SOP for STANDARD WORKING PROCEDURE FOR ESTIMATION OF CHROMIUM

PRINCIPLE:

            Metal in solution are determined by Atomic Absorption Spectrophotometer. Suspended metals are separated by membrane filtration or suspension is dissolved and analyzed.

Limit of Detection: 0.02 mg/L

REAGENTS

1. Nitric acid : (1:1)

2. Nitric acid : (1:499)

3. Hydrochloric acid : (1:1) 

4. Chromium Stock Solution: Merck 1000 mg/L NIST Certified Reference Material or Weigh accurately 1.923 g chromium oxide into a beaker and add dissolving medium (Water + 10 ml nitric acid). When metal is completely dissolved, transfer quantitatively into 1000 ml volumetric flask and dilute to volume with distilled water.

5. Working Solution: Prepare daily. Prepare series of standards covering the required range (0.2, 0.4, 0.6, 0.8 and 1.0 mg/L). Add 1.5 ml nitric acid per liter to all working standard solution before diluting to volume. Add 1 ml lanthanum chloride for every 10 ml making the working standard solution.

6. Lanthanum Stock Solution: Slowly add 250 ml HCl to 58.65 g lanthanum oxide (La₂O₃, purity 99.9% by mass) dissolve and dilute to 1000 ml.

PROCEDURE:

Preparation of Sample:

Transfer 1000 ml well-mixed sample to the evaporation dish and add 3 ml nitric acid. Heat and evaporate to 20 ml (do not boil). Cool and add 3 ml nitric acid and heat until digestion is complete, Add 2 ml HCl (1:1 v/v) and heat gently to dissolve the residue. Wash the evaporation dish for 3 to 4 times with water and filter. Wash the filter and discard. Dilute the filtrate & make up 100 ml volume with water. (10 times concentrate the sample).

Analysis: Operate the instrument as per SOP. Aspirate nitric acid (1:499) as blank. Then aspirate series of standards and then aspirate the sample solution. Read the concentration in mg/L of Chromium directly.

(Apply the dilution factor 0.1 because the sample is concentrate 10 times)

REFERENCE: IS 13428: 1998 (Annex J)