AIDS FOR ANALYTICAL CHEMISTS
A comparison of several available commercial instruments for the high-temperature determination of sulfur in virtually any material ( I ) has revealed that the latest models particularly rely on an exact timing of the measurement operations. This holds true not only for the complex, fast, and expensive instruments but also for the widely used coulometric titrators. In certain analytical situations, however, their principle of measurement, based on the electrochemical generation of OH- ions for the neutralization of the sulfuric acid formed, may not suffice, so an iodometric titration alternative is desirable. In the North American steel industry particularly, the Leco automatic sulfur determinator ( 2 ) for the spectrophotometric iodometric titration of sulfur has become widely used. It was considered that an exact timing of its operation would prove beneficial in several ways. The present contribution describes the additions to this instrument necessary to achieve this goal.
CIRCUIT DESIGN Controls. The instruction manual ( 2 ) describes the controls on the instrument. They are: an on-off switch, a toggle switch with three positions-“neutral,” “titrate,” and “end point”-a potentiometer for adjusting the end point, a photo-cell for the spectrophotometric measurement of the density of the color of the solution which, in turn, controls the addition of the titrant, a push-button switch for the manual addition of the titrant, and a neon signal lamp for indicating the titrant flow. Operation. The operation in a titration is as follows. The absorption solution is colored by adding a small, predetermined quantity of the titrant. This is achieved by setting the toggle switch in the “end point” position and regulating the potentiometer until the desired density of color is reached. For the titration proper, the toggle switch is set to the “titrate” position. In both positions (except in “neutral”), the signal lamp is on, indicating that the titrant will flow, if the solution is bleached, for any reason such as the incoming sulfur dioxide from the sample. After the titration, the toggle switch has to be manually returned to the “neutral” position. There is no provision for timing. Circuitry. Our modification (dotted lines, Figure 1) incorporates an electric-clock-timed contact in which the main component is a time-delay relay (Model 450, 60-min time range, Cramer Division, Giannini Controls Corporation, Old Saybrook, Conn.). Figure 2 shows the detailed connection of the time-delay relay. The relay contacts are normally open and are connected across the toggle switch which is left in the “neutral” position. After closing the clock switch, the circuit is controlled only by the photo(1) 6.Nebesar, J. Chem. E d u c , 48, A 751 (1971); 49, A 9 (1972); 49, A 63 (1972). (2) Instruction Manual for Operation of Leco Sulfur Determinators, Models 517, 518, and 532, Form No., 133A, Laboratory Equipment Corporation, St. Joseph, Mich.
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ANALYTICAL CHEMISTRY, VOL. 46, NO. 8 , JULY 1974
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