Manual Distillation is not really a distillation as you may interpret it. While you may be heating the solution to boiling you are not passing the solution from container A and condensing it in container B. In cyanide manual distillations you are boiling the sample in strong acid for a prolonged period of time (about 1 hour usually). A cold finger condenser drips the steam back into the original container as water. A carrier stream of laboratory air bubbles through the boiling sample and pulls the vapor (theoretically the HCN) into an impinger containing a sodium hydroxide solution. It is the prolonged boiling with strong acid that decomposes the cyanide metal bonds, allows the CN to convert to HCN, The boiling (heat) combined with the purging of air adds enough energy so that the HCN gas pass into the carrier stream. Cyanide will not transfer readily into the carrier stream without the heat. However, without the prolonged boiling in strong acid the metal cyanide complexes do not convert to cyanide.

In a Continuous Flow automated cyanide method the “distillation” is almost instantaneous. A portion of the sample is vaporized and recollected after passing through a condenser that converts the steam to water. There is no prolonged boiling with acid, and the process is too fast to decompose strong metal cyanide complexes. Thus, a continuous flow in-line distillation method is only capable of analyzing weak to moderate metal cyanide complexes without extra energy supplied.

These weak to moderate metal cyanide complexes are exactly the same cyanide species that are measured by gas diffusion. Instead of the rapid distillation where a thin film of liquid hitting a hot surface converts immediately to steam, the HCN generated upon acidification by acid passes through a hydrophobic membrane and is absorbed into a dilute sodium hydroxide solution. It is the exact same process, and the same cyanide complexes, simply separated from the acidified solution in a slightly different way.

So if both flash distillation and gas diffusion separate the exact same weak to moderately strong metal cyanide complexes, how do they analyze for total cyanide (strong metal cyanide complexes)? The answer is by UV irradiation. All continuous flow methods for total cyanide require UV irradiation to liberate total cyanide so that it can be separated from the matrix by flash distillation or gas diffusion.

I hope you can see that the flash distillation adds nothing extra to the analysis except extra complexity. A flash distillation requires heat and a condenser. The heat requires an extra module and the condenser requires flowing water. All these extras are not recovering more cyanide. Total is dissociated by UV irradiation and all other cyanide complexes are equally recovered by gas diffusion or flash distillation. 

ASTM D7284-08 is a distillation method that uses OIA1677 and/or ASTM D6888 as the determination step. Using gas diffusion amperometry eliminates the use of hazardous and smelly pyridine plus it overcomes interferences with colorimetry, such as sulfide or sulfite. Quit making pyridine barbituric acid reagent. Quit wondering if your chloramine T is fresh. Use ASTM D7284 and measure cyanide more accurately.

But, why distill at all? Why not use ASTM D7511-09? This method utilizes UV irradiation to liberate strong metal cyanide and passes the HCN generated through a gas diffusion membrane into a basic acceptor solution where cyanide is measured with a highly sensitive and selective amperometric detector. No heat, and no distillation is required. It is the heat involved during distillation combined with the oxidizing power of concentrated sulfuric acid that produces cyanide out of the organic nitrogen compounds in your samples. The definition of total cyanide does not include organics and thiocyanate. Why do you want to?

Save time by not distilling, Save money with decreased labor. Be more accurate with gas diffusion amoperometry. Measure what you’re supposed to measure and do it quickly. 

Much work has been done on the Table of approved inorganic methods to try and make it more readable, and also ofn Part 136.6. Part 136.6 is “method flexibility” and the part has been expanded to include example situations of what is allowed.