that rely on distillation are not |
| These losses of cyanide result from sample storage | | | | accurate and introduce both significant positive and |
| as well as the distillation so treatment applies to both | | | | negative bias. This bias will never be known because |
| distilled and non distilled methods. | | | | the exact matrix of each sample stream can never |
| Treating sulfide with lead, cadmium, zinc, or bismuth is | | | | be determined every time. Non-distillation total |
| not recommended. Without an on-line sulfide | | | | cyanide methods, though not perfect, out perform |
| abatement such as in ASTM D6888-04 it is | | | | distillation in that interferences are limited to a few |
| recommended that samples containing greater than | | | | compounds and these interferences can be minimized |
| 50 parts per million sulfide (ASTM D6888-04), or | | | | to known values. Even in worst case scenarios, |
| greater than 20 parts per million sulfide (distillation) be | | | | non-distillation methods produce less false positives in |
| diluted to the point that sulfide no longer interferes. | | | | real world samples than distillation methods. Better |
| All precipitation or volatilization treatment methods | | | | yet, there are no known compounds (other than |
| result in significant cyanide losses. Preservatives used | | | | from reactions that occur in the sample bottle itself) |
| also have an effect on cyanide recoveries. We | | | | that cause false negatives in non-distillation methods. |
| recommend not using ascorbic acid prescribed in | | | | Non-Distillation methods for available cyanide are |
| most methods since any excess can cause 200 parts | | | | virtually interference free (using sulfide abatement |
| per billion cyanide to be not detected after 72 hours. | | | | on-line) and quantitatively recover cyanide in |
| Use of Sodium Arsenite results in no decrease. Also | | | | moderately strong metal cyanide complexes rapidly |
| we know that distillation methods do not produce | | | | with low quantitation limits. Unless strong metal |
| accurate results if there is thiocyanate, nitrate, nitrite, | | | | complexes are know to be present and need to be |
| sulfite, ozone, peroxide, hypochlorite, metallic sulfides, | | | | measured, available cyanide methods should be used |
| thiosulfate, and trace organics in the sample. This | | | | to accurately measure the potential toxicity of |
| means that if any or all of the mentioned parameters | | | | cyanide from an effluent. |