The single point standard addition method was used in the determination of phosp

Question

The single point standard addition method was used in the determination of phosphate by the molybdenum blue method. A 2 mL urine sample was treated with molybdenum blue reagents to produce a species absorbing at 820 nm, after which the sample was diluted to 100 mL. A 25 mL aliquot gave an absorbance of 0.425 (Solution 1). Addition of 1 mL of a solution containing 0.0800 mg of phosphate to a second 25.1 mL aliquote gave an absorbance of 0.531 (Solution 2). Use these data to calculate the ppm of phosphate in the sample. (Assume the density of the solution is 1 mL/g).

Explanation / Answer

A = e x C x l is formula

A2/A1 = C2/C1

let initial Phosphate amount = x mg in 25 ml   , A = 0.425, ,

final phosphate is ( x + 0.08) mg in 26.1 ml , A = 0.531 ,

then (0.531/0.425) = ( x+0.08)/26.1/(x/25)

1.304x = (x+0.08) ,

x = 0.263 mg in 25 ml

hence for 100 ml solution x = 0.263 x4 = 1.052 mg

conc in ppm (mg/liter)= ( 1.052 mg / 0.1) = 10.52 ppm

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