INSTRUMENTATION

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INSTRUMENTATION The advantages of the manual polarograph include low cost, high precision educational value, and utility in the performance of amperometric titrations by Ralph ^T^HE instrumental needs of modern polarography are well met by a variety of polarographs. These range from elaborate photographic or pen-and-ink recording instruments to simple homemade systems. As the experts in this field have pointed out repeatedly, fundamental and precise information can be obtained with relatively simple electrical components and when these are carefully calibrated, the results are in no wise inferior to those obtained by the more elaborate de­ vices. It is also impressive to note the extent to which many routine industrial analyses are being conducted polarographically by short cuts which represent abridg­ ments or compromises between rigorous theory or in­ volved instrumentation. In every case, such applica­ tions are successful only if the analytical problem is relatively simple and if it has been studied carefully by more exhaustive methods. Manual

H.

Müller

pounds and the a.c. power consumption is less than 5 watts. The measuring circuits are powered from three 1.5-volt dry cells. The Model III polarograph provides means for the manual application of voltage across the dropping mer­ cury electrode and for indicating the resultant current.

Polarograph

A manual polarograph possesses several advantages, among which are low cost, high precision, instructional and educational value, and utility in the performance of amperometric titrations. Even in large industrial laboratories, several manual instruments may be rele­ gated to the performance of repetitive analyses for which the appropriate conditions have been developed by the time-saving automatic instruments. All polarographers will welcome the appearance of the manual polarograph Model I I I developed by Ε. Η. Sargent and Co., 4647 West Foster Ave., Chicago 30, 111. As shown in Figure 1, this is a compact welldesigned instrument with but seven simple controls. I t is mounted on a cast aluminum base and is enclosed in a sheet metal housing provided with a hinged top and finished in black enamel. All controls are mounted on a polished aluminum panel set in the front casting be­ low the galvanometer scale. It is 10 inches high, 16.75 inches wide, and 17 inches deep. The weight is 32

F i g u r e 1.

Sargent Manual Polarograph

A ten-turn helical potentiometer accurate to 0 . 1 % is used as a voltage divider and it is provided with an integrating dial graduated to read thousandths of the total span voltage. A single-turn potentiometer, coupled with an on-off switch, serves to select the volt­ age across the divider and its value is indicated on a panel-mounted voltmeter, the accuracy of which is 1% of full scale. The diffusion current in the polarographic cell is registered by a sensitive galvanometer. A line light 21 A

22 A

ANALYTICAL CHEMISTRY

INSTRUMENTATION

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image is reflected from a mirror to a translucent scale located above the instrument panel. The scale is graduated in millimeters from —15 to 0 to +300 and is curved in the arc of a circle whose radius is equal to the length of the light path from the galvanometer. This construction eliminates the tangent error and other drawbacks of a flat scale. Current range is adjustable by means of a ten-position Ayrton shunt between 1 to 1 and 1000 to 1. A ten-turn helical potentiometer is also provided for up-scale and down-scale compensation and for the adjustment of galvanometer zero position. This control can be inactivated by a toggle switch. As in most polarographs, the compensation feature has been included, but as polarographers well know, there are instances in which it is useful in suppressing a previous wave of large magnitude in favor of a smaller following wave, and there are other cases in which this scheme is quite useless. After considerable deliberation, the manufacturers of this instrument decided to eliminate facilities for referring the span potential to a standard cell. This was done in the interest of reduced cost and on the sensible assumption that the 1% accuracy afforded by the voltmeter is sufficient for most uses. For this and other reasons, it is recommended that for the occasional measurements where accurate thermodynamic data are required, the cell potential should be measured directly with a potentiometer. A plug-in resistor of 250,000 ohms is furnished for insertion in place of the cell leads whereby an Ohm's law check can be made for linearity of the system. The general specifications for the Model III polarograph can be summarized as follows: galvanometer sensitivity : approximately 0.006 microampere per mm. ; galvanometer shunt ratios: 1, 2, 5, 10, 20, 50, 100, 200, 500, and 1000 to 1, respectively; voltmeter accuracy: 1% of full scale; compensator ranges: upscale of 1 times full scale, downscale 9 times full scale; bridge voltage span selection continuously variable from 0 to 3 volts. This instrument should find extensive use in educational, institutional, and industrial laboratories. Its simplicity and low cost should discourage all but the most incurable shop-minded chemists. Because the cost is approximately one fifth that of a recording polarograph such as the Sargent Model X X I or the new Leeds & Northrup Electrochemograph, the use of several Model I l l ' s will be attractive to larger laboratories in which repetitive as well as original polarographic determinations are encountered. We draw personal pleasure from seeing another well designed instrument, not the least of which arises from seeing precision components instead of radio-store parts in critical controls.