4.5  Determination of chloride in precipitation

When an ion chromatograph is not available, chloride may be determined spectro­photometric as described below.

4.5.1  Spectrophotometric mercury thiocyanate-iron method

4.5.1.1  Field of application

The method can be used for direct determination of the chloride ion content in precipitation samples within the range 0.05 to 5 mg/l.

4.5.1.2  Principle

Chloride ions will substitute the thiocyanate ions in undissociated mercury thiocyanate. The released thiocyanate ions react with ferric ions forming a dark red iron-thiocyanate complex.

The absorbance is measured at 460 nm.

          2 Cl- + Hg(SCN)2® HgCl2 + 2SCN-
          SCN- + Fe3+® Fe(SCN)2+

4.5.1.3  Instrumentation

4.5.1.4  Chemicals

During the analysis, use only chemicals of recognized analytical grade and only double-distilled or deionized and distilled water.

4.5.1.5  Reagents

  1. Perchloric acid, 1:1
    Mix 1 volume 72% perchloric acid with 1 volume of water.
  2. Mercury (II) thioccyanate solution, saturated:
    Shake 1 g Hg(SCN)2 with 1000 ml ethanol. Filter the solution after 24 hours. The solution may be stored in a glass bottle at room temperature.
  3. Iron (III) nitrate solution, 6%:
    Dissolve 6 g Fe(NO3)3 · 9H2O in 100 ml 1:1 perchloric acid. Filter the solution after 24 hours.
  4. Standard chloride solution I, 1000 mg/l:
    Dissolve 412.5 mg NaCl dried at 140-200 °C, in water and fill it up to 250 ml with water.
  5. Standard chloride solution II, 10 mg/l:
    Dilute 10.0 ml standard chloride solution I to 100 ml with water.

4.5.1.6  Calibration

Preparation of the calibration curve:

  1. Transfer 2.5, 5.0, 7.5, 10, 15, 20 and 25 ml of standard chloride solution II to 50 ml volumetric flasks, and fill up to the mark with water. These solutions contain 0.5, 1.0, 1.5, 2.0, 3.0, 4.0 and 5.0 mg Cl/l.
  2. Transfer 25 ml of the calibration solutions to 100 ml Erlenmeyer flasks. to each flask add with pipettes 5 ml mercury (II) thiocyanate solution and 2 ml iron (III) nitrate solution. Mix well between and after the additions. After 20 minutes, measure the absorbance in 50 mm cells at 460 nm.
    As reference, use 25 ml water mixed well with 5 ml of reagent (2) and 2 ml of reagent (3).
    Plot the readings against the concentrations and draw the calibration curve.

4.5.1.7  Analytical procedure

Transfer 25 ml of the precipitation sample to a 100 ml Erlenmeyer flask. Proceed according to 4.5.1.6 (2). Read the chloride content of the sample from the calibration curve.

4.5.1.8  Interferences

Bromide and iodide will give the same absorbance as the equivalent amount of chloride.

4.5.1.9  References

Iwasaki, I., Utsumi, S., and Ozawa, T. (1952) New colorimetric determination of chloride using mercuric thiocyanate and ferric ion. Bull. Chem. Soc. Japan, 25, 226.

Zall, M., Fisher, D., and Gamer, Q. (1956) Photometric determination of chlorides in water. Anal. Chem., 28, 1665-1668.


Last revision: November 2001