When an ion chromatograph is not available, chloride may be
determined spectrophotometric as described below.
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.
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+
- Spectrophotometer,
equipped with 50 mm optical cells
- Pipettes
- Volumetric
flasks
- Erlenmeyer
flasks
During the analysis, use only chemicals of recognized
analytical grade and only double-distilled or deionized and distilled water.
- Perchloric
acid (HClO4)
72%
- Mercury
(II) ( thiocyanate (Hg(SCN)2)
- Iron
(III) nitrate nonahydrate (Fe(NO3)3 · 9H2O)
- Sodium
chloride (NaCl)
- Ethanol
(C2H5OH)
- Perchloric acid, 1:1
Mix 1 volume 72% perchloric acid with
1 volume of water.
- 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.
- 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.
- 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.
- Standard chloride solution II, 10 mg/l:
Dilute 10.0 ml standard chloride
solution I to 100 ml with water.
Preparation of the calibration curve:
- 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.
- 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.
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.
Bromide and iodide will give the same absorbance as the
equivalent amount of chloride.
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