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    I'm a bit confused about my procedure for the Determination of phosphate in coke using visible spectrophotometry. I think its the wording of the procedure that I don't understand!

    Basically it says:

    Calibration Curve

    Prepare a standard solution using potassium dihydrogenphosphate (KH2PO4) p.a. The calibration curve solutions contain 0 - 10mg/L phosphate (PO4^3-). Use 50 mL volumetric flasks. Don't make up to the mark yet.

    Sample Preparation

    The Coca Cola sample contains approximately 500mg/L phosphorous (PO4^3-). Transfer the correct amount of the degassed Coca Cola sample in a 50 mL volumetric flask. The absorption of the sample should be the midpoint of the calibration curve. Do not make up to the mark yet.

    I'm just unsure of how to make up the standard solutions and the coke samples (how do I work out the correct amount that has to be transferred)??

    I probably sound really stupid but I'd be grateful if someone could explain it better


    This is one of those things that gets quite fiddly and easy to slip up, but be persistent and you'll get there.

    For the calibration curve, you need to work backwards. At the end of the day, you want ten flasks, each of volume 50ml, containing between 0 and 10 mg/l phosphate. So work out using c=n/v what the number of moles of phosphate is in each, and how much potassium dihydrogen phosphate you need to add.

    *question: have you been given the potassium dihydrogen phosphate as a solid or solution? If solid then you'll need to convert moles to mass and work out how much to weigh in, if solution you'll need to use the concentration given to work out a volume to add to each flask*

    Once you've achieved this, you then take the absorbance readings for each solution, and plot a graph.

    Now, your coke sample measurement will be accurate if and only if the concentration of phosphate lies somewhere on that graph ie between 0 and 10mg/l. You've been told it contains roughly 500 mg/l, so you need to dilute it by an appropriate ammount. In this case they specify you want to get it to exactly 5mg/l (the midpoint of the graph) so you need to dilute it 100fold. So you should be able to work out what volume to add to a 50ml flask and what volume of water to add to that.

    Easy enough eh?
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