A-5. DETERMINATION OF CHLORIDES (LOW RANGE)—COLORIMETRIC METHOD
A-5.1
Outline of the Method—Solutions of ferric ammonium sulphate and mercuric thiocyanate are added to the sample. The chloride ion reacts with the mercuric thiocyanate to produce thiocyanate ion which in turn combines with ferric to form red ferric thiocyanate. The intensity of the colour which is proportional to the concentration of the chloride ions, is measured photometrically at a wavelength of 463 nm, or by visual comparison with standard solutions.A-5.2
Interferences—Bromides, iodides, cyanides, thiosulphates, hydrazine, and nitrites interfere in this method. Colour, if present in, the sample, will interfere with visual comparison and depending upon its spectral absorbance may interfere with the photometric measurement.*Methods of sampling and test (physical and chemical) for water used in industry.
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A-5.3 Apparatus
A-5.3.1
Nessler Tubes or Photometer—A set of 50-ml matched Nessler tubes or a photometer suitable for measurements at a wavelength of 463 nm, may be used for evaluating the intensity of the colour produced.A-5.4 Reagents
A-5.4.1
Ferrous Ammonium Sulphate—Dissolve 5.0 g of ferrous ammonium sulphate [Fe (NH4)2 (S04)2.6H2O] in 20 ml of water. Add 38 ml of concentrated nitric acid (HNO3 sp gr 1.42) and boil to oxidize the iron and remove the oxides of nitrogen. Dilute to 100 ml with halide-free water.A-5.4.2
Mercuric Thiocyanate Methanol Solution (3 g/l)—Dissolve 0.30 g of mercuric thiocyanate [ Hg (CNS)2 ] in 100 ml of methanol. Store in amber bottles. Allow to stand for at least 24 h before using. Do not use if more than 4 weeks old.Note 1—Caution—Mercuric salts are very poisonous. Due precautions should be observed when using this material.
Note 2—A slight precipitate may form and settle out after 24 h. Care should be taken so that this precipitate is not resuspended when using the reagent. Only the clear, supernatant liquid should be used.
A-5.4.3
Sodium Chloride Solution Standard (10 mg Cl/l)—Dry several grams of sodium chloride (NaC1) for 1 h at 600°C. Prepare a stock solution by dissolving exactly 1.649 g of the dry salt in water and dilute to 1 litre. Prepare the standard solution as needed by diluting 10 ml of the stock solution to 1 litre with halide-free water. The resulting standard contains 10 mg of chloride ion per litre.A-5.5 Calibration
A-5.5.1
Prepare a series of reference standards by diluting suitable volumes of the standard chloride solution with halide-free water. The temperature of the solutions used for calibration should be the same as that of the sample to be tested.A-5.5.2
Treat each reference standard as described in A-5.6.A-5.5.3
Prepare a calibration curve by plotting the readings on the photometer versus the concentration of chlorides. When the scale of the photometer reads directly in absorbance, plot the curve on rectilinear paper. When the scale reads in transmittance, it is convenient to plot the results on semilog paper, using the single cycle log axis to plot transmittance and the linear axis to plot the concentrations.21
A-5.6 Procedure
A-5.6.1
Transfer 25 ml of sample to a glass stoppered cylinder and add successively 5 ml of ferric alum solution and 2.5 ml of mercuric thiocyanate solution. Mix thoroughly and allow to stand for 10 minutes.A-5.6.2
Measure the intensity of the colour formed either by comparison with suitable reference standards in Nessler tubes or by a photometer. Adjust the zero setting of the photometer by using 25 ml of halide-free water tested in accordance with A-5.6.1.Note—Since chloride ion is a very common contaminant, exercise extreme care in the collection and processing of the sample.
Soak all new glassware in hot nitric acid (1 + 19) for several hours. To be certain that new glassware is conditioned for the test run a chloride determination on halide-free water. After the run rinse the glassware thoroughly. Soak the glassware in halide-free water between tests. Discard all glassware that appears etched or scratched.
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