4. Spectrometric determination of phosphates
in apple juice

EXERCISE

Determine if the phosphate concentration in apple juice is between the permitted RSK values (100 – 350 mg PO43–/L).
 

PRINCIPLE OF THE METHOD

Phosphates react with molybden ions in acid medium to phosphomolybdate, which is reduced with ascorbic acid to molybdic-blue. The final solution is photometrically measured at 720 nm.
 

MATERIALS AND REAGENTS
 

  • Measuring flasks: 50 in 100 mL
  • Pipettes: 2 mL, 4 mL, 20 mL, 25 mL
  • Pipetor 200 μL
  • Measuring cylinder
  • Ammonium heptamolybdate
    (NH4)6Mo7O24·4 H2O
  • Disodium-hydrogenphosphate
    (Na2HPO4·12 H2O)
  • L-ascorbic acid
  • Hydrochloric acid (2 mol/L)
  • Sulphuric acid (1 mol/L)
  • Phosphate stock solution P
    (γ = 1000 mg/L)


SAMPLE PREPARATION

Transfer 25 mL of juice into an incinerating dish, evaporate on the water bath and ignite the residual at 550°C, until white ash appears. Add 2-3 mL of hydrochloric acid and transfer to the 50 mL measuring flask. Quantitative transfer is achieved by rinsing with distilled water. Fill the measuring flask with distilled water to the marker line and mix well.
 

SAMPLE PREPARATION FOR PHOTOMETRIC DETERMINATION

 

Reagents (Pipeted in  100 mL measuring flask)

Sample

Standard low–
Standard 1

Standard high–
standard 2

Ash solution

5 mL

-

-

Phosphate stock solution

-

100 µL

300 µL

Distilled water (ca.)

50 mL

50 mL

50 mL

Sulphuric acid

20 mL

20 mL

20 mL

Ammonium heptamolybdate

4 mL

4 mL

4 mL

Ascorbic acid

2 mL

2 mL

2 mL

Heat an opened measuring flask on a water bath for 15 minutes, cool it down and fill it with distilled water to the marker line. Pipette 350 µL of well mixed solution into the blister and measure  transmittance photometrically using the red LED.
 

EVALUATION

Measurements (transmittance readings) are signed in the table and absorbances are calculated.

 

T (%)

A

γPO43- (mg/L)

Standard 1

 

 

1.00

Standard 2

 

 

3.00

Sample

 

 

 

 γsample = Asample . γstandard . 122.6 / Astandard 

Result and final conclusion:........................................................................


SOLUTION PREPARATION

Ammonium heptamolybdate  (NH4)6Mo7O24·4 H2O (γ =2 g/100 mL)

Dissolve 2.0 g of ammonium heptamolybdate  in 60 mL of hot (60°C)  distilled water. Fill in the 100 mL measuring flask with distilled water to the marker line.

Disodium-hydrogenphosphate (Na2HPO4·12 H2O) (γ =11.5000 g/L)

Dissolve 11.5000 g of disodium-hydrogenphosphate in distilled water and fill the 1000 mL measuring flask to the marker line.

L-ascorbic acid (γ =0.353 g/100 mL)

Dissolve 0.353 g L-ascorbic acid in distilled water and fill the 100 mL measuring flask to the marker line.

Hydrochloric acid (2 mol/L)

Sulphuric acid (1 mol/L)
 

HAZARDS 
 

Ammonium heptamolybdate (NH4)6Mo7O24·4 H2O
R22, R36-38, R40

Disodium-hydrogenphosphate Na2HPO4·12 H2O
R 36/37/38

Hydrochloric acid
R34, R37

Sulphuric acid
R35


Developed and prepared by:: Alenka HMELAK GORENJAK, Živilska šola Maribor