(b) A soft drink was analysed for soluble reactive phosphoras (SRP) by the molyb
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Question
(b) A soft drink was analysed for soluble reactive phosphoras (SRP) by the molybdenum blue/tartrate/ascorbic acid spectrophotometric technique. The operating range of the method is 0-13 mgP dm. A standard selution ot 7439 mgP drn was prepared and diluted to give working standards as indiced in the table. Twenty-tive em of the sot drink was dsluted to 100 em. The necessary 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 7439 100 cm 100 cm 100 cm 0.478 0.357 15cm 10cm 100 em Diluted sample 0.246 i) What massofro, is required to prepare 250 ern, ofthe 7430 mgP dma standard solution? How would you prepare this standard? ii) Plot the appropriate data and calculate the concentration of SRP in the soft drink, (10) giving your answer in both mgP dm 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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