1. William Lipps Analytical & Measuring Instrument Division August, 2017
Total Cyanide may not always be free, but it is often available – cyanide methods and interferences
William Lipps
Analytical & Measuring Instrument Division
August, 2017

2. Regurgitated

3. Contents Fundamental CN Chemistry Methods and Interferences Conclusion
Sampling and Sample Preservation
Methods and Interferences
Conclusion
In this presentation we will cover the methods promulgated to measure the new free cyanide chemistry. Before we do that we will examine what free cyanide is, and isn’t. WE will look at sampling requirements, closely examine the methods, and come to a conclusion on what method is best for regulatory and lab purposes.

4. Contents Fundamental CN Chemistry
In this presentation we will cover the methods promulgated to measure the new free cyanide chemistry. Before we do that we will examine what free cyanide is, and isn’t. WE will look at sampling requirements, closely examine the methods, and come to a conclusion on what method is best for regulatory and lab purposes.

5. A generalized summary of cyanide and it’s metal – cyanide species
Transition metals - strong bonds
Alkali metals - ionic bonds
Cyanide is a polyatomic ion consisting of a carbon and a nitrogen atom. The bond between the carbon and the nitrogen is very strong making the molecule behave as a single atom. There is one electron pair on the nitrogen atom and on extra electron associated with the carbon atom. It is this extra electron that dictates the chemistry of the cyanide molecule. When the molecule is surrounded by alkali or alkali earth metal it behaves as an anion, such as chloride for instance. When there are transition metals present, the cyanide will bond covalently to the transition metals. So, cyanide can form both ionic or covalent bonds depending on the positively charged ions it comes in contact with.

6. Alkali and alkaline earth metals form water soluble salts with the cyanide anion
Cyanide can behave as an anion when there are no other metals present. These ionic bonded salts freely dissociate cyanide in water. This is a crystalline salt of sodium cyanide. When dissolved in solution:

7. HCN is the toxic form of cyanide
HCN is the toxic form of cyanide. This small uncharged molecule passes freely through cell membranes and becomes toxic to aquatic life. According to the 40 CFR Part Table 1b, free cyanide is the amount of HCN measured when the sample pH = 6.
Get your Free CN here

8. A tetragonal coordination complex anion with CN ligands
Transition metals form stronger bonds with cyanide resulting in a coordination complex in which cyanide is a ligand. The coordination complex itself is a negatively charged molecule and acts as an anion in solution. The transition metals listed here form weak to moderate bonds with the cyanide ligand that are readily dissociated in excess of hydrogen ions, or protons. This dissociation is where the term WAD cyanide came from. Tetragonal metal-cyanide complex anions are what we call available, amenable, or WAD cyanide. In operation, methods that measure available cyanide also measure the cyanide ion and free cyanide.
Zn, Cd, Cu, Ni, Hg

9. Iron and Cobalt CN are hexagonal coordination complexes with CN ligands
Iron, Cobalt, gold, and platinum group metals form very strong bonds with cyanide. These complexes are very stable and are essential to the use of cyanide in precious metal extraction. Because iron is so prevalent in the environment, iron is the most common strong metal cyanide complex in environmental samples. Iron, cobalt, and the platinum group metals form hexagonal coordination complexes with cyanide. These complexes have a net negative charge and serve as anions in many soluble cyanide salts. They do not readily dissociate into cyanide ion or HCN in acid solution. Only the combination of acid and extra energy will break these very strong bonds. This extra energy is what we call distillation. Another way to dissociate these strong bonds is to use photons of energy below 350nm.

10. If a metal-cyanide complex forms a salt with a transition metal it is not soluble
If potassium, for instance, is replaced by another transition metal forming a metal-metal cyanide complex it is not soluble in water.

