Phosphates Colorimetric Determination

In summary, the method used to determine the phosphate concentration in phosphate mixtures in powder form is not applicable when the main component is polyphosphate. This is because the method is specific for orthophosphate and the sample treatment preparation process may not break down the more complex phosphates into orthophosphate. It is important to measure the pH of the hydrolysis solution and use a slightly basic buffer solution to ensure the conversion of polyphosphate to orthophosphate.
  • #1
Snitzel
2
0
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
 
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  • #2
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.
 
  • #3
Would the sample treatment preparation process break the more complex phosphates into orthophosphate?
 
  • #4
symbolipoint said:
Would the sample treatment preparation process break the more complex phosphates into orthophosphate?

thats how I understand it (I am not a chemist by the way..)
 
  • #5
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.

edit:

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:

Related to Phosphates Colorimetric Determination

1. What is the purpose of phosphates colorimetric determination?

The purpose of phosphates colorimetric determination is to measure the concentration of phosphates in a sample using a colorimetric method. This is important in various fields of science, such as environmental monitoring, water quality testing, and agricultural research.

2. How does colorimetric determination of phosphates work?

In colorimetric determination, a reagent is added to the sample which reacts with phosphates to produce a colored compound. The intensity of the color is directly proportional to the concentration of phosphates in the sample. This color is then measured using a spectrophotometer, which gives a quantitative measurement of the phosphates concentration.

3. What are the advantages of using colorimetric determination for phosphates?

Colorimetric determination is a simple, quick, and cost-effective method for measuring phosphates. It does not require specialized equipment and can be performed in the field. It also has a wide detection range and is highly accurate, making it a reliable method for phosphates analysis.

4. Are there any limitations to using colorimetric determination for phosphates?

One limitation of colorimetric determination is that it can be affected by the presence of other substances in the sample that may interfere with the color reaction. Also, the color intensity may vary depending on the pH of the sample, so proper calibration and standardization are important.

5. How is the data from phosphates colorimetric determination analyzed?

The data obtained from colorimetric determination is usually compared to a standard curve, which is created by measuring known concentrations of phosphates and their corresponding color intensities. This allows for the determination of the exact concentration of phosphates in the sample. Statistical analysis can also be used to validate the results and ensure accuracy.

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