Determination of Hexavalent Chromium in Waters by a Colorimetric Method

Determination of Hexavalent Chromium in Waters, by a Colorimetric Method

Version: 02

Page: 1 of 3

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 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 (16.7MW/cm resistivity; 0.06mS.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 the solution 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. Label with the following information:

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

· date;

· preparer's initials;

· symbol and concentration (mg/L) of each element in the solution; and

· discard date (12 months after preparation).

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

(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. 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

· discard date (3 months after preparation).

(e) Store on laboratory bench.

5. 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 – 0.45mm

5.8. Analytical balance

6. 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.

7. 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) Filter approximately 120mL sample through a 0.45mm 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.

7.3. Colour Development

(a) Add 2.0mL diphenylcarbazide solution and mix well.

(b) Allow 10 minutes for colour development.

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) / mg Cr6+
1 / 2.0 / 10
2 / 5.0 / 25
3 / 20.0 / 200

(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 mg 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+

A* = A - B

Where: A = Absorbance of treated sample

B = Absorbance of untreated sample

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

8.2. Waters

Cr6+ mg/L =

Where: mg in sample = concentration of Cr6+ in test solution, in micrograms

D = dilution factor (if any) of the test solution for measurement

V = 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%.

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.

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/visible spectrophotometry.

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

(d) Spectrophotometer operators manual.

06-096