SOP for ESTIMATION OF HEXAVALENT CHROMIUM

PRINCIPLE:

The hexavalent chromium is determined calorimetrically by reaction with diphenylcarbazide in 

acid solution. A red-violet colored complex of unknown composition is produced. The reaction

 is very sensitive, the molar absorptivity based on chromium being about 40000 Lg ^–1 cm^-1 

at 540 nm.

MINIMUM DETECTION LIMIT:    0.05 mg/l

REAGENTS:

1.        Stock chromium solution: Dissolve 141.4 mg K₂Cr₂O₇ water and dilute to 100 ml; 

1.00 ml = 500 µg Cr.

2.        Standard chromium solution: Dissolve 1.00 ml stock chromium solution to 100 ml; 

1.00 ml = 500 µg Cr.

3.        Phosphoric acid, H₃PO_4­, conc.

4.        Diphenylcarbazide solution: Dissolve 250 mg 1,5- diphenylcarbazide 

(1,5-diphenylcarbazide) in 50 ml acetone. Store in a brown bottle. Discard when solution

 becomes discolored. Chromium solution (5 mg/L) ranging from 2.00 to 20.0 ml, to give 

standards for 10 to 100 µg Cr, into 250-ml beakers or conical flasks. Proceed with 

subsequent treatment

PROCEDURE:

Preparation of calibration curve: To compensate for possible slight losses of  

chromium during digestion or other analytical operations, treat standards by the 

same procedure as the sample. Accordingly, pipette measured volumes of standard

 of standards as if they were samples, also carrying out cupferron treatment of standards 

if this is required for samples. Develop color as for samples, transfer a suitable portion of 

each colored solution to a 1-cm absorption cell, and measure absorbance at 540 nm, 

using reagent water as reference. Correct absorbance readings of standards by 

subtracting absorbance of a reagent blank carried through the method.

Color development and measurement: Add 0.25 ml (5 drops) H₃PO₄. Use 0.2 N 

H₂SO_4­  and a pH meter to adjust solution to pH 1.0 ± 0.3 the matter of pH range is 

currently being considered by standard methods. Transfer solution to a 100-ml volumetric 

flask, dilute to 100 ml, and mix. Add 2.0 ml diphenylcarbazide solution, mix, and let stand 

5 to 10 min for full color development. Transfer an appropriate portion to a 1-cm 

absorption cell and measure its absorbance at 540 nm, using reagent water as reference. 

Correct absorbance reading of sample by subtracting absorbance of a blank through 

the method. From the corrected absorbance, determine micrograms chromium 

present by reference to the calibration curve.

CALCULATION

HEXAVALENT CHROMIUM (mg/l)   =   ABS / SLOPE

REFERENCE:  APHA – 3500 Cr - B  Colorimetric Method