3. A SPECTROMETRIC DETERMINATION OF
CHLORIDE IN A WATER SAMPLE

3.1 Objective

Use a one-point calibration and determine the concentration of chloride ions in a water sample. Express the results in mmol/L and mg/L.

3.2 Background
 

Hg(SCN)2 + 2 Cl–

HgCl2 + 2 SCN–

 

SCN– + Fe3+

Fe(SCN)2+

 

 

orange-brown coloration


3.3 Equipment and reagents*

  • A SpektraTM spectrometer.
  • A colour-forming reagent for determining the concentration of chloride.
  • A standard solution of NaCl.

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

Hazards

Ethylene glycol monomethyl ether (2-Methoxyethanol) is a flammable liquid.
R: 60-61-10-20/21/22.1, S: 53.1-4

Mercury(II) thiocyanate  is toxic. Repeated exposure can cause damage to the liver, the kidneys and the central nervous system.
R: 26/27/28-32-33, S: 13-28-36-45

Nitric acid is corrosive. Avoid skin contact and inhalation.
R: 8-35, S: 23-26-36-45

3.4 Procedure

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

 

Blank

Standard

Sample

Deionised water

6

/

/

Standard solution of sodium chloride

/

6

/

Sample

/

/

6

Colour- forming reagent

3

3

3

Mix thoroughly, measure after five minutes using the blue LED.

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

 

T [%]

A

cCl- [mmol/L]

γCl- [mg/L]

Standard

    

0.200

  

Sample

        

Calculation of the result:

Calculate the amount and the mass concentration of chloride ions in the water sample. Write the results in the table “Results – Major Components”, under the serial number of the sample.

 csample =

 Asample . cst.

γ= c . M

 Ast.

MCl = 35.45 g/mol

The guide value for chloride in drinking water which is set by the EU Drinking Water Directive (Council Directive 98/83/EC) is 250 mg/L. According to the Directive, chloride is classified as an indicator parameter, but it is also hydro-geologically originated. The concentrations are usually low - few milligrams per litre - and analytical procedure, which is used here, was optimised for determining chloride in this concentration range.