2 A Colorimetric Method for the Determination of Chloroform BY W. G. MOFFITT PH.D. A.I.C. (Read at the Meeting October 5 1932) THE colour reaction described by Lustgarten (Monatsh. Chem. 1882 3 715) between chloroform and a- or /?-naphthol in strong potassium hydroxide solution lends itself to the rapid colorimetric determination of chloroform. The method whilst of limited accuracy can be used with advantage where methods based upon the action of alcoholic potash on chloroform break down owing to the presence of other readily decomposed chloro-compounds. When this reaction is used for the detection of chloroform Lustgarten recom-mended that it should be carried out at a temperature of 50" C. when it gives rise to an intense Prussian blue colour. For comparative work it is better to allow the reaction to proceed at room temperature when the colours are found to be of satisfactory depth and with 0.001 to 0.003 ml.of pure chloroform to form a well-graded series increasing in intensity with the concentration of the chloroform. The utility of the method is determined chiefly by the strength of the potassium hydroxide solution. This controls the sensitivity of the reaction the rate at which the colours fade and in part the value of the reaction as a specific test for chloroform. The reaction has been studied with both a- and /?-naphthol; although the former appears to be the more sensitive reagent /?-naphthol gives better colours and is less liable to interference by other compounds. SOLUTIONS REQUIRED-1. 2.0 grms. of /3-naphthol dissolved in 100 ml.of 40 per cent. cold potassium hydroxide solution. 2. A standard solution (0.5 per cent. by volume) of chloroform dissolved in industrial methylated spirit (95 per cent. of ethyl alcohol). PROCEDURE-Ten ml. of the /?-naphthol solution are measured into each of several Nessler glasses ; measured volumes of the standard chloroform solution are then added to each tube and sufficient industrial methylated spirit to make the total volume 11.0 ml. (In practice the methylated spirit is added before the chloroform and both are delivered with the tip of the pipette dipping slightl NOFFITT A COLORIMETRIC METHOD FOR THE DETERMINATION OF CHLOROFORM 3 below the surface of the liquid.) for 5 to 10 minutes. The tubes are then shaken and allowed to stand Example.Number of tube 1. 2. 3. 4. 5. Naphthol solution ml. . . . . 10.0 10.0 10.0 10.0 10.0 Industrial methylated spirit ml. . . . . 0.8 0.7 0.6 0.5 0.4 Chloroform (0.5 per cent. solution) ml. . . 0.2 0.3 0.4 0-5 0.6 Colour intensities as ratios . . 1.8 3.0 4.0 5.0 6.2 The ratios of the colour intensities were measured by means of a Duboscq colorimet er. Experiments were carried out to determine how far other chloro-compounds interfered with the method. The following compounds were studied :-Methylene chloride (b.pt. 41.6" C.) ; acetylene dichloride (b.pt. 55" C.) ; ethylidene chloride (b.pt. 59.9" C.) ; carbon tetrachloride (b.pt. 76.7" C.) ; ethylene chloride (b.pt., 83.7" C.) ; trichloroethylene (b.pt. 87.1" C.) ; dichloroethyl ether (sym.) (b.pt., 178" C.).With carbon tetrachloride a blue colour was obtained with a-naphthol; this colour was intensified by the presence of acetone. No colour was obtained with 13-naphthol and any of these compounds. Acetone was found to have no interfering effect when 13-naphthol was used. Slight variations were observed in the chloroform tubes when some of the above compounds were present in large excess but these variations were observed with 13-naphthol only after the solutions had been standing for more than ten minutes. ANALYSES OF FRESHLY-PREPARED MIXTURES No. Composition (by vol.) 1. Chloroform Industrial methylated spirit 2. Chloroform Industrial methylated spirit 3. Chloroform Methylene chloride Acetylene dichloride Industrial methylated spirit 4.Chloroform Met hylene chloride Carbon tetrachloride Industrial methylated spirit 5. Chloroform Lin. belladonnae 6. Chloroform Lin. belladonnae Lin. aconiti Per Cent. 8.0 92.0 5.4 94.6 3.3 6.6 3.3 86.8 8.3 20.0 20.0 51-7 12-5 87.5 20.0 40.0 40.0 Chloroform (by vol.) found Per Cent. 8.0 5.4 (i) 3.4 (ii) 3-5 8.3 (i) 12-2 (ii) 12.8 19. 4 NICHOLLS THE DETERMINATION OF BENZOYL PEROXIDE IN FLOUR A small volume (usually 3 to 5 ml.) of the sample under examination should first be distilled with 75 ml. of methylated spirit. At least 50 ml. of distillate should be collected to ensure the distillation of all the chloroform. The distillate should then be treated in the manner described above and the colours compared with those obtained with the standard solutions of chloroform treated at the same time. (It is essential that comparison be made within a few minutes.) SUMMARY-Conditions are given for the colorimetric determination of chloroform by means of 13-naphthol in strong potassium hydroxide solution. None of seven chloro-compounds was found to have an appreciable influence on the reaction. I am indebted to the Government Chemist for permission to publish this note and to Mr. G. F. Sheppard for advice. THE GOVERNMENT LABORATORY, LONDON W.C.