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Homework Help: Phosphates Colorimetric Determination

  1. Feb 9, 2009 #1
    Hello all,
    I am using the method below to determine the phosphate concentration of phosphate mixtures in powder form(sodium triphosphate, potassium triphosphate, sodium di and potassium diphosphate). The mixture also contains Na2O. The concentration of the mixture in P2O5 should be between 55 and 57%. Since I started analysing the samples I was always getting results that were around 45-50%.
    the method I use is

    Reagents- KH2PO4
    - NaOH 10N
    - concentrated HCl
    - Solution of acids : 30ml of concentrated H2SO4 and 0,4ml of concentrated HNO3 in 100ml water
    - Phenolphthalein indicator
    - Solution of vanadomolybdate phosphoric acid :
    20gr (NH4)6Mo7O¬24.4H¬2O in 100ml H2O (SA)
    0,47gr NH4VO3 in 50ml H2O (SB)
    Mix SA and SB, add 140ml of concentrated HNO3 in a 1L volumetric flask and fill with H2O.
    (store in dark bottle)
    All the solutions are prepared by using distilled water.

    - Common laboratory equipment
    - Photometer

    Calibration curve
    Prepare the reference solution by dissolving 2,195gr of KH2PO4 in 1000ml water. Add 10ml of this solution in a 100ml volumetric flask and fill with water (S1). Prepare the following solutions according to the table below.

    Solution ml of S1 ml of H2O μg of total phosphorous (P)
    1 0 10 0
    2 1 9 50
    3 2 8 100
    4 4 6 200
    5 6 4 300

    Add 10ml of vanadomolybdate phosphoric acid in each solution and wait for 10min. Measure the absorption at 430nm.

    Sample preparation
    Weight 0.5-3.0 gr of the sample (the weighted amount of the sample depends on the expected phosphate concentration) and add it in about 100ml of distilled water. Add drops of phenolphthalein indicator and then the sufficient volume of solution of acids untill decolorization. Then add 2 ml of concentrated hydrochloric acid and boil the solution for at least 90 min under stirring (add water if necessary) for hydrolysis. After heating the desired volume of the solution should be about 25-50ml. Neutralize the solution by adding 10N NaOH and then add water to a final volume of 1L. Add 10ml of the prepared solution in a 100ml volumetric flask and fill with water.

    Experimental procedure
    Mix 10ml of the prepared sample solution and 10ml of vanadomolybdate phosphoric acid solution. Wait for 10min and measure the absorption at 430nm.

    Calculation of P2O5
    Use the calibration curve to convert the absorption at 430nm to μg of P. Then calculate the %P2O5 using the formula:
    %P2O5 = x*142 / 620*W
    where x= μg of P calculated from the calibration curve
    W = gr of sample weight

    after discussing the results with my supplier he told me that the method is not valid because the dilution factor is too high, and that the method is valid for impurities only.
    Could someone explain to me why this method is not applicable when phosphates are the main component of the mixture

    Thanks in advance
  2. jcsd
  3. Feb 9, 2009 #2


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    When you write '...sodium triphosphate, potassium triphosphate, sodium di and potassium diphosphate...' you are referring to the polyphosphates, yes?

    Your test method is specific for orthophosphate.
  4. Feb 9, 2009 #3


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    Would the sample treatment preparation process break the more complex phosphates into orthophosphate?
  5. Feb 9, 2009 #4
    thats how I understand it (I am not a chemist by the way..)
  6. Feb 10, 2009 #5


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    That is a good question. Have you ever measured the pH of the hydrolysis solution before you heat it? If the pH is low (<2) the method should convert all polyphosphate to orthophosphate. Phenolpthalein will decolorize at about pH 8-10 so you start with a slightly basic buffer solution. The buffer solution is the partly protonated polyphosphates. I'm not sure if 2 mL of HCl (37%) is enough to lower the pH of your hydrolysis solution but it sounds like it should. Still, it is a good idea to check the pH of the hydrolysis solution before you start heating it (after you add the 2 mL of HCl) just to be sure. I would use methyl orange (pH of change is 3-4) in place of phenolphthalein just to be sure I've overcome any buffering capacity of the polyphosphates.


    pKa's for phosphoric acid (H3PO4):
    pKa1 = 2.16
    pKa2 = 7.21
    pKa3 = 12.32

    pKa's for pyrophosphoric acid (H4P2O7):
    pKa1 = 0.91
    pKa2 = 2.10
    pKa3 = 6.70
    pKa4 = 9.32
    Last edited: Feb 10, 2009
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