the analytical methods and technologies of cyanide chemistry training

52
Reducing Interferences in Cyanide Analysis William Lipps Market Specialist – Water Analyzer Products OI Analytical

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Page 1: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Reducing Interferences in Cyanide Analysis

William LippsMarket Specialist – Water Analyzer Products

OI Analytical

Page 2: The Analytical Methods And Technologies Of Cyanide Chemistry Training

This talk presents problems and solutions in cyanide analysis

1. What we measure2. Problems

HCN

CN-

3. Solutions

Page 3: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Cyanide methods measure the various cyanide “species”

Total CN

Strong Metal Complexes

Fe, Co, Pt, Au

Available CN

Weak to Moderately Strong Metal ComplexesAg, Cd, Cu, Hg, Ni, Zn

Free CN

HCN

CN-

Page 4: The Analytical Methods And Technologies Of Cyanide Chemistry Training

If all we had was CN- in dilute NaOH it would be easy

Correlation of CATC and Direct Colorimetry on Standards

y = 0.8874x + 0.0085

R2 = 0.9921

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

CATC mg/L CN

Dir

ec

t C

olo

rim

etr

y m

g/L

CN

Page 5: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Direct colorimetry does not correlate with distillation results

Correlation of CATC with Direct Colorimetry on Real Samples

y = 0.503x + 0.0109

R2 = 0.1127

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

CATC mg/L CN

Dir

ec

t C

olo

rim

etr

y m

g/L

CN

Page 6: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Direct ISE does not correlate with distilled real sample results

Comparison of CATC with Direct ISE

y = 1.2501x - 0.0303

R2 = 0.2528

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

CATC mg/L CN

ISE

mg

/L C

N

Page 7: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Cyanide methods require separation of CN from matrix

• Separated from interferences, cyanide measurement is no different than running standards.

Page 8: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Distillation most common technique to remove interference

Macro Distillation MIDI Distillations

Page 9: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Distillation actually creates CN interferences

• Boiling acid

• Automated UV-Distillation– Boiling acid

Page 10: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Interferences with distillation are in almost every sample

• Thiocyanate

• Sulfur

• Thiosulfate/Sulfite

• Oxidizers

Page 11: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Thiocyanate + Nitrate results in positive bias

• The addition of Sulfamic acid does not sufficiently reduce this interference.– A real POTW sample with 0.1 mg/L SCN-

and 63.5 mg/L NO3- detected total CN- at

0.10 mg/L even after the addition of Sulfamic Acid

Page 12: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Sulfur compounds react rapidly with CN

• Elemental Sulfur– 8CN- + S8 SCN-

• Metal Sulfides – Cu2S, FeS, PbS, CuFeS2, CdS,

ZnS, etc.– S reacts with CN- to form SCN-

Page 13: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Thiosulfate reacts with cyanide during distillation

• 0.200 mg/L CN- + 200 mg/L S2O3-2

– Cyanide Found = 0.160 mg/L– Recovery = 80%*

– * Double Chloramine T added, or recovery would be lower.

Page 14: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Sulfite reacts rapidly with CN in basic solutions

• 0.200 mg/L CN- + 200 mg/L SO3-2

– Cyanide Found = 0.000 mg/L– Recovery = 0%

• This reaction occurs in absorber solution, or in preserved sample

Page 15: The Analytical Methods And Technologies Of Cyanide Chemistry Training

There is no way to “know” if sulfur compounds are present

• No “spot” tests that adequately detect the sulfur compounds

• Sodium sulfite and sodium thiosulfate are both added to samples for dechlorination.

Page 16: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Oxidizers destroy cyanide before or during distillation

• Hypochlorite

• Peroxide

• Caro’s Acid

• Chloramines

Page 17: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Footnote 6 (MUR 2007) allows other methods to be used

• More accurate?– Spikes?

• “challenge matrix” distilled

Page 18: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Matrix spikes cannot be used to demonstrate accuracy

MethodAmount

Detected (ppb)Recovery

335.4 32 98 %

335.3 16 98 %

Both methods detected CN- in a synthetic sample with no CN-.

Page 19: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Verify Accuracy Using Interference Free Methods

• Use methods demonstrated by literature and multiple users to be interference free– OIA 1677 or ASTM D6888-04– ASTM D 7284-08– OIA 1678 (ASTM D7511-09)

Page 20: The Analytical Methods And Technologies Of Cyanide Chemistry Training

The old versus the new

HCN

CN-

Purging and boiling in acid

Diffusion at room

temperature

Page 21: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Electrochemistry techniques integrate matrix removal

• Very sensitive with large dynamic range.

Page 22: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Unlike colorimetry, GD amperometry is easy to visualize

• CN- + H+ HCN

• HCN + OH- CN- + H2O

• Ag + 2CN- Ag(CN)2- + e-

measure

Page 23: The Analytical Methods And Technologies Of Cyanide Chemistry Training

This flow diagram illustrates the simplicity of GD-amperometry

Acid

Page 24: The Analytical Methods And Technologies Of Cyanide Chemistry Training

A series of GD-amperometry CN methods to meet needs

• Free cyanide

• Available cyanide

• Total distilled cyanide

• Total non-distilled cyanide

Page 25: The Analytical Methods And Technologies Of Cyanide Chemistry Training

The only fully automated free cyanide method

Method Description Measurement

ASTM D 7237 FIAGas Diffusion-Amperometry

Page 26: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Direct colorimetry is not measuring free cyanide

Page 27: The Analytical Methods And Technologies Of Cyanide Chemistry Training

The free cyanide method by GD-amperometry

Buffer

ASTM D7237-06

Page 28: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Real Data Comparison – Free CN

Page 29: The Analytical Methods And Technologies Of Cyanide Chemistry Training

GD-amperometry methods for available cyanide – no CATC!

