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Determination of Hexavalent Chromium in Waters, by a Colorimetric Method

Version: 02

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Issue Date: 13 October 1999

LABORATORY METHODS MANUAL Authorised by: Marjorie Dickenson

DETERMINATION OF HEXAVALENT CHROMIUM IN

WATERS BY A COLORIMETRIC METHOD

1. SCOPE

This method applies to the determination of hexavalent chromium (Cr6+) in water.

2. PRINCIPLE

Water samples are acidified and filtered, treated with diphenylcarbazide solution and analyseddiphenylcarbazide

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 thatfor 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 causetrouble. Iron in concentrations greater than 1mg/L may produce a yellow colour but the ferric (Fe 3+)colour is not strong and no difficulty is encountered normally if the absorbances are measuredspectrophotometrically 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;

0.06S.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 high-purity 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 thesolution becomes discoloured.

4.5. Potassium Dichromate (K2Cr207) AR grade

4.6. Stock Chromium Standard Solution (500mg/L).

(a) Record all preparation information on laboratory proforma.

(b) 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. Labelwith the following information:

identification (i.e. Method 06-058 Composite Solution);

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Determination of Hexavalent Chromium in Waters, by a Colorimetric Method

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Issue Date: 13 October 1999

LABORATORY METHODS MANUAL Authorised by: Marjorie Dickenson

date;

preparer's initials;

symbol and concentration (mg/L) of each element in the solution; and discard date (12 months after preparation).

(c) Store on laboratory bench.

4.7. Intermediate Standard Chromium Solution (5mg/L)

(c) Record all preparation information on laboratory proforma.

(d) 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. Labelwith 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

discard date (3 months after preparation).(e) Store on laboratory bench.

5. APPARATUS

5.1. Volumetric Flasks 100mL,

5.2. PipettesA-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 0.45m

5.8. Analytical balance

6. SAFETY

6.1. General

(a) This method includes the use of harmful and corrosive substances. Beforeproceeding, 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 laboratorys safety precautions andemergency procedures, and with the safety instructions in the handbooks from theinstrument manufacturers.

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Determination of Hexavalent Chromium in Waters, by a Colorimetric Method

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Issue Date: 13 October 1999

LABORATORY METHODS MANUAL Authorised by: Marjorie Dickenson7. PROCEDURE

Use laboratory proformasto 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-rangesolutions.

7.1. Quality control

7.1.1. General

While the procedure in Clause 8.2 is written in terms of a single determination, anyconvenient number of test samples, with accompanying replicates, blanks and referencematerials, 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 partthereof.

(b) Test samples should be prepared in replicate at the rate of one, plus one in ten orpart 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) Filter approximately 120mL sample through a 0.45m membrane filter.(b) 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.(c) 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) into100mL volumetric flasks.

(b) Treat standards by the same procedure as the sample to compensate for possibleslight losses of chromium.

StandardVolume 5mg/L Cr6+

(mL)g Cr6+

1 2.0 10

2 5.0 25

3 20.0 200

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Determination of Hexavalent Chromium in Waters, by a Colorimetric Method

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Issue Date: 13 October 1999

LABORATORY METHODS MANUAL Authorised by: Marjorie Dickenson(c) Develop the colour as detailed in Section 7.4.

(d) Prepare a calibration curve (concentration versus absorbance).

8. 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 thecalibration curve.

NOTE: If the solution is turbid after dilution to 100mL, take an absorbance reading before addingdiphenylcarbazide solution and correct absorbance reading of final coloured solution by subtractingthe absorbance measured previously.

Calculate A*, the true absorbance due to Cr6+

A* = A - B

Where: A = Absorbance of treated sampleB = Absorbance of untreated sample

Read the concentration in the sample (A*) from the concentration versus absorbance graph.

8.2. Waters

Cr

6+

mg/L =

W

Dxsampleing

Where: g in sample = concentration of Cr 6+ in test solution, in microgramsD = dilution factor (if any) of the test solution for measurementV = sample volume of test sample, in millilitres

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 reportresults in the job file.

(b) Report Cr 6+ concentrations to two (2) significant figures.(c) The detection limit for Cr6+ is 0.05mg/L for waters.

11. REFERENCES

(a) ISO 3696:1987 - Water for analytical laboratory use; specification and test methods.

(b) AS 3753-2001 - Recommended practice for chemical analysis by ultraviolet/visiblespectrophotometry.

(c) APHA Standard Methods for the Examination of Water and Wastewater 20th edition (1998),Method 3500-Cr (D) - Colorimetric Method.

(d) Spectrophotometer operators manual.

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