11. Bonds between the metal and cyanide are moderate to strong
[Hg(CN)4]-2
[Cd(CN)4]-2
[Zn(CN)4]-2
[Ag(CN)2]-
[Cu(CN)4]-3
[Ni(CN)4]-2
Hg(CN)2
[Fe(CN)6]-4
[Fe(CN)6]-3
[Co(CN)6]-3
Weak
These are the transition metals that can bind with cyanide as coordination complexes. They are arranged in increasing strength of the metal cyanide bond. The weaker bonds, such as those between zinc and cyanide dissociate readily in neutral aqueous solutions. The moderate metal cyanide complexes require more protons (stronger acid), and the strong metal complexes require acid plus some other form of energy, such as heat or UV irradiation, to cause the metal and the cyanide to dissociate
Moderate
Strong
[Hg(CN)4]-2 --> Hg(CN)2+ 2CN-

12. Cyanide methods measure the various cyanide “species” dissolved in water
Total Cyanide Fe Co
Available Cyanide Ag Cu Hg Ni Cd Zn
Free Cyanide
HCN
CN-
Free cyanide refers to the sum of HCN and CN ion in a sample. Free cyanide is bioavailable and about 1000 times more toxic to aquatic organisms than it is to humans.
Weak to moderately strong metal-cyanide complexes are compounds that dissociate and release HCN under mildly acidic conditions. The WAD, CATC, and OIA1677 (D6888) methods are developed to quantify available cyanide. These methods measure the weak and moderately strong metal cyanide complexes plus free cyanide.
Strong metal cyanide complexes require strongly acidic conditions to dissociate and release hydrogen cyanide gas. Strong metal cyanide species include complexes of iron, cobalt, and platinum group metals.
The EPA method defined “Total cyanide” includes the strong metal-cyanides, weak to moderately strong metal-cyanides, and free cyanide. The EPA defined “total cyanide” does not include thiocyanate or organic cyanides.

13. Collect Sample Preserve and Process Analyze
As we all know, we first collect a sample for CN, preserve it in the field, ship to the lab and they analyze it within 14 days. For the most part, our requirements will not change. Holding time and preservation studies, if ever even done, are almost always done using simple cyanide salts.

14. Contents Sampling and Sample Preservation
In this presentation we will cover the methods promulgated to measure the new free cyanide chemistry. Before we do that we will examine what free cyanide is, and isn’t. WE will look at sampling requirements, closely examine the methods, and come to a conclusion on what method is best for regulatory and lab purposes.

15. Cyanide formation after preservation at pH 12

16. Store samples at 2 – 6 °C in the dark at pH 11
Collect the sample in a brown bottle. This is very important as the brown bottles exclude any light less than 350 nm. Remember that UV light will dissociate iron and available cyanide complexes into free cyanide. If samples are not kept in the dark, the free cyanide concentration will be over reported. The method update rule also specifies a pH 11 rather than the pH greater than 12 specified previously. The CFR over rides any other sample preservation pH existing in any method. Why a low pH? Free cyanide can readily form in solution at high pH by reaction of organic nitrogen compounds, thiocyanate, chloramine, etc. At regulatory limits of 5 ppb it is important to not over preserve and generate free cyanide when it is not there.

17. Sulfide reacts with free cyanide lowering its concentration
Sulfur containing compounds, particularly sulfides and colloidal sulfur, rapidly react with cyanide to form thiocyanate causing results to be low. It is important to remove any sulfide that may be present prior to pH adjustment. This is done by adding lead carbonate powder and filtering. This needs to be done in the field if sulfide is detected on lead acetate test strips.

18. Holding Time Study – Sulfide Bearing Samples
Sulfide is a major interferent for cyanide testing and if high concentrations are not treated at the time of sampling cyanide loss is virtually assured. Test strips for sulfide are only sensitive to about 50ppm.
Cyanide was added as KCN. Data shows cyanide concentrations dropping rapidly in the presence of a lot of sulfide, however, in samples lower than sensitivity of the lead acetate test samples are stable for 48 hours.
This is why we say it is best to sample and analyze within 48 hours if S is < 50 ppm.

19. Recovery of 100 ppb CN in up to 50 ppm Sulfide using on-line sulfide abatement
We have found that if you use lead carbonate and remove sulfide to below detection on the lead acetate test strips, the new methods for available and total cyanide in the coming method update rule quantitatively recover cyanide in the presence of up to 50 ppm sulfide.

20. Oxidizers react with free cyanide lowering its concentration
Residual chlorine must be treated in the field or it will react with cyanide and form cyanate causing results to be low. Residual chlorine is detected using starch iodide test strips and sodium thiosulfate is added till chlorine is not detected. Do not use ascorbic acid or sodium bisulfite.

21. Loss of 200 ppb CN due to ascorbic acid addition
Ascorbic acid is most often used to remove residual chlorine, however, if ascorbic acid is added and the sample pH adjusted to 12, there is rapid loss of cyanide.