Descriptive Name

Method Number

Description Measurement

Available Cyanide

OIA 1677Ligand Exchange / Flow Injection Analysis

Gas Diffusion - Amperometry

ASTM D 6888

Ligand Exchange / Flow Injection Analysis

Gas Diffusion - Amperometry

No distillation or pyridine required

Page 30: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Sulfide does not interfere

Page 31: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Quantitative recovery compared to other methods such as CATC

OIA1677OIA 1677

Page 32: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Precise and more accurate than WAD methods

OIA1677

Page 33: The Analytical Methods And Technologies Of Cyanide Chemistry Training

GD-amperometry available CN has fewer interferences

CATC WAD OIA 1677

N-organicsExcessive

Iron CyanideNone

SCN,NH3,NO2Concentration

Dependent

S2O3, H2O2

Concentration Dependent

Page 34: The Analytical Methods And Technologies Of Cyanide Chemistry Training

GD-amperometry available CN saves time and labor

CATC WAD OIA 1677

Sample Preparation

2 distillations

2 – 3 hours

1 distillation

2 – 3 hours

No distillation

Analysis1 – 2 minutes

1 – 2 minutes

1 – 2 minutes

Total Time3 – 4 hours 3 – 4 hours 1 – 2

minutes

Page 35: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Less manipulation means better Available CN data

• No distillation

• 0.5 ppb MDL

• Up to 90 samples per hour

• Ease of Operation

• Very simple chemistry

Page 36: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Easy to use, understand, and environmentally friendly

Acid Reagent

OIA 1677, ASTM D6888-04 or ASTM D7284-08

No pyridine

Page 37: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Total cyanide methods using manual distillation

Descriptive Name

Method Number

Description Measurement

Total Cyanide

EPA 335.4Midi Distillation – MgCl2

Automated Colorimetry

ASTM D 7284Midi / Micro Distillation – MgCl2

Gas Diffusion - Amperometry

Page 38: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Most total cyanide analyses are by EPA 335.4 or similar

• Manual distillation• Prolonged heating (125 °C) , strong

acid (pH <2) • Purging into basic absorber solution• Colorimetry

Page 39: The Analytical Methods And Technologies Of Cyanide Chemistry Training

ASTM D7284 manual distillation GD-amperometry method

• Distill samples

• Use GD-amperometry

• No pyridine

• Fewer interferences

Page 40: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Once again, sulfide does not interfere with GD-amperometry

Page 41: The Analytical Methods And Technologies Of Cyanide Chemistry Training

But wait, I thought distillation was bad?

Page 42: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Comparison of Measurements with Interferences Present

Sample (Total CN)

EPA 335.4 ASTM D7284

20 ppb 18.7 18.4

20 ppb +SCN + NO3 227 229

200 ppb + SO3 147 159

200 ppb + Ascorbate

152 154

200 ppb + Ascorbate + Ammonia

73 71

200 ppb + Ascorbate + Ammonia + OCL-

47 49

Page 43: The Analytical Methods And Technologies Of Cyanide Chemistry Training

What exactly do “total” cyanide methods measure?

• Iron Cyanide + Available CN

Page 44: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Iron cyanide is not toxic.

• Sunlight causes iron cyanide to release HCN

• Sunlight = UV irradiation

[Fe(CN)6]-3 + H+ _ 6 HCN + Fe+3

hv

Page 45: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Automated total cyanide methods use UV to liberate HCN from Fe

Descriptive Name

Method Number

Description Measurement

Total Cyanide

ASTM D4374 (Kelada 01)

High power UV- Auto distillation

Alkaline pH

Automated colorimetry

EPA 335.3Low power UV- Auto distillation

pH <2

Automated Colorimetry

OIA 1678/ASTM

D7511

Low power UV- pH <2

Gas Diffusion - Amperometry

Page 46: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Comparison of Kelada and ASTM D7511-09

Kelada 01ASTM D7511

Pump Tubes

15 5

Reagents Pyridine No Pyridine

Distillation Yes No

SCN Interaction

0.25 – 0.5 % 0.01 – 0.03 %

Page 47: The Analytical Methods And Technologies Of Cyanide Chemistry Training

For total cyanide, simply add UV irradiation to the GD method

No pyridine

OIA1678 / ASTM D7511-09

Page 48: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Same results with and without flash distillation

Distillation is not necessary

Page 49: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Same, but more precise results as manual distillation

OIA 1678 UV GD-Amperometry

EPA 335.4 Semi-automated Colorimetry

Page 50: The Analytical Methods And Technologies Of Cyanide Chemistry Training

ASTM D7511 and D7284 get the same result if no interferences

Page 51: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Comparison of Total CN methods

335.4ASTM D7284

ASTM D7511

Sample Preparation

2 – 3 hour distillation

1 – 3 hour distillation

No distillation

Analysis1 – 2

minutes1 – 2

minutes1 – 2

minutes

Total Time 3 – 4 hours 2 – 4 hours1 – 2

minutes

Page 52: The Analytical Methods And Technologies Of Cyanide Chemistry Training

Benefits

Distillation / Colorimetry

GD-Amperometry

In summary, distillation/colorimetry should be replaced