Free, Available, and Total Cyanide Analysis Using Gas Diffusion Amperometry

Introduction

Flow Injection Analysis followed by gas diffusion amperometry was specially designed to accurately measure all forms of cyanide. This technique simplifies cyanide analysis by eliminating time-consuming distillations steps and hazardous pyridine reagents.

The instruments are designed to provide trouble free analysis of cyanide and accomplish this task by using as few reagents as possible. The reagents are also either very stable, or can be easily prepared as needed from stock solutions.

Since the instruments are stable, and use sensitive amperometric detectors, QC almost always passes. Simply recalibrate at the beginning of the new days testing and start running. The gas diffusion method is specifically designed to overcome sulfide and other interferences that plaque colorimetric detection causing matrix spikes to fail.

Only gas diffusion methods enable the determination of the important cyanide species. Free and available cyanide analysis is possible without any reconfiguration of the cartridge. The two methods simply replace one reagent with another. All other reagents remain the same. You do not have to change detectors, or pump tubes to easily measure free and available cyanide.

If you choose to distill, you may also measure total cyanide by ASTM D7284 using the same exact configuration and reagents used to measure available cyanide.

To run total, by either ISO Modified (described later) or the ASTM D7511 method, simply change a few pump tubes and connect the cartridge to the UV digester. Only one extra reagent is needed. The other reagents are the same reagents you use to run available cyanide.

A close up view of the analyzer demonstrates the simplicity of the gas diffusion amperometry methods. The entire hardware includes only a valve, a diffusion manifold that doubles as a reagent mixing manifold and a flow through amperometric detector. Injected sample merges with reagents in the gas diffusion manifold and the CN passes through a solid state amperometric detector flow-cell.

Free Cyanide

ASTM D7237 determines free cyanide as defined by the ASTM International Standard Guide for Understanding Cyanide Species. These methods measure Hydrogen Cyanide and simple salts of cyanide, such as those of potassium and sodium. Free cyanide is the species that is toxic to aquatic life and these methods should be what are used in analysis of cyanide in ambient waters such as lakes, and rivers.

ASTM D7237 is an automated method for the determination of free cyanide in aqueous solution. Sample is injected into a flowing stream of buffer solution. The HCN generated diffuses through a membrane into a flowing stream of base reagent and is measured by the very sensitive amperometric detector.

Please note that free cyanide cannot be accurately measured by simply injecting sample solutions into color reagents without any interference removal. Many laboratories do this, but cannot guarantee that results are due only to cyanide.

The ASTM D7237 free cyanide cartridge is exactly the same cartridge (flow configuration) used for OIA 1677 or ASTM D6888. In other words, if you can do OIA-1677 you can run free cyanide as well.

Available Cyanide

To perform OIA-1677 a sequestering agent, or ligand, is added to a basic sample at room temperature. The ligands form very stable complexes with various transition metals releasing cyanide from the metal – cyanide complex. The ligands release CN ion from all transition metal complexes except those, such as iron, gold, and cobalt, that are only determined with total cyanide methods. Once the CN ion is released, the sample is injected into an acidic stream where the CN ion is quantitatively converted to HCN. The HCN in the acidified sample passes through a micro porous hydrophobic gas diffusion membrane into a basic trapping solution that carries it to the very sensitive amperometric detector.
Complete and quantitative recoveries of all species intended to be measured by CATC and WAD methods are obtained by this method. The whole process only takes 1 – 2 minutes per sample, reagents are non-hazardous and easy to prepare, and the analytical cartridge is simple, intuitive, and easy to use.

Data demonstrates that OIA 1677 quantitatively recovers all metal cyanide complexes expected to be recovered by CATC and WAD methods. None of the methods recover Iron Cyanide (total Cyanide). Data also illustrates the incomplete recoveries obtained by the distillation methods.

WAD recoveries are low for both mercury cyanide complexes at 0.2 and at 2 ppm CN, and CATC recoveries are low for silver cyanide.

Why use methods loaded with interferences when the easiest to use out of all of them has very few, or none. An easy to use method means less error, and more up time.

Again, why distill when you don’t have to? Quit wasting time and money on interference prone inaccurate distillations. Invest in your future with OIA-1677.

The cartridge configuration for OIA-1677, or the equivalent ASTM D6888 method, is identical to the free cyanide configuration, and demonstrates the simplicity of the method.
This same cartridge can be used for total cyanide as well by ASTM D7284-08.

This, easy to use, configured only once cartridge can be used to measure free and available cyanide with no intervention other than replacing one reagent.

Total Cyanide after distillation

All manual distillation methods for total cyanide are essentially the same, and will obtain essentially the same result. All share the same interferences that occur during the distillation process. ASTM D7284-08 achieves superior results on samples where distillates interfere with the pyridine (or isonicotinic acid) barbituric acid reagent. The method has the further plus in that it eliminates the use of pyridine barbituric acid.

Remember, that methods that use isonicotinic acid in replace of pyridine are still subject to the same interferences.

A scatter plot of results from the analysis of impinger solutions by colorimetry and GD-amperometry following distillation yields a slope of exactly one demonstrating that the two measurement techniques produce the same result. This is important to remember.

Gas diffusion Amperometry and Colorimetry obtain identical results when there are no interferences present. If results of the same solution are different there is interference. Since the silver electrode only reacts with CN- interferences are more likely with the non specific color reaction.

