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COD determination: Removing Cl- Interference in S tSeawaters
Amitte M. Gulamhussen SOLVAY PORTUGALSOLVAY PORTUGAL
03/07/2013
The Problem
IntroductionIntroduction
Chemical Oxygen Demand (COD) is defined as the amount of a specific oxidant (expressed in its oxygen equivalence) that reacts with a sample with organic substances under controlled conditions‐Dichromate ion (Cr2O7
2‐) is reduced to its chromic ion (Cr3+).
Chemical Oxygen Demand (COD) is defined as the amount of a specific oxidant (expressed in its oxygen equivalence) that reacts with a sample with organic substances under controlled conditions‐Dichromate ion (Cr2O7
2‐) is reduced to its chromic ion (Cr3+).
The most common interference is the chloride ion (Cl‐), present in many effluents (soda‐ash, Chlorine electrolyses…).The most common interference is the chloride ion (Cl‐), present in many effluents (soda‐ash, Chlorine electrolyses…).
In Portugal it is one of the parameters of the Solvay’s IPPC for the Global Effluent.
SO 0 2002 S 220 329 996 S 2 2 06 h d
In Portugal it is one of the parameters of the Solvay’s IPPC for the Global Effluent.
SO 0 2002 S 220 329 996 S 2 2 06 h dISO 15705:2002, SMEWW 5220, NP 4329:1996, ASTM D 1252‐06, EPA method 410.3:1978, DIN 38409 H41 and H43, were studied for accreditation submission.ISO 15705:2002, SMEWW 5220, NP 4329:1996, ASTM D 1252‐06, EPA method 410.3:1978, DIN 38409 H41 and H43, were studied for accreditation submission.
L602‐(01‐03‐2012)CQO‐DIN 38409 part 41:1980L602‐(01‐03‐2012)CQO‐DIN 38409 part 41:1980
IntroductionIntroduction
Cr2O72‐ is commercially available in pure salts as K2Cr2O7 and is easy to prepare.
Due to a higher capacity of oxidation, manipulation and range of application Cr2O72‐
is most commonly used rather than KMnO4, K2S2O8, HClO4, HIO3 or HNO3.
To measure the COD, usually there are 3 steps: A) Interferences elimination (Cl‐)B) Digestion, C) Q tifi tiC) Quantification
Interferences
A) Interference elimination ChlorideA) Interference elimination Chloride
Chloride reacts with AgSO4 precipitating AgCl reducing or inhibiting the Silver catalyticti it (1) Cl i di h t t idi d t hl i d i l hi h
Chloride reacts with AgSO4 precipitating AgCl reducing or inhibiting the Silver catalyticti it (1) Cl i di h t t idi d t hl i d i l hi hactivity (1). Cl‐ in dichromate gets oxidized to chlorine gas producing erroneously high
COD levels (2) [1] .
Ag+ + Cl‐ → AgCl (pp) (1)
activity (1). Cl‐ in dichromate gets oxidized to chlorine gas producing erroneously highCOD levels (2) [1] .
Ag+ + Cl‐ → AgCl (pp) (1)Ag+ + Cl → AgCl (pp) (1)
6 Cl‐ + Cr2O72‐ (aq) + 14 H+ (aq) → 3 Cl2 + 2 Cr3+ (aq) + 7 H2O (líq) (2)
Ag+ + Cl → AgCl (pp) (1)
6 Cl‐ + Cr2O72‐ (aq) + 14 H+ (aq) → 3 Cl2 + 2 Cr3+ (aq) + 7 H2O (líq) (2)
A) Interference elimination Chloride-OLD PROCEDURE A) Interference elimination Chloride-OLD PROCEDURE
ISO 15705: 2002 Determination of the chemical oxygen demand index (ST‐COD) –small‐scale sealed‐tube methodISO 15705: 2002 Determination of the chemical oxygen demand index (ST‐COD) –small‐scale sealed‐tube methodsmall‐scale sealed‐tube method.
The most frequent procedure to remove Cl‐ with a maximum weight ratio 10:1, HgSO4 + 2 Cl‐ → HgCl2 + SO4
2‐ (3)
small‐scale sealed‐tube method.
