Total Cyanide Analysis
Total Cyanide - USEPA Method 335.4
The FS3700 also measures total cyanide in post-distillation samples by a colorimetric reaction involving chloramine-T and pyridine/barbituric acid reagents, followed by spectrophotometric measurement at 570 nm. The USEPA notes a potential drawback of method 335.4 in their publication; Solutions to Analytical Chemistry Problems with Clean Water Act Methods stating; “If sulfite and thiosulfate are present there is no way to accurately determine cyanide if heat is applied. In these situations a non-distillation method such as D6888-04, or method OIA-1677 may be used.” A schematic drawing of the FS3700 configured to perform USEPA Method 335.4 is presented below.
Learn about the products used in this analysis:
1
HCN
Principle of Operation Samples to be analyzed must undergo a preliminary 2 hour sulfuric acid distillation step to breakdown cyanide complexes and release HCN gas, which is trapped in a sodium hydroxide scrubber solution (1). This NaOH solution is injected into a carrier stream and reacted with chloramine-T (2), converting the cyanide ion to cyanogen chloride. A pyridine and barbituric acid reagent is then reacted with cyanogen chloride (3) producing a red-colored complex which passes through a 37 ˚Cheater and into the flow cell of a photometric detector for measurement at 570 nm (4). WinFlow software displays the detector response in real-time as a peak. The resulting peak height is proportional to the cyanide concentration present in the sample.
NaOH
Pump
Sample
Reaction Manifold
Waste Carrier Phosphate Buffer Chloramine T
2
Valve
Pyridine/ Barbituric Acid
4
Detector
Waste
37 ˚C
3
2 Hour Sulfuric Acid Distillation MCN - + H 2 SO 4 HCN (g) in NaoH
1 2 3 4
HCN + NH 2 Cl
NCCl
NCCl+ C 5 H 5 N / C 4 H 4 O 3 N 2
(red-colored complex)
Photometric Detection at 570 nm
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