(c) You are going to use External Calibration method for the spectrophotometric analysis of chromium content in the soil

[answerhappygod]

(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]