Determination of Hexavalent Chromium in Waters, by a Colorimetric Method
LABORATORY METHODS MANUAL
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Marjorie Dickenson
DETERMINATION OF HEXAVALENT CHROMIUM IN WATERS BY A
COLORIMETRIC METHOD
1.
SCOPE
This method applies to the determination of hexavalent chromium (Cr 6+) in water.
2.
PRINCIPLE
Water samples are acidified and filtered, treated with diphenylcarbazide solution and analysed
diphenylcarbazide
3.
REACTIONS AND INTERFERENCES
The reaction between diphenylcarbazide is nearly specific for chromium. Hexavalent molybdenum
and mercury salts react to form a colour with the reagent but the intensities are much lower than that
for chromium at the specified pH. Concentrations as high as 200mg Mo or Hg/L can be tolerated.
Vanadium interferes strongly but concentrations up to ten (10) times that of chromium will not cause
trouble. Iron in concentrations greater than 1mg/L may produce a yellow colour but the ferric (Fe3+)
colour is not strong and no difficulty is encountered normally if the absorbances are measured
spectrophotometrically at the appropriate wavelength.
4.
REAGENTS
4.1.
General
Unless otherwise specified, all reagents shall be of analytical reagent grade, and high-purity
water used shall comply with Type I water as specified in ISO 3696 (16.7M/cm resistivity;
0.06S.cm conductivity).
4.2.
Acetone – AR grade
4.3.
Sulfuric acid (H2SO4)
4.3.1.
Sulfuric acid (6N)
Slowly, and with continuous stirring, add 10mL concentrated sulfuric acid to 50mL of highpurity water in a clean 100mL beaker. Cool before use.
4.3.2.
Sulfuric acid (0.2N)
Slowly, and with continuous stirring, add 17mL 6N sulfuric acid solution (4.2.1) to 500mL
high-purity water in a clean 600mL beaker.
4.4.
Diphenylcarbazide
reagent
–
Dissolve
250mg
1,5-diphenylcarbazide
(1,5diphenylcarbohydrazide) in 50mL acetone. Store in amber bottle and discard when the
solution becomes discoloured.
4.5.
Potassium Dichromate (K2Cr207) – AR grade
4.6.
Stock Chromium Standard Solution (500mg/L).
(a)
(b)
Record all preparation information on laboratory proforma.
Dissolve 141.4mg potassium dichromate (K2Cr207) in high purity water and make to
volume in a clean 100mL volumetric flask. Mix well and store in a glass bottle. Label
with the following information:
 identification (i.e. Method 06-058 Composite Solution);
06-096
Determination of Hexavalent Chromium in Waters, by a Colorimetric Method
LABORATORY METHODS MANUAL
Version:
Page:
Issue Date:
Authorised by:
02
2 of 4
13 October 1999
Marjorie Dickenson
 date;
 preparer's initials;
 symbol and concentration (mg/L) of each element in the solution; and
(c)
4.7.
 discard date (12 months after preparation).
Store on laboratory bench.
Intermediate Standard Chromium Solution (5mg/L)
(c)
(d)
Record all preparation information on laboratory proforma.
To a clean 100mL volumetric flask pipette 1mL 500mg/L stock chromium solution (4.6)
and make to volume with high-purity water. Mix well and store in a glass bottle. Label
with the following information:
 identification (i.e. Method 06-058 Intermediate Chromium Standard L);
 date;
 preparer's initials;
 symbol and concentration (5mg/L) of each element in the solution; and
(e)
5.
6.
 discard date (3 months after preparation).
Store on laboratory bench.
APPARATUS
5.1.
Volumetric Flasks – 100mL,
5.2.
Pipettes – A-grade, 1mL, 2mL, 5mL, 20mL
5.3.
Spectrophotometer Conforming to AS 3753-2001
5.4.
pH meter
5.5.
Conical beakers – 250mL, 100mL
5.6.
Vacuum filtration unit
5.7.
Membrane filters – 0.45m
5.8.
Analytical balance
SAFETY
6.1.
General
(a)
This method includes the use of harmful and corrosive substances. Before
proceeding, familiarity should be gained with the Material Safety Data Sheets (MSDS)
on each reagent used with the method.
(b)
Users of this method must be familiar with the laboratory’s safety precautions and
emergency procedures, and with the safety instructions in the handbooks from the
instrument manufacturers.
