2. A SPECTROMETRIC DETERMINATION OF
HYDROGEN PHOSPHATE IN A WATER SAMPLE

2.1 Objective

Use a one-point calibration and determine the concentration of hydrogen phosphate in a water sample.

2.2 Background

 
cationic dye
malachite green
HPO42– + 12 MoO42– + 23 H+  
 PMo12O403– + 12 H2O  
green coloration


2.3 Equipment and reagents*

  • A SpektraTM spectrometer.
  • A colour-forming reagent for determining the concentration of hydrogen phosphate.
  • A standard solution of Na2HPO4.

*For reagent preparation please contact natasa.gros@fkkt.uni-lj.si. 

Hazards

The colour-forming reagent contains sulphuric acid, which is corrosive and causes skin burns. Continued contact can cause tissue necrosis.
R: 35, S: 26-30-45

 
2.4 Procedure

For the blank, the standard and the sample, follow the procedure as given below.  

 

Blank

Standard

Sample

Deionised water

8

/

/

Standard solution of disodium hydrogen phosphate

/

8

/

Sample

/

/

8

Colour- forming reagent

1

1

1

Mix thoroughly, measure after twenty minutes using the red LED.

Write the measurements in the table below and calculate the absorbance.

 

T [%]

A

γHPO42- [mg/L]

Standard

 

 

0.5

Sample

 

 

 

Calculation of the result:

Calculate the concentration of hydrogen phosphate ions in the sample and write the result in the table “Results – Contaminants”, under the serial number of the sample.

 γsample =  

 Asample . γst.

 Ast.

The guide value for phosphate in drinking water which is set by the EU Drinking Water Directive (Council Directive 80/778/EEC) is 0.4 mg/L - expressed as P2O5.