Discrepancies in phosphate vs phosphate analysis using a Hach kit on Alcojet

Problem:
When using a Hach water testing method for phosphate analysis, values of 26-28% phosphate were detected when a value of 9% or 8.7% was expected.

Solution:
I can explain the observations about detecting 26%, 27% and 28% phosphate in Alcojet using a Hach analytical method when you are expecting 9%. The kit you are using is detecting ortho-phosphate (PO4 -3) which has a molecular weight of 94.97. The 9% you refer to is elemental P which has a molecular weight of 30.97. Actually if you look on the Alcojet box it contains 8.7% elemental P. This elemental P is present as polyphosphates which needs to be converted to ortho phosphate to be detected by the Hach method. If all the P in Alcojet has been converted to ortho-phosphate, then it will be 26.7% ortho-phosphate as calculated by 94.97/30.97*8.7. Assuming you have acidified your solution according to the Hach sample preparation method which involved boiling in excess acid for 0.5 hrs and the neutralizing with a slight deficiency of base, then you are getting the correct results expressed as ortho-phosphate. I would comment that the variation in your results might be attributed to a couple of causes that you might want to be aware. of. One, you need to be careful to boil in acid with the full 5.25 N H2SO4 for a full 0.5 hour. If you do not do this, then some of the polyphosphates may not be converted to ortho-phosphate and you will get a false low reading. Another possible source of variation is failure to take a representative large enough sample of Alcojet. You should use at least 10 g of Alcojet to make your stock solutions from in order to avoid sampling error from this non-homogeneous dry blended powder. If you take too small a sample of Alcojet, you run the risk of getting a few extra particles or few particles short of polyphosphates which will cause variation in your results. By taking a 10 g sample of Alcojet to start with, a few particles here and there is not enough to cause variation in your results at the level of precision of this method. By converting the reported results from % ortho-phosphate to % phosphorous, perfectly usable data results as follows: 28% ortho-phosphate = 9.1 % phosphorous; 27% ortho-phosphate = 8.8 % phosphorous; 26% ortho-phosphate = 8.5% phosphorous. The average value is 8.8% phosphorous which is quite in agreement with the 8.7% expected value. The variation may be acceptable, or by improving the boiling in acid step or the size of the sample from the Alcojet box you may be able to reduce the variation in your results. It may simply be that the precision of this analytical method is such that you will get this level of variation. From having worked with similar kits in the past, I suspect you can get less variation in your results if you want to work on it.

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