Answer Happy

(c) You are going to use External Calibration method for the
spectrophotometric analysis of chromium content in the soil sample.
(i) The linear working range of the spectrophotometer is 0.05 to 5
ppm (mg/L) of the Cr(VI) content. Given a 500 ppm of Cr6+ stock
solution (prepared from potassium dichromate (K2Cr2O7¬)), design
the concentrations (ppm, mg/L) of the five Cr6+ working solutions
for the calibration and demonstrate, with calculations, the amounts
(in volume) of 500 ppm of Cr6+ stock solution needed for the
preparation of five designated working solutions in 25.0 mL of
volumetric flasks. [10 marks]
(ii) You are given the following reagents and apparatus: 25 mL
of volumetric flasks; 500 ppm of Cr6+ stock solution, a solution of
mixture of 5% NaF and 0.5% 1.5– diphenylcarbazide; 0.2 M of H3PO4
solution (solvent medium), the pre-treated soil sample solution,
other general laboratory glassware, etc. Based on the given
measurement principle of the Cr(VI) analysis, describe the detailed
procedural steps on preparing the five working solutions. [4
marks]