Iron iron cyanide complexes, namely Prussian Blue and/or Turnbull's Blue are very insoluble compounds that form when soluble iron cyanide complexes contact dissolved iron in aqueous solution. These insoluble iron cyanide compounds become part of the biosolids during water treatment, or part of the sediment in stream deposits. The EPA has ruled that these compounds, though not toxic, be measured as "total cyanide" because it is possible that they can release toxic hydrogen cyanide upon exposure to sunlight. The problem with the EPA ruling is that the EPA does not define analytical methods capable of quantitatively recovering these very insoluble cyanide species.

The EPA's decision that prussian blue should be considered toxic cyanide was in part because some measureable cyanide was generated when prussian blue was analyzed by distillation/colorimetric methods. Acid distillation, however, when followed word - for - word according to most accepted methods does not quantitatively recover cyanide from iron iron cyanide complexes. The Method Update Rule published in the March 12, 2007 Federal Register requires adjusting the pH of an aqueous sample thought to contain insoluble cyanide complexes to 12 - 13 and warming at room temperature for a minimum of 4 hours prior to removal of an aliquot for distillation. The problem with this is, although we have been doing this for years, that pH adjustment to 12 often causes significant loss of the more soluble cyanide species, and these are the ones that are actually toxic.

Since iron iron cyanide complexes are insoluble, it is best to filter samples in the field at the time of collection reserving the filter for subsequent extraction, and the filtrate for analysis of soluble cyanide. I would recommend that the filtrate not be treated with NaOH and only for residual chlorine, sufide (if more than 50ppm) and aldehydes. Treatment with NaOH could result in significant available cyanide loss.

The filter paper should be extracted with strong base solution and the extract recombined with the original sample, or analyzed separately. More information is obtained when analyzed separately because more information on cyanide speciation results. If it can be demonstrated that there is no significant cyanide present in the solids from a particular sampling site, then future extraction and analysis of the filtered solids can be omited.

There are other advantages to extracting solid samples besides the obvious that insoluble cyanides are more soluble in base than in strong acid solution. The extraction in base significantly reduces interferences in the subsequent analysis because many of the interfering species with cyanide distillations are insoluble in base reagent: the filtration first separates CN from dissolved interferences, then separates them from insoluble ones. These interferences include metallic sulfides, native sulfur, and sulfur (IV) oxides. All of these sulfur compounds are extremely likely in sediment samples, and biosolids and all of them are significant interferences with distillation.