phoric acid added, and the solution titrated with ferrous solution, using the automatic titration apparatus. APPARATUS
The electrode system, a lithium glass electrode and a platinum wire spiral, was connected to a millivoltmeter and recorder, while titrant was added by means of a variable-speed titration unit connected to a volume counter and recorder and their associated power supply. The titration unit could be switched off automatically a t the end point by a phototransistor control ( 2 ) on the millivolt recorder. The titration unit comprises a variable-speed motor driving a n Agla micrometer s-yringe (Burroughs, Wellcome 8: Co., Inc., Tuckahoe, N. Y.) through suitable reduction gearing to give final delivery rates of 0.01 to 0.3 ml. per minute, although this could be extended by using larger syringes fitted
to the same micrometer head by a n adapter. The volume of titrant delivered was metered by causing the micrometer head to actuate a n electromagnetic counter with delivery of each 0.01 ml. When the potential of the electrode system is being recorded, an electromagnetic pen is connected in parallel with the volume counter, so that volume of titrant and system potential are simultaneously recorded.
The average deviation from the mean on samples titrated was 1.85% using the ferrous method and 3.94% using the Winkler method. The average value of the end point potential using the glass-platinum electrode system was 524 mv., while the average rate of change of potential at the end point was 4500 mv. per ml. of 0.0125N titrant.
RESULTS
(1) Griffiths,V. S., Stock, D. I., J . Chern. SOC.1956, 1633. (2) Jackman, M. I., Lab. Practice 7, 526 (1958). (3) Lykken, L., Tuemmler, F. D., ANAL. CHEY. 14, 67 (1942). (4) Ohlweiler, 0. A,, Meditsch, J. de O., Eng. e quim. 5 , 10 (1953). (5) d’ombrain, G. L., Griffiths, V. S., Stock, D. I., Proc. Conf. Soc. Inst. Technol., Swansea, England, September 1957. ( 6 ) Rinkler, L. W,, Ber. 21, 2843 (1888).
LITERATURE CITED
Simultaneous determinations of dissolved oxygen were made on water samples by the ferrous ammonium sulfate method and the conventional Winkler (6) method. The range studied was from 0.1 to 8 p.p.m. The results obtained by each method and their spread are shown on the graph, together with the line of theoretical slope.
A Modification of the Fisher-Serfass Electronic Relay A. C. Andrews, E. B. Beetch,’ and C.
H. Whitnah,
Chemistry Department, Kansas State College, Manhattan, Kan.
of the Fisher-Serfass elec0 tronic relay a t current levels near the maximum causes arcing with subsePERATION
quent burning of the contact points, especially where refrigeration compressors are used in conjunction with a mercury contact regulator for temperature control in constant temperature water baths. A description of a modified Fisher-Serfass electronic relay to eliminate the above difficulties is reported here. This device has been used with very satisfactory results. Essential features of the wiring diagram are shown in Figure l. The portion of the circuit involved in the modification is outlined by the dashed lines. The relay, A , was changed by removing the upper contact point. The contact point on arm E of relay A was wired t o one side of a 60-volt direct current source, while the lower contact point F was connected to one terminal of the actuated electromagnet of a single-pole, double-throw rocking mercury switch H. The remaining terminal on the electromagnet was connected to the other side of the 60-volt direct current source, thus completing the circuit as shown in Figure 1. A mercury switch operating on exactly 60-volt direct current is not essential, as switches with voltage ratings of 40- to 60-volt direct current were satisfactory. The 60-volt type was incorporated here, because of 1 Present address, Research Department, Rahr Malting Co., Manitowoc, Wis.
162
ANALYTICAL CHEMISTRY
il00
I
A
7 0.1 UFO.
5.1ME
0.51 MEG.
-_________-__J
L
Figure 1. Schematic wiring liagram showing details of alterations on FisherSerfass electronic relay -
its availability. The function of the pilot light, D, in the circuit iTas‘not changed. Incorporation of the single-pole, double-throw rocking mercury switch made available a normally energized 110-volt outlet or a normally deenergized 110volt outlet depending upon the desired application. Terminals of the mercury switch were connected to 110 volts as shown in Figure 1; two terminals were connected to a common lead and the other two terminals were connected through outlets Cl and CZ. The mercury contact regulator, or other controlling element, is attached to the jacks, 0. I n the arrangement as outlined, relay A merely actuates the
mercury which closes the 110-volt alternating current circuit. Thus, the 110volt circuit is isolated from the contact points of the relay, a n important factor when high operating currents are encountered. Many of the electronic relays designed for automatic control are similar, therefore the suggested modifications presented here might be applied t o other circuits. An immediate practical application of this type of apparatus is in modifying circuits similar in type to those of the Fisher-Serfass electronic relay. TAKENfrom the Ph.D. dissertation of E. B. Beetch, Kansas State College, Manhattan, Kan., 1957.
