ANALYTICAL EDITION
SEPTElTBER 15, 1938
it should be pseudounimolecular, with the rate of reaction proportional to the first power of the ethylene dibromide concentration. Thus, as the amount of ethylene bromide present is reduced to a few milligrams or less, it is to be expected that the reaction will be slow. Experimentally it was found that it was not practical to extend the reflux period beyond 180 minutes and that a variation of 10 minutes in this period introduced no significant error.
329
Inspection of the data shows that the average deviation from the mean over the whole range of ethylene bromide concentration is less than 1 per cent of the determined value. However, it is apparent that this method, particularly for amounts of ethylene bromide below 25 mg., may not be used without error unless a correction is applied. It is believed that the precision and reproducibility are of sufficiently high order to justify the use of the method when a calibration or correction curve is plotted.
Procedure
Summary
The preliminary work led t o the adoption of the following standard technic: Ten milliliters of 20 to 30 per cent potassium iodide solution and 50 ml. of alcohol are placed in a 250-ml. flask and the sample to be analyzed is added. The flask is then fitted to a watercooled condenser by a ground joint and the liquid is heated sufficiently to maintain a gentle reflux for 180 minutes. A t the end of this period the source of heat is removed, the flask and contents are allowed to cool to room temperature, and the condenser tube is rinsed with a few 10-ml. portions of water. The liberated iodine is then titrated with 0.01 or 0.1 N sodium thiosulfate. Sufficient water is added to the flask to bring the total volume t o approximately 200 ml., t o minimize tJhe effect of t,he alcohol upon t,he starch-iodine end point.
Results and Discussion The results are given in Table I anrl are illustrated graphically in Figure 1.
Simple Electrode Support for Electrometric Titrations
b simple method proposed for the quantitative determination of ethylene dibromide involves reaction of the sample with potassium iodide and titration of the liberated iodine. The method does not give complete recovery. and many substances interfere. However, it is capable of giving reproducible results, and in the absence of interference thew determined values may be corrected by use of a correction curve. Literature Cited 11) Dillon, R T., J . Am. Chem. Soc., 54, 952-60 (1932)
(2) Dillon, R. T., Young. W. G.. and Lucas, H J., I h d , 52, 19.53 64 (1930). (3) Duin, C. F., van, Rec. trav. chim., 45, 345-62 (1926). (4) Patterson. T. S.,anrl Robertson, ,J., J. Chem Sor., 125, 1526 (1924). R~PCEIYED April 30, 1938.
MAURICE E. STANSBY Technological Laboratory, U. S. Bureau of Fisheries, Seattle, Wash.
T
HE simple device shown in the sketch has proved very useful for electrometric titrations. Glass tube B has
AGAR I R I D C E
I
A
’OPPER
TO
REFEREPICE
HALF
CELL
several small holes in i t to prevent unstirred solution from collecting within. This tube is held to tube A by rubber bands. The electrode wire, C, is a thin wire, made of the same metal as the electrode. This system has the following advantages : The electrode can be easily removed and cleaned by ignition in a flame. This cannot be readily accomplished using the ordinary glass seal-mercury connection, since heating may produce small cracks in the glass which, as shown by Morgan, Lammert, and Campbell ( I ) , can materially alter the potential obtained. The electrode and agar bridge are held close to the edge of the beaker, preventing contact with the stirrer. The electrode and agar bridge are held close together, diminishing the resistance in the circuit. This is of especial importance when working with solutions of low conductivity. The device can be easily prepared in a few minutes’ time from simple material, available in any laboratory. It can be very easily attached to or removed from the beaker, without the use of clamps or other supports.
Literature Cited (1) Morgan, Lammert, and (1931).
C BEAUER
ACAR
BRIDGE
Campbell, J . A m . Chem.
Soc., 53, 454
RECEIVEDMay 4, 1938. Published with the permission of the Commissioner, E. S. Bureau of Fisheries.
