Determination of Chromium by the Diphenylcarbazide method (Spectrophotometric)

DETERMINATION OF CHROMIUM BY THE DIPHENYL CARBAZIDE METHOD
(PHOTOMETRIC)

Scope
This method covers the determination of 5 to 120 µg Cr/g U of chromium in nuclear grade uranyl nitrate solutions.

Summary of Method
Chromium is oxidized with ammonium persulfate in the presence of silver nitrate. The hexavalent chromium gives a soluble red-violet complex with diphenylcarbazide. Photometric measurement is made at 540 nm.

Equipments
· pH meter with glass and calomel electrodes
· Spectrophotometer

Reagents
· Ammonium persulfate solution (100 g/L)-Dissolve 10 g of (NH4)2S2O8 in water and dilute to 100 mL. Prepare fresh as needed.
· Chromium, Standard Solution A 1 mL = 200 µg/Cr (VI) – Dissolve 0.565 g of potassium dichromate (K2Cr2O7) in water. Transfer to a 1 L volumetric flask, dilute and mix.
· Chromium, Standard Solution B 1 mL = 10 µg/Cr (VI) – Transfer a 25 mL aliquot of chromium solution A to a 500 mL volumetric flask and dilute to volume.
· Diphenylcarbazide Solution (5g/L) – Dissolve 0,50 g of diphenylcarbazide in 100 mL of acetone. Prepare fresh as needed
· Silver Nitrate Solution (2.5 g/L) – Dissolve 2.5 g of AgNO3 in water and dilute to 1 L.
· Sodium Hydroxide Solution (80 g/L) – Dissolve 80 g of NaOH in water and dilute to 1 L.
· Sulfuric Acid Solution (2 N) – Add 56 mL of H2SO4 to water and dilute to 1 L.
· Uranium Solution (1 mL = 0.05 g U) – Dissolve 10.0 g of uranium metal or 11.8 g of U3O8 in HNO3. Add 25 mL H2SO4 and heat to copious fumes. Cool, transfer to a 200 mL volumetric flask and dilute to volume.

Procedure
· Transfer 3 to 4 g of sample solution to a 100 mL flask and dilute to volume with water.
· Take a 5 mL aliquot of this dilution for analysis and place in a 150 mL beaker.
· Evaporate on a steam bath to dryness.
· Add 4 mL of 2 N H2SO4 and warm to dissolve the residu.
· Adjust the volume to about 40 mL.
· Adjust the pH with NaOH solution from 0.9 to 1.1 using pH meter.
· Add 4 mL of AgNO3 solution and 4 mL of (NH4)2S2O8 solution and cover.
· Boil gently for 25 min to destroy the excess (NH4)2S2O8 Maintain a minimum volume of 40 mL during heating.
· Cool to room temperature and transfer to a 100 mL volumetric flask.
· Add 2 mL of diphenylcarbazide solution, adjust to volume with water and mix.
· Measure the solution immediately in a 1 cm cell at 540 nm using water as the reference.

Note – The colour of the chromium complex develops almost immediately and starts to fade within a short time. Photometric measurements must be made within5 min after adding the diphenylcarbazide

· Carry a background color blank containing the addition of diphenylcarbazide.
· Carry reagent blank through step 4 up to step 11 at procedure.
· Prepare the standard curve by carrying 0 to 120 µg of Cr through step 3 to step 11 (at procedure) in the presence of 150 mg of uranium ( 3 mL of uranium solution).

Calculation
Calculate the chromium content as follows :

Chromium, µg/g = ( A-B-C)/D

There
A = Chromium found in the sample solution aliquot, µg
B = Chromium equivalent found in the reagent blank, µg
C = Chromium equivalent found in the background color blank, µg
D = Uranium contained in the sample solution aliquot, g

Precision
The limit of error at the 95% confidence level for a single determination is ± 5 %

Literature
ASTM-C799, Standard Methods for “Chemical, Mass Spectrometric, Spectrochemical, Nuclear, and Radiochemical analysis of Nuclear-Grade Uranyl Nitrate solution”