(b) A soft drink was analysed for soluble reactive phosphorus (SRP) by the molyb
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Question
(b) A soft drink was analysed for soluble reactive phosphorus (SRP) by the molybdenum blue/tartrate/ascorbic acid spectrophotometric technique. The operating range of the method is 0-1.5 mgP dm. A standard sol 7.439 mgP dm in the table. Twenty-five cm of the soft drink was diluted to 100 cm. Ihe was prepared and diluted to give working standards as indicated colouring reagents were added to 50 cm of each of the diluted sample and the working standards and their absorbances recorded after 20 minutes; the data are given in the table. Volume of Diluted to Absorbance 7.439 dm 20 cm100 cm3 0.478 100 cm3 10cm 100 cm 0.235 15cm 0.357 5 cm 0 cm3 100 cm 0,117 0.009 0.246 100 cm Diluted sample i) What mass of KH PO, is required to prepare 250 em of the 7.439 mgP dm standard solution? How would you prepare this standard? i) Plot the appropriate data and calculate the concentration of SRP in the soft drink, giving your answer in both mgP dm3 and mmol dmExplanation / Answer
(b) Analysis of soft drink
(i) preparation of KH2PO4 standard
7.439 mg/dm^3 has = (7.439 mg/dm^3)/136.086 g/mol
= 0.055 mmol/dm^3 KH2PO4
total volume = 250 cm^3
= 0.25 dm^3
moles of KH2PO4 needed = 0.25 dm^3 x 0.055 mmol/dm^3
= 0.01375 mmol
mass KH2PO4 needed to prepare standard = 0.01375 mmol x 136.086 g/mol
= 1.8712 mg
(ii) concentration of standard in diluted solution for,
when 20 cm^3 of 7.439 mg/dm^3 is diluted to 100 cm^3
concentration of standard in diluted solution = 7.439 x 20/100
= 1.4878 mg/dm^3
when 15 cm^3 of 7.439 mg/dm^3 is diluted to 100 cm^3
concentration of standard in diluted solution = 7.439 x 15/100
= 1.11585 mg/dm^3
when 10 cm^3 of 7.439 mg/dm^3 is diluted to 100 cm^3
concentration of standard in diluted solution = 7.439 x 10/100
= 0.7439 mg/dm^3
when 5 cm^3 of 7.439 mg/dm^3 is diluted to 100 cm^3
concentration of standard in diluted solution = 7.439 x 5/100
= 0.37195 mg/dm^3
Plot, concentration on x-axis and absorbance on y-axis
concentration of SRP in diluted sample = 1.11585 x 0.246/0.117
= 1.16808 mg/dm^3
concentration of SRP in original soft drink = 1.16808 mg/dm^3 x 2 x 100cm^3/25cm^3 = 9.34464 mg/dm^3
= 9.34464/31 = 0.30144 mmol/dm^3
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