The most frequent procedure to remove Cl‐ with a maximum weight ratio 10:1, HgSO4 + 2 Cl‐ → HgCl2 + SO4
2‐ (3)
It was possible to eliminate chloride up to 8 g/L adding 0,5 g HgSO4 with COD Standards (50, 60 e 70 mg O2/L) and Cl‐ spiking of 2, 4, 8, 15 and 20 g/L (similar to E2 outlet).It was possible to eliminate chloride up to 8 g/L adding 0,5 g HgSO4 with COD Standards (50, 60 e 70 mg O2/L) and Cl‐ spiking of 2, 4, 8, 15 and 20 g/L (similar to E2 outlet).
Recoveries : 91,8% ‐118,6% adding 0,5 g of HgSO4.
In samples with more than 8 g/L of chloride it was necessary to perform a dilution. In those cases
Recoveries : 91,8% ‐118,6% adding 0,5 g of HgSO4.
In samples with more than 8 g/L of chloride it was necessary to perform a dilution. In those cases COD values were very close to LD leading to high errors.
This method for our wastewaters was not effective due to low precision comparing with legislation requirements
COD values were very close to LD leading to high errors.
This method for our wastewaters was not effective due to low precision comparing with legislation requirementsrequirements.
According to the literature [1], wastewaters with more than 2g/L of Cl‐ should have alternative procedures.
requirements.
According to the literature [1], wastewaters with more than 2g/L of Cl‐ should have alternative procedures.
A) Interference elimination Chloride-NEW PROCEDURE DIN38409A) Interference elimination Chloride-NEW PROCEDURE DIN38409
•Ionic membrane exchange.
•Numeric corrections by chlorides calibration curves.
•Subtraction of the apparent COD and the corrected COD determined by the liberated Iodine against y gthiosulfate titration solution with starch indicator
•Stripping of chloride with a sulfuric acid solution in a closed vapor chamber and bound by Ca(OH)2 (DIN 38409)
B) DigestionB) Digestion
Sample’s digestion at 148 ± 3ºC under reflux is essential to prevent the loss of volatile Sample’s digestion at 148 ± 3ºC under reflux is essential to prevent the loss of volatile p g porganic mater. The H2SO4 medium with excess of K2Cr2O7 is necessary to oxidize all organic matter
p g porganic mater. The H2SO4 medium with excess of K2Cr2O7 is necessary to oxidize all organic matter
Cr2O72‐ is reduced to Cr3+, H2O and CO2 produced by oxidation of the organic matter:
Cr2O72‐ (aq) + 14 H+ (aq) + 6 e‐ → 2 Cr3+ (aq) + 7 H2O (líq) Eº = 1,33 V
Cr2O72‐ is reduced to Cr3+, H2O and CO2 produced by oxidation of the organic matter:
Cr2O72‐ (aq) + 14 H+ (aq) + 6 e‐ → 2 Cr3+ (aq) + 7 H2O (líq) Eº = 1,33 V
C) Quantification: C1-Redox VolumetryC) Quantification: C1-Redox Volumetry
After digestion the quantification can be performed by:
C1 Redox Volumetry
After digestion the quantification can be performed by:
C1 Redox VolumetryC1-Redox Volumetry C2-Molecular Absorption Spectroscopy
After digestion, the residual Dichromate is determined by a back titration using
C1-Redox Volumetry C2-Molecular Absorption Spectroscopy
After digestion, the residual Dichromate is determined by a back titration using g y gAmmonium Iron Sulfate as titrant and ferroin as indicator
6 Fe2+ + Cr2O72‐ (aq) + 14 H+ (aq) → 6 Fe3+ + 2 Cr3+ (aq) + 7 H2O (líq)
g y gAmmonium Iron Sulfate as titrant and ferroin as indicator
6 Fe2+ + Cr2O72‐ (aq) + 14 H+ (aq) → 6 Fe3+ + 2 Cr3+ (aq) + 7 H2O (líq)
The dichromate consumed for the destruction of the organic matter can then be calculated and is expressed in terms of its oxygen equivalent (COD in mg O2/L)The dichromate consumed for the destruction of the organic matter can then be calculated and is expressed in terms of its oxygen equivalent (COD in mg O2/L)
To see or not to seeTo see or not to see…
DIN Implementation Result- LD, LQ & UncertaintyDIN Implementation Result- LD, LQ & Uncertainty
RP PC
N 146 30 26
MÉDIA 2,1 103% 102%
CV (%) 5,6% 8,9% 5,6%
Uncertainty= 20%
LD=5 mg/L O2
Uncertainty= 20%
LD=5 mg/L O22
LQ=15 mg/L O2
2
LQ=15 mg/L O2
Interlaboratory results- CODInterlaboratory results- COD
Ensaio IL - CQO
4
1
2
3
-1
0
1
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18Z-sc
ore
4
-3
-2
-4nº do Ensaio
Audits NP EN ISO IEC 17025:2005Audits NP EN ISO IEC 17025:2005
12 Jun.08 CQO 1,0 RELACRE• The laboratory has shown good technical knowledge and an excellent knowledge of the specific difficulties of their specific samples namely interferences
13 Out.08 CQO 0,3 RELACRE
14 N 09 CQO 0 5
interferences.
