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Methods in Water Science and Technology
Scientific research
Cross validation of two different COD test kits (Kit with Hg and kit without Hg)
Written by: Eric Clayderman CAZOLI
December 2014
University of Stavanger Department of Mathematics and Natural Sciences
4036 Stavanger
Abstract: COD measurements on COD standard solutions and wastewater samples
were done by using two different COD test kits (kit with Hg and kit without Hg). Data
obtained from both kits were analyzed by looking essentially at statistical parameter
mean X and standard deviation σ. The objective of this work was to compare the
analyzed data in order to check the cross validation of the two kits. Results of this
work showed that the two kits are the same when measuring COD standard
solutions, while they do not really overlap when measuring wastewater samples.
Key words: COD, wastewater, test kits, mean, cross validation
1. Introduction:
Chemical Oxygen Demand (COD) is a term used in both water and
wastewater treatment to measure the amount of a specified oxidant reacting with a
given sample under controlled conditions (Al-‐Momani, 2003). The diochromate ion
(Cr2O72-‐) is the specified oxidant in colorimetric method and its amount is
expressed in terms of its oxygen equivalence.
Under the presence of catalysts (sulphuric acid H2SO4, mercuric sulphate AgSO4 and
sulfamic acid H3NSO3), the dichromate (Cr2O72-‐) oxidizes organic material in a
sample after incubation of 2h at 150°C. This oxidation reduces Cr2O72-‐ (hexavalent)
into Cr3+ (trivalent). Each of these chromium species has a direct relationship with
oxygen consumed (Association, Association, Federation, & Federation, 1915).
Colorimetric method for COD analysis is a time consuming since you have to
prepare both digestion and catalyst solutions. Nowadays, COD test kits are available
for an easy and quick COD analysis. A COD test kit is like a small glass tube (vial) on
which there is a unique barcode label that is automatically read by a
spectrophotometer to identify the appropriate method and take the COD
measurement. COD test kits also contain both digestion and catalyst solutions (like in
the colorimetric method) that react with samples to be measured. A COD test kit
may present a risk for the environment when it contains harmful chemicals. A kit
containing Hg is harmful for the environment compared to the one that has Hg free.
Regarding the environmental aspect and the price of kits, it would be recommended
to use the one with Hg free and the one that is less costly. For this reason, I did
some COD analysis on COD standard solutions and wastewater samples by using two
types of COD test kits (kit with Hg and kit without Hg). For this COD analysis, three
known COD concentrations of standard solution were prepared from Potassium
hydrogen phthalate (C8H5KO4).
The objective of this work was to check the cross validation of the two kits by
analyzing the statistical parameter mean and standard deviation of obtained data.
Data obtained from this work and the results of statistical analysis are presented and
explained in the result section of this paper.
2.Theory
A COD test kit contains all necessary chemicals that digest organic matters in
a given sample and catalyse reactions that happen inside. The following
stoichiometry shows these reactions and the relationship as well as the theoretical
ratios between chromium species and O2.
Oxidation: C6H12O6 + 6 H2O === > 6 CO2 + 24 e-‐ + 24 H+
Reduction: 24 e-‐ + 24H+ + 32 H+ + 4 Cr2O72-‐ === > 8 Cr3+ + 28 H2O
Redox reaction: C6H12O6 + + 32 H+ + 4 Cr2O72-‐ === > 8 Cr3+ + 6 CO2 + 22 H2O
In reality, O2 is the electron acceptor: C6H12O6 + 6 H2O === > 6 CO2 + 24 e-‐ + 24 H+ 24 e-‐ + 24 H+ + 6 O2 === > 12 H2O
From the reactions above, we can see that 1 mole of O2 takes up 4 e-‐ and 1 mole of
Cr2O72-‐ takes up 6 e-‐ .
= > 4 e-‐ /6 e-‐ * mole Cr2O72-‐/ mole O2 = 4 mole Cr2O72-‐/ 6 mole O2 = > Δ Cr2O72-‐ =
3/2*ΔO2 = COD
= > 8 Cr3+ /4 Cr2O72-‐ * 4 Cr2O72-‐/6 mole O2 = > 4 mole Cr3+/3 mole O2 = > Δ Cr3+=
¾* ΔO2 = COD
Practically, we measure Cr3+ for the high range COD (100 and 900 mg/L). The
relationship between theoretical COD and ΔCr3+ is obtained by a standard curve
calibration.