22. Contents Methods and Interferences
In this presentation we will cover the methods promulgated to measure the new free cyanide chemistry. Before we do that we will examine what free cyanide is, and isn’t. WE will look at sampling requirements, closely examine the methods, and come to a conclusion on what method is best for regulatory and lab purposes.

23. These compounds are in almost every sample and interfere significantly
Thiocyanate
Sulfur and Sulfide
Sulfur (IV) oxides
Oxidizers
Interferences
These are some of the compounds that interfere most with cyanide distillation/digestion. We already discussed sulfide and oxidizers as interferents as the sample is held in the bottle waiting for testing. They also interfere during the distillation process resulting in either negative bias, positive bias, or both. Oxidizers can be compounds other than residual chlorine, and when combined with other compounds such as ammonia can co-exist in solution with CN. Once the solution is heated to boiling, the combination of the high heat, oxidizing conditions, high acid concentration, can synthesize cyanide or destroy it.

24. Thiocyanate Create CN Destroy CN
We will now discuss interferences from thiocyanate. Thiocyanate interferes with distillation and with the UV methods we will discuss later. Its interesting that thiocyanate interferes because the originally described colorimetric cyanide methods first distilled in an effort to separate cyanide from thiocyanate. I think the issue is greater now because the concentrations we are measuring are so much lower.
Thiocyanate is in almost every sample. Thiocyanate forms during the combustion process as a reaction of CN and S. It forms in the human body as a way to detoxify the cyanide that we breathe in every day. It is in wastewater from a few tenths of a ppm to several hundred ppm depending. There are no spot tests for thiocyanate and you will rarely know if it is present.

25. Recovery of CN from the ASTM “challenge matrix”

26. Recovery of Prussian Blue (particulate) Cyanide by distillation

27. Recovery of Prussian Blue from sand - distilled or extracted

28. Free Cyanide Analysis

29. Auto-diffusion at pH < 2
Method Number
Description
Measurement
ASTM D 4282
Micro-diffusion at pH 6
Colorimetry
ASTM D 7237
Auto-diffusion at pH 6
Amperometry
OIA *
Auto-diffusion at pH < 2
ASTM D4282 is a manual diffusion method that uses manual pyridine barbituric acid colorimetry as the determination step. After diffusion you can use your regular method you have been using to detect cyanide after they have been distilled. For example, you can follow the diffusion procedure for D 4282 and then measure CN using continuous flow colorimetry. This modification is allowed at 40 CFR Part Sulfide interferes with the method at greater than 1 ppm and must be treated.
ASTM D7237 uses automated gas diffusion followed by amperometry resulting in a fully automated and selective measurement of free cyanide. Sulfide interferes with the method and must be treated. As discussed sulfide also interferes by depleting the cyanide concentration as it sits in the sample bottle waiting for analysis.
OIA-1677 is an available cyanide method that if used will overcompensate free cyanide by measuring free cyanide plus available cyanide complexes. This method was added to the rule because it is capable of selective analysis of cyanide in samples containing up to 50 ppm of sulfide. OIA and ASTM D7237 use the same piece of equipment. The only difference is one reagent.
* Use if sulfide is present

30. Recovery of metal-cyanide complexes by ASTM D7237

31. Available Cyanide Analysis

32. Amenable Cyanide—CATC methods measure “available cyanide”
Method Number
Description
Measurement
SM 4500-CN G
Alkaline Chlorination/ Manual Distillation
Colorimetry
ASTM D 2036
Alkaline Chlorination/ Manual distillation
Colorimetry,
Gas Diffusion -Amperometry
The most common treatment to detoxify cyanide is alkaline chlorination. These methods were designed to provide quantitative estimates on the amount of cyanide that will be destroyed in the chlorination process. In the CATC method, one half sample is adjusted to pH 12 and reacted (in the dark) with excess hypochlorite solution. After the 1 hour reaction the chlorinated half and the unchlorinated half are distilled and analyzed for CN by manual colorimetry. Note that the ASTM method allows Gas Diffusion amperometry as a measurement step. The cartridge for the GD-amperometry is the same as the 1677 cartridge. The EPA FIA cartridge could be used to determine CN in CATC distillates by pyridine barbituric acid colorimetry.
ASTM D7365 recommends using D6888

