ANALYTICAL CHEMISTRY
1244
I n the use of instruments operating a t different frequencies, care must be used in selecting the proper frequency for a titration, as these regions of linear response and high sensitivity are dependent upon frequency (S). ACKNOWLEDGMENT
This iTork was supported in part by a grant-in-aid from E. I. du Pont de Kemours & Co., Inc., and in part by a grant from the Kisconsin Alumni Research Foundation. LITERATURE CITED
Anderson, W. K., Bettis, E. S., and Revinson, D., ANAL. CHEW,22, 743 (1950). (2) Bever, R. J., Crouthamel, C. E., and Diehl, H., Iowa State C022. (1)
J . Sci., 23, 289 (1949).
(3)
Blaedel, W.J., and Malmstadt, H. V., ANAL. CHEM.,22, 734 (1950).
Falkenhagen, H., “Electrolytes,” London, Oxford University Press, 1934. (5) Forman, J., and Crisp, D., Trans. Faraday SOC.,42A, 186 (4)
(1946).
Hall, J. L., and Gibson, J. A., Jr., AXAL.CHEM.,23, 966 (1951). Hare, G., and Hawes, R. C., private communication from Beckman Instruments Co., Pasadena, Calif., May 26, 1950. (8) Jensen, F. K., and Parrack, A. L., IND.ENG. CHEW,ANAL. (6) (7)
ED.,18, 595 (1946).
Tang, K. T.,“Alternating-Current Circuits,” Scranton, Pa., International Textbook Co., 1940. (10) Weissberger, A , “Technique of Organic Chemistry,” Vol. I, Part 11. Xew York, Interscience Publishers, 1949. (11) JTest, P. W., unpublished work from Coates Chemical Laboratories, Louisiana State University. (12) West, P. W,, Burkhalter, T. S., and Broussard, L., ASAL. CHEM.,22, 469 (1950). (9)
RECEIVED for review November 30, 1951
Accepted April 14, 1952.
Practical High-Frequency Titration Apparatus for General laboratory Use JAMES L. HALL West Virginia University, Morgantown, W. Va. An instrument has been constructed to meet the need for a reliable low-cost high-frequency titration apparatus which will respond to both effective overall capacitance and effective over-all conductance changes of a titration cell. This instrument is based on a simple crystal oscillator circuit and a commercially obtained vacuum-tube voltmeter. The oscillator operates at 2 Mc. It is easily constructed and is simple in operation. The cost is low enough and its performance is reliable enough to permit the introduction of the high-frequency method into student analytical laboratories. The effective capacitance changes of a titration cell are significant only in working with solutions of 0.1 N or less, but effective conductance differences are significant for many reactions at much higher concentrations.
S
0 MAKY circuits have been described for use in high-fre-
quency titration that the presentation of a new one requires justification. The apparatus described here has a number of advantages for routine laboratory use. The cost is low. The instrument is stable in operation, and the tuning adjustments are easily reproduced to a high degree of precision. Its construction requires no great knowledge of electronics and no techniques are required beyond the ability to use a soldering iron. This circuit, for the first time in a simple apparatus, provides direct indication of both effective over-all conductance and effective over-all capacitance changes of a titration cell. These are read from a calibrated capacitor dial or from an auxiliary vacuumtube voltmeter. A coupling arrangement is included in the circuit t o allow its use with a variety of cells. The low cost and simple design of this instrument make it possible t o introduce the high-frequency method into the undergraduate and graduate student analytical laboratories. The instrument described here is not intended to take the place of the highly versatile impedance-measuring circuit described by Hall and Gibson ( 6 ) , but it has been found very satisfactory for a large variety of determinations. The circuit used for this instrument is, with only slight modi-
fication, the crystal oscillator circuit described by Alexander ( 1) and also used by Bender ( 2 ) and by Fischer ( 4 ) for the deterniination of dielectric constant. An auxiliary vacuum-tube voltmeter is required for follon ing effective over-all conductance changes of the cell. The principle of operation is simple. If the tuning capacitor of a crystal oscillator is set a t a value too great for oscillation to take place, and then the capacitance is gradually decreased, a point is reached a t which oscillation abruptly starts. This point may be precisely reproduced. This feature enables one to follow accurately the over-all capacitance changes of a high-frequency titration cell. If the tuning capacitor of a crystal oscillator is adjusted until maximum resonant voltage is attained, the grid bias voltage is a t a maximum. The maximum grid bias voltage at each point of a titration provides an excellent indication of effective conductance changes of the cell. The grid bias voltage may be measured by a vacuum-tube voltmeter of suitable range. Large changes of grid bias voltage are not directly proportional to conductance changes, but this does not interfere Rith the accuracy of end-point determinations. CIRCUIT
The complete circuit diagram is shown in Figure 1. The 250-volt power supply is not shown, as it was exactly as described by Bender ( 2 ) . Any poaer supply delivering 6.3 volts alternating current and 250 volts direct current would be satisfactory. The 20 volts direct current was obtained from a separate power supply or from a 22.5-volt B battery. The vacuumtube voltmeter was a Silver Vomax Model 900 meter or an RCri Model WV-97A Volt Ohmyst. Any good vacuum-tube meter having direct current ranges up to 30 volts could be used. The oscillator was built into a 10 X 7 X 8 inch steel cabinet. 4 photograph of the instrument including cell and voltmeter is shown in Figure 2. The capacitors, C, and CA,were made by removing some of the plates from capacitors obtained from surplus navy GP-7 radio equipment, I n the S a v y equipment, these were fitted with geared drives, and M-ith scales dividing the 180 degrees of rotation into 800 divisions; these were retained in the present apparatus. Equivalent capacitors and suitable vernier scales may be obtained from radio supply houses. A shunt feed for the plate supply voltage to the tube was used, in order that both the rotors and the stators of the tuning ca-
V O L U M E 2 4 , NO. 8, A U G U S T 1 9 5 2 Dacitors could he a t direct current mound mtential.
1245 This sim-
Banana-pjug ja& were provided on one side of the oscillator box for the cell terminal block, and on the other side the valtmeter was connected hy means of phone-tip jacks.
voltmeter jumped from zero. This point was very sharp and easily reproduced t o within 0.04 wf., which corresponds to one scale division on the dial of the 30-ppf. variahle capacitor. The limiting factor here i s the small amount of play in the gear driving the scale. The dial reading was recorded at this paint and was platted t o indicate the changes of the effective over-all ea-
the tuninx cauacitor
WBS
&&ed
until the maximum vac;uml
The-v&,ble resistor,-&, and the additional 20-volt d&ct current source provide a convenient method for obtaining more accurate readings where the hias voltage change is not great. When the vssiable tap is a t the ground end of the resistor, the voltmeter indicates the true grid bias voltage. The t a p may be adjusted to give a voltmeter reading of zero or any other convenient value a t the beginning of any titration, and the voltmeter may he shifted to a lower scale to give more accurate determination of small changes. INSTRUMENT CHARACTERISTICS
Figure 3 shows the response of the instmment as the concentration of sodium chloride solution in the eell is varied from zero, and a oomparison with the absolute values for the same cell as mBasured by the Twin-T circuit a t the same frequency.
Figure 1. Oscillator Cirouit Ci. 100-wf. variable capacitor Ca.
30-ppf. variable oapsoitor mica
c*. 100.""f.
C