Reagents: According to (Association et al., (1915), different reagents are needed
during the set up of analysis in order to have a complete oxidation reaction and also
to remove any possible interferences. Specifically, these reagents are mercuric
sulphate, sulfuric acid and sulfamic acid. Mercuric sulphate is added to remove
complex chloride ions present in the sample. Without the mercuric sulphate, the
chloride ions would form chlorine compounds in strong acid media used in the
procedure. These chlorine compounds would oxidize the organic matter in the
sample, resulting in a COD value lower than the actual value. Sulfamic acid is added
to remove interferences caused by nitrite ions. Without sulfamic acid, the COD of
the sample would measure higher than the actual value. Potassium dichromate is
used as the oxygen source with concentrated sulfuric acid added to yield a strong
acid medium.
3. MATERIALS AND METHODS
3.1. Materials
Different materials (devices, chemical and instruments) were used during the
work in a laboratory. These materials are listed in the following table (tab. 1).
Table 1: list of materials that were used during the experiments, and their functions
Material Function
Potassium hydrogen phthalate (C8H5KO4) Chemical used to prepare COD standard solutions
Distilled water Used for dilution
Analytical balance Used to weight the amount of C8H5KO4 to be used
2 types of COD test kits: kit with Hg and Kit without Hg
Contain chemicals needed to react with samples
Pipettes Used to take a precise volume of sample
Flask Erlenmeyer Used for mixing chemicals and solutions
Gloves Hands protection
Glass Yes protection
Incubator Used to cook COD kits containing samples
Spectrophotometer Measures COD concentration of sample as a function of the color intensity.
Stop watch Record time
3.2. Methods
First of all, I prepared three known concentrations of COD standard solution
from Potassium hydrogen phthalate (KHP= C8H5KO4). This preparation was done
according the American standard for COD analysis (Association et al., 1915). KHP has
a COD of 1.176 gO2/g KHP. This value is obtained by the following reaction and
calculations:
C8H5KO4 + aO2 + H+ = > 8 CO2 + K+ + 3 H20 === > a= 15/2 * mole
O2/mole KHP
= > 7.5 mole O2/mole KHP = > γO2/KHP = 7.5 mole O2/mole KHP * (32gO2/moleO2) / (204 gKHP/mole
KHP) = > 1.176 gO2/gKHP = 1.176 gCOD/gKHP
(1 gCOD/L)/ (1.176 gCOD/gKHP) = 0,85 gKHP/L.
Preparation of the three known COD concentrations:
C1= stock= 1500 mg/L (add 1275mg KHP into 1000mL distilled water)
C2= 525 mg/L (add 35 mL stock into 100 mL volumetric flask
containing distilled water)
C3= 150 mg/L (add 10 mL stock into 100 mL volumetric flask
containing distilled water)
The second part of this work was to measure the COD of these three known
COD concentrations and measure the COD of wastewater samples. The procedures
for COD measurement are explained in the appendix i.
The third part of this work was to analyse the data obtained from the COD
measurement. Mean and standard deviation were the two main statistical
parameters analysed to compare the data obtained from both Kits. This analysis was
done under Excel software.
4. Results and discussion
4.1. Raw data
The raw data obtained from all measurement during this work is presented in table
2.
For simplification, let´s use the terms “Hg” and “Hg free” respectively for kit with Hg
and kit without hg.
Table 2: Raw COD data obtained from the two kits.
Expected concentrations of the three known COD standard solutions (mg COD/L)
C1 C2 C3 1500 525 150
COD concentrations of blanks (CODmg/L) Blank 1 Blank 2
Hg 83 65
Hg free 49 0
COD concentrations of COD standard solution after the measurement (mgCOD/L)
C1 C1 C1
Hg 1556 1569 1551
Hg free 1528 1561 1515
C2 C2 C2 C2 C2 C2 Hg 569 571 573 571 577 570 Hg free 530 533 537 531 534 530
C3 C3 C3
Hg 205 202 200
Hg free 153 158 157
COD concentrations of wastewater samples (mgCOD/L)
sample 1 sample 2 sample 3 sample 4 sample 5 sample 6 Hg 1304 409 388 400 354 392 Hg free 441 422 637 465 354
4.2. Analysed data
4.2.1. Mean (X) and standard deviation (σx)
During the statistical analysis, parameter mean X and standard deviation σ were
calculated. Table 3 and 4 show respectively the mean and the standard deviation of
COD values obtained from the two kits. The average COD concentrations of standard solutions and wastewater samples were subtracted by COD content of blanks.