Recall that Amperometry detection only requires one NaOH reagent. Why make several reagents that expire quickly when you can use only one reagent?

Automated Methods for Total Cyanide

The Kelada total cyanide method and EPA method 335.3 differ only in the wattage of the UV lamp. While EPA 335.3 (and all the other UV total cyanide methods) use 8 – 12 Watts, the Kelada method specifies a minimum of 450 Watts. It is my opinion that since the Kelada specifies this wattage in the method, and the method is very insistent on it, the wattage cannot be lowered according the method flexibility of 40 CFR Part 136.6. This means that UV digestion followed by colorimetric determination for total cyanide, such as EPA 335.3 are not EPA approved. Your choice for total automation is the Kelada, requiring a 450 W lamp, 15 pump tubes, on-line distillation, and pyridine barbituric acid reagent, or ASTM D7511 that requires only three reagents and easily measures total cyanide without pyridine or distillation.

The ISO 14403 method uses a citric acid solution to digest samples using UV light. Once digested, HCN can be separated by either distillation or diffusion and measured via colorimetry or amperometry. The ISO method, as written, does not compensate for sulfide interference and sulfite interferes at concentrations above 1 ppm. Validation of the method was made on fairly clean drinking water, surface water, and wastewater matrices. No interferences were tested and the metal cyanide complexes were predominantly zinc.

The ASTM D7511 method has undergone extensive testing and validation for more than 10 years. It has been tested with more matrices and its behavior tested with more potential interferents and sample types than any other cyanide method. I believe that this ASTM D7511 is the only true way to accurately measure total cyanide in complex matrices.

ASTM D7511 or ISO 14403 M are analyzed using the same pump tubing and flow configuration. The ISO procedure may be modified to include an extra reagent that handles sulfide concentrations up to 50 ppm. The advantage of gas diffusion amperometry, besides the obvious added simplicity, is that sulfite does not interfere.

The ISO 14403 colorimetric method uses about 15 pump tubes. Since pump tubes add complexity, the ISO colorimetric method is about 2-3 times more complex than the gas diffusion method. Recall that diffusion Amperometry and Colorimetry obtain identical results when there are no interferences present.

The ISO 14403 standard as written has no mechanism to overcome sulfide interference. Referenced literature states the interference occurs at sulfide concentration of 10 ppm and higher. The ISO 14403 standard states that sulfite greater than 1 ppm interferes.

The ISO 14403 standard requires > 90% recovery on a 100 ppb ferric cyanide complex, but did not even obtain greater than 90% recovery from ferric complexes in either of two Interlaboratory studies. Acceptance criteria written in the standard does not agree with its own validation data.

Because D7511 uses gas diffusion amperometry combined with a complexing reagent for sulfide, it can compensate for samples containing sulfide up to 50 ppm and sulfite does not interfere with amperometry. Sulfite concentrations above 1 ppm interfere with colorimetric detection causing results to be low. Ten parts per million sulfite can result in cyanide recoveries as low at 60%. There are no test kits for the detection of sulfite essential oil diffuser B0BN8G88YY.

Sulfide is a significant interference and concentrations above 50 ppm can lower the holding time from 14 days to 24 – 48 hours. (Sulfide does not really react with cyanide, HS- does, but regardless sulfide in concentrations detectable by lead acetate test strips must be corrected). Traditional methods for sulfide removal also result in reduction of the analyte, rarely recovering more than 80% of the original cyanide present before the removal procedure.

Sulfide concentrations up to 50 ppm do not interfere with gas-diffusion amperometry if proper reagents are used. 50 ppm is important because 50 ppm is about the detection threshold of sulfide test strips. If there is no detect on the test strip the method is essentially interference free.

Sulfide correction is important because sulfide below 50 ppm is not normally detected in field screening but can cause serious errors with your sample. Also, treating samples for sulfide can also cause serious loss of cyanide. Any method for CN should be capable of compensating for sulfide at the concentrations below reliable field screening. Sulfide interferes with colorimetric methods. While there is no sulfide interference for gas-diffusion amperometry, there is a positive interference using the colorimetric reagents along with gas diffusion (or distillation) as in the ISO 14403 method. Since this sulfide interference begins at about 15 ppm sulfide, and the test strip cannot detect below about 50 ppb sulfide, you would not know whether the results are from cyanide or sulfide. The only way to be confident in your results is to employ gas-diffusion amperometry detection combined with the sulfide abatement acidification reagent.

The modification of the ISO method uses the ASTM D7511 gas diffusion amperometry configuration replacing the TA1 reagent with the citric acid digestion reagent from the ISO procedure. This procedure is similar to the gas diffusion method in the appendix of ISO 14403; however, it inserts the TA2 reagent after digestion and before diffusion. The TA2 reagent complexes sulfide up to 50 ppm and use of gas diffusion amperometry overcomes sulfite interference to the color reaction.

The ASTM D7511 procedure can be modified to use a single Teflon coil in place of two quartz coils, replace all mixing coils and transmission tubing with Teflon, and omit surfactant. The TA1 reagent can be enhanced to boost recoveries of ferrous and ferric iron cyanide complexes to 100%. Prussian blue is recovered at about 85%.

The modified ISO 14403 recoveries remain similar to the regular ISO method because there is no change to the digestion reagent, modifying the method only minimizes sulfide and sulfite interferences.