•The validated Internal or Normalized methods have a very good internal quality control program, well defined, organized and implemented in routine with good14 Nov.09 CQO 0,5 RELACRE
15 Mar.10 CQO 0,6 Aquacheck - CQO total
control program, well defined, organized and implemented in routine with good performance in IL trials
•The technical competence was verified with interviews to all the personal
16 Mar.10 CQO -0,1 Aquacheck - Não filtravél
p f pinvolved, CV’s, competence matrices and qualifications. The audit team would like to address the excellent knowledge in management and technical requirements
17 Mar.10 CQO -0,5 Aquacheck - Decantada
CQO 0 418 Abr.10 CQO 0,4 RELACRE
Generating InformationShould we look to the Tree or to the Forest?InformationShould we look to the Tree or to the Forest?
What should we compare?What should we compare?
1 L i l i i1‐Legislation requirements2‐Intrinsic method characteristics (precision, accuracy, uncertainty)3‐Analysts exposure 4 Hazard wastes4‐Hazard wastes5‐Costs
ConclusionsConclusions
DIN 38409 part 41:1980 was implemented for the measurement of COD in samples containing >20g/L.
It is the most efficient method due to several reasons:
DIN 38409 part 41:1980 was implemented for the measurement of COD in samples containing >20g/L.
It is the most efficient method due to several reasons:It is the most efficient method due to several reasons: Systematic compliance with Legislation requirementsBetter Intrinsic characteristics (precision, accuracy, uncertainty)Less Analysts exposure
It is the most efficient method due to several reasons: Systematic compliance with Legislation requirementsBetter Intrinsic characteristics (precision, accuracy, uncertainty)Less Analysts exposure Less Hazard wastes Less CostsLess Hazard wastes Less Costs
The SMEWW 5220-B method is not appropriate for samples containing more than 2g Cl-/L, Sample dilution produce massive errors.The SMEWW 5220-B method is not appropriate for samples containing more than 2g Cl-/L, Sample dilution produce massive errors.
According with BAUMANN et al. [2] samples with 18g/L of Cl- and results of 142-147 mg/L O2 have an error of 73-89 mg/L O2 (>50%). True values are in the range of 53-74mg/L O2.According with BAUMANN et al. [2] samples with 18g/L of Cl- and results of 142-147 mg/L O2 have an error of 73-89 mg/L O2 (>50%). True values are in the range of 53-74mg/L O2.
ReferencesReferences
[1] Eaton, A., Clesceri, L., Greenberg, A., ―Standard Methods for the Examina on of Water and Wastewate II, 20th edition, (5) American Public Health Association,Water and Wastewate II, 20th edition, (5) American Public Health Association, Washington, p 13 – 18, (1998)
[2] Baumann, Frank J.; A proposed Method for Chloride Correction in Highly Saline Wastes; Analytical Chemistry, Vol. 46 (9), p 1336 – 1338,(1974)
[3] DIN 38409: 1990; Part 41; Method H 41‐2.
[4] Burns, E. R., Marshall, C., Journal WPCF, Vol. 37, p 1716‐1721(1965).
[5] Semi‐Micro Reflux Procedure for Minimization of Chloride Interference by COD D t i ti Cl 36 (1) 66 69 (2008)Determination; Clean, 36 (1), p 66 – 69 (2008)
[6] HACH PROCEDURES HANDBOOK, Method 8000 – Oxygen Demand, Chemical (2005).
[7] K YW Ch M H K C ti f H d P id i t f[7] Kang, Y.W., Cho, M., Hwang, K., Correction of Hydrogen Peroxide interference on standard chemical oxygen demand test, Wat. Res. Vol.33 (5), p 1247‐1251 (1999).
[8] Duncan, M.; Haran, N.J.,―Handbook of water and wastewater microbiology ,[8] Duncan, M.; Haran, N.J., Handbook of water and wastewater microbiology‖, Academic Press, Elsevier, (4) p. 167‐169. (2003)