06-096
Determination of Hexavalent Chromium in Waters, by a Colorimetric Method
Version:
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Issue Date:
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LABORATORY METHODS MANUAL
7.
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PROCEDURE
Use laboratory proformas to record values associated with this method as necessary. This form must
be used to check off the addition of the caesium/lanthanum solution and dilution of over-range
solutions.
7.1.
Quality control
7.1.1.
General
While the procedure in Clause 8.2 is written in terms of a single determination, any
convenient number of test samples, with accompanying replicates, blanks and reference
materials, should be batched for maximum efficiency.
7.1.2.
Reference material
Suitable approved certified reference materials must be carried through the procedure.
7.1.3.
Blank determination
Use high-purity water and follow the procedure in Clause 8.2 as for test samples.
7.1.4.
Number of determinations
(a)
Reference materials must be processed at the rate of one, plus one in twenty or part
thereof.
(b)
Test samples should be prepared in replicate at the rate of one, plus one in ten or
part thereof.
(c)
Spike & spike duplicate Prepare a spike and spike duplicate sample at a rate of one,
plus one in twenty or part thereof.
7.2.
Preparation
(a)
(b)
(c)
Filter approximately 120mL sample through a 0.45m membrane filter.
Use 0.2N H2SO4 and a pH meter to adjust the solution pH to 1.0  0.3. Transfer
100mL of solution to a 100mL volumetric flask.
Develop the colour as detailed in Section 7.4.
7.3.
Colour Development
(a)
Add 2.0mL diphenylcarbazide solution and mix well.
(b)
Allow 10 minutes for colour development.
(c)
Transfer an appropriate portion to a 1cm absorption cell, and measure absorbance at
540nm on the spectrophotometer.
7.4.
Standards
(a)
Pipette the following volumes intermediate standard chromium solution (5mg/L) into
100mL volumetric flasks.
(b)
Treat standards by the same procedure as the sample to compensate for possible
slight losses of chromium.
Standard
Volume 5mg/L Cr6+
(mL)
g Cr6+
1
2.0
10
2
5.0
25
3
20.0
200
06-096
Determination of Hexavalent Chromium in Waters, by a Colorimetric Method
Version:
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Issue Date:
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LABORATORY METHODS MANUAL
8.
(c)
Develop the colour as detailed in Section 7.4.
(d)
Prepare a calibration curve (concentration versus absorbance).
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CALCULATIONS
8.1.
Corrected Absorbance
Correct absorbance reading of standards and samples by subtracting absorbance of a blank carried
through the method. From the corrected absorbance, determine g Cr6+ present by reference to the
calibration curve.
NOTE: If the solution is turbid after dilution to 100mL, take an absorbance reading before adding
diphenylcarbazide solution and correct absorbance reading of final coloured solution by subtracting
the absorbance measured previously.
Calculate A*, the true absorbance due to Cr6+
Where:
A
B
A*
=
A
-
B
=
=
Absorbance of treated sample
Absorbance of untreated sample
Read the concentration in the sample (A*) from the concentration versus absorbance graph.
8.2.
Waters
Where:
Cr6+ mg/L
=
 g in sample x D 


W


g in sample
=
=
=
concentration of Cr6+ in test solution, in micrograms
dilution factor (if any) of the test solution for measurement
sample volume of test sample, in millilitres
D
V
9.
QUALITY CONTROL
(a)
Update the InfoLAB Quality Control chart for the reference material(s).
(b)
The Relative Percent Difference (RPD) of replicate results should be within 10%.
(c)
Percent (%) recovery of spike results shall be in the range of 80-120%.
10.
REPORTING
(a)
File all worksheets and printouts associated with each application of this method and report
results in the job file.
(b)
Report Cr6+ concentrations to two (2) significant figures.
(c)
The detection limit for Cr6+ is 0.05mg/L for waters.
11.
REFERENCES
(a)
(b)
(c)
(d)
ISO 3696:1987 - Water for analytical laboratory use; specification and test methods.
AS 3753-2001 - Recommended practice for chemical analysis by ultraviolet/visible
spectrophotometry.
APHA Standard Methods for the Examination of Water and Wastewater 20 th edition (1998),
Method 3500-Cr (D) - Colorimetric Method.
Spectrophotometer operators manual.
06-096