33. WAD Cyanide methods measure “available cyanide”
Method Number
Description
Measurement
SM 4500-CN I
Buffered pH 4.5 manual Distillation
Colorimetry
ASTM D 2036
Buffered pH 4.5 manual distillation
Colorimetry,
Gas Diffusion -Amperometry
Hydrogen Cyanide is liberated from a slightly acidic sample (pH 4.5) via a 1 – 2 hour heating and purging with air (distillation). Hydrogen Cyanide generated is captured in a basic absorber solution and the sample is analyzed by any of the cyanide measurement techniques. Zinc acetate is used in the buffer to precipitate iron cyanide complexes and enhance the selectivity of the method. This method is used in the precious metals extraction and milling industry to estimate potential cyanide recovery from barren solution, or as an estimate on how much cyanide will be destroyed in the cyanide detoxification process.
The photometric post distillation (EPA 335.4) FS3100 cartridge can be used to analyze WAD CN after distillations. Also, similarly to CATC, ASTM D2036 allows gas diffusion amperometry as well.
ASTM D7365 recommends using D6888

34. Ligand Exchange methods measure available cyanide
Method Number
Description
Measurement
OIA 1677
Ligand Exchange / Flow Injection Analysis
Gas Diffusion - Amperometry
ASTM D 6888
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.
GD-amperometry methods do not require distillation

35. ASTM D6888 flow diagram
Acid Reagent
This is a flow diagram for the available cyanide methods. We will see a colorimetric diagram later. Remember, however, that 1677 and D6888 determine the same cyanide species without a preliminary distillation. The CATC method first does the chlorination, the distillation, and then an analytical step.

36. Ligand Exchange GD-amperometry methods get better recovery
WAD
CATC
This data demonstrates that OIA 1677/D6888 quantitatively recovers all metal cyanide complexes expected to be recovered by CATC and WAD methods. None of the methods recover Iron Cyanide (total Cyanide). The data also illustrates the incomplete recoveries obtained by the distillation methods.
The last two (dark red and dark blue) columns for each metal cyanide species represent recoveries by method 1677/D6888.
Note that WAD (light green and yellow) recoveries are low for both mercury cyanide complexes at 0.2 and at 2 ppm CN, and that CATC (light blue) is low for the 2 ppm silver cyanide.
Recoveries by CATC and WAD are species and concentration dependent. OIA 1677/D6888 recovers all available cyanide species regardless of concentration.

37. Recovery of metal-cyanide complexes by ASTM D7237 and D6888

38. Total Cyanide Analysis
Manual Distillation Methods

39. Total cyanide methods using manual distillation
Descriptive Name
Method Number
Description
Measurement
Total Cyanide
EPA 335.4
Midi Distillation – MgCl2
Automated Colorimetry
ASTM D2036
Midi / Micro/macro Distillation – MgCl2
Colorimetry/ISE/amperometry/IC
ASTM D 7284
Midi / Micro Distillation – MgCl2
Gas Diffusion - Amperometry
All these methods are essentially the same, and will obtain essentially the same result. All share the same interferences that occur during the distillation process. ASTM D achieves superior results on samples where distillates interfere with the pyridine barbituric acid reagent. The method has the further plus in that it eliminates the use of pyridine barbituric acid and uses a reagent that compensates for sulfide up to 50 ppm.

40. Semi-automated colorimetric cyanide analysis flow diagram
EPA method utilizes manual distillation followed by automated colorimetry. The diagram illustrates the chemistry involved in the colorimetric test.

41. Semi-Automated GD-amperometric by ASTM D7284
This method uses gas diffusion amperometry and analyzes cyanide in the absorber solution after distillation. Sulfide and sulfite, interference with colorimetry, do not interfere.
Same reagents as D6888 but no ligands

42. Recovery of metal-cyanide complexes by ASTM D7237, D6888 and D7284

43. FIA diagram of ASTM D7511
The ASTM D7511 diagram is almost identical to that of OIA1677 and/or ASTM D6888. A UV digester inserted in the path irradiates the acidified sample solution which then enters the 1677 device. Note the fewer pump tubes. Again, the fewer pump tubes make the method easier to run and maintain. As discussed earlier, the gas diffusion accomplishes the same task as distillation but without the complexity.

44. Recovery of metal-cyanide complexes by ASTM D7237, D6888, D7284 and D7511

45. Thank You wclipps@shimadzu.com
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