Table 3: Average COD concentrations of standard solutions (C1, C2, C3) and wastewater samples
COD concentrations ( mgCOD/L)
Blank C1 C2 C3 Wastewater Hg 74 1485 498 128 467
Hg free 26 1510 508 131 439 Table 4: Standard deviation of COD values (σx)
C1 C2 C3 Wastewater Hg 9 3 3 374
Hg free 24 3 3 105
4.2.2. Rejection of data
By looking at the table 2, we can see that some of the COD concentrations of
wastewater samples look specious. The value 1303 mgCOD/L (from kit with Hg) and
the value 637 mgCOD/L (from kit without Hg) seem anomalously large. By applying
the Chauvenet´s criterion, we can decide the rejection of these two values.
Assuming provisionally all COD measurement of wastewater samples is
legitimate.
a. N=6 (1303, 409, 388, 400, 354, 392); the mean X is here 467 and the standard
deviation σx is 374. The difference between the suspect Xsus=1303 and the mean X=
467 is 836, or 2.2 standard deviations; that is,
Tsus= (xsus-‐x)/ σx = (1303-‐467)/374 = 2.2
Referring to the table in Appendix ii, the probability that a measurement will differ
from X by 2.2σx or more is:
Prob(outside 2.2σx) = 1-‐ Prob(inside 2.2σx)
= 1-‐ 0.972 = 0.028
In 6 measurements, I would expect to find 0.168 of one measurement as deviant as
the suspect result. Since 0.168 is less than the 0.5 set by Chauvenet´s criterion, I
should reject the suspect Xsus= 1303 mgCOD/L. So, the new mean and standard
deviation for COD of wastewater, which was measured by kit with Hg, would be
respectively 315 mgCOD/L and 21.
b. N=5 (441, 422, 637, 456, 354); the mean X here is 439 and standard deviation σx is
105. The difference between the suspect Xsus=637 and the mean X= 439 is 198, or
1.88 standard deviations; that is,
Tsus= (xsus-‐x)/ σx = (637-‐439)/105 = 1.88
Referring to the table in Appendix ii, the probability that a measurement will differ
from X by 1.88σx or more is
Prob(outside 1.88σx) = 1-‐ Prob(inside 1.88σx)
= 1-‐ 0.939 = 0.061
In 5 measurements, I would expect to find 0.305 of one measurement as deviant as
the suspect result. Since 0.305 is less than the 0.5 set by Chauvenet´s criterion, I
should reject the suspect Xsus= 637 mgCOD/L. So, the new mean and standard
deviation for COD of wastewater, which was measured by kit without Hg, would be
respectively 421 mgCOD/L and 48.
4.2.3. Correct estimator for all measurements
If we assume 95 % confidence for our measurements, the average COD for
our samples would be:
COD= X ± (σx/√n)*tα/2,n-‐1
where:
x: mean σx : standard deviation
n: number of measurements α: accepted error= 5%= 0.05
For 95% confidence, the real mean of COD (mg/L) for the standard solutions and the
wastewater sample are the following:
Conclusion: By looking at the real mean of COD for standard solutions, we can see
that the two test kits overlap. Therefore, we can say that the two tests are the same.
For the wastewater samples, we can see that the two kits almost overlap. The
reason, why they do not exactly overlap for wastewater samples, could be from the
subtraction of the COD concentrations of samples by the mean of the COD of blanks
that seem incorrect.
Reference:
Al-‐Momani, F. (2003). Combination of photo-‐oxidation processes with biological
treatment: Universitat de Barcelona. Association, A. P. H., Association, A. W. W., Federation, W. P. C., & Federation, W.
E. (1915). Standard methods for the examination of water and wastewater (Vol. 2): American Public Health Association.
Taylor, J. (1997). Introduction to error analysis, the study of uncertainties in physical measurements (Vol. 1).
C1 C2 C3 wastewater Hg 1485± 22 498± 3 128± 7 315± 26
Hg free 1510± 60 508± 3 131± 7 421± 77
Appendix i a. Procedures for COD analysis using Kit with Hg
Source: http://www.google.no/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CB8QFjAB&url=http%3A%2F%2Fenvirotest.com.my%2Fproductsupport%2Fdownload%2F57&ei=LGKuVLKWLYLyaNyegig&usg=AFQjCNESMTZBXvJApC7q3eBZfUkcSuhTvg&sig2=88DGxtDdmga_9a9tpWl1EA
b. Procedures for COD analysis using Kit without Hg (Hg free)
Source: http://www.google.no/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CB8QFjAB&url=http%3A%2F%2Fenvirotest.com.my%2Fproductsupport%2Fdownload%2F57&ei=LGKuVLKWLYLyaNyegig&usg=AFQjCNESMTZBXvJApC7q3eBZfUkcSuhTvg&sig2=88DGxtDdmga_9a9tpWl1EA
Source: (Taylor, 1997)
Appendix ii
