- #1
Snitzel
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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.
• Equipment
- 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 until 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
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.
• Equipment
- 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 until 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