Harold E. Brower
Martin's College Olympia, Washington
A Glass Electrode pH Meter Using
St.
a Vacuum Tube Voltmeter
A
simple pH meter can be constructed for less than ten dollars when used in conjunction with a vacuum tube voltmeter (VTVM) of 1.5 volt full scale sensitivity. This makes an interesting, elementary project in chemical instrumentation. Vacuum tube voltmeters generally have an input impedance in the range of 10-22 megohms, whereas instruments using an electrometer such as a micro microammeter, ionization chamber, or pH meter may have an input impedance of lo4 to loR megohms.' Thus it was possible to construct a modular unit consisting of a single electrometer peutode with grid current of the order of 10-'"amperes and direct coupling this to a VTVM to provide the necessary input impedance and a slight voltage amplification of about 1.4. A CK 5886 subminiature electrometer pentode (Raytheon) was selected for its low cost ($5) and ready availability from most large electronic supply houses. Of course the accuracy of the whole unit cannot be any greater than that of the VTVM which is commonly *3% of the full scale voltage; however, our instrument was found to be accurate to 0.2 pH unit and reproducible to 0.1 pH unit. The electrometer tube was triode connected since this gives better linearity than the pentode connecti~n.~ Resistors R, and R) are to protect the grid from accidental voltages. Battery B, provides enough negative bias on the grid to keep the grid current at amperes; if the bias becomes more positive than -1.5 volts grid current increases exponentially. Battery B, provides a bucking voltage so that it is possible to zero the meter with the zero adjust control on the VTVM. The grid circuit should be carefully wired to avoid leakage paths. Switch SI may be a source of trouble due to its capacitance to ground in an ultra high 'PARTRIDGE, G . R., "Principles of Electronic Instruments," Prentice-Hall, Inc., New Jersey, 1958, p. 109. 2Terhnical Information and Transfer Curves, CK 5886, Raytheon Co., Inc., Industrial Tnhe Division, Newton, Mass., 1956.
resistance circuit and also due to contact p ~ t e n t i a l s . ~ Better results were obtained when the single pole double throw connection on a microphoue input jack was used which puts the glass electrode into the grid circuit when the plug is inserted and switches to "standby" when it is removed. The whole assembly was placed in a 4 in. X 2'/2 in. X 21/4 in. Bud "Minibox." The operation and calibratiou of the meter was as follows: a cardboard scale was constructed and placed on the plastic meter cover. Three buffer solutions, for example pH 4, 7, and 9 were sufficient to calibrate the scale. The zero adjust control on the VTV.34 was used to center the needle a t pH 7, with the sensitivity set a t 1.5 volts full scale. It will be found that the scale is reversed from the conventional direction; i.e., pH units will be 14 to 0 from left to right. I t can readily be shown that the potential of the glass-S.C.E. electrode pair becomes more positive with increasing acidity according to the equation I3 = 0.09 - 0.059 pH a t 2 5 T , accounting for this ob~ervation.~Ko temperature compensation was employed since this would lead to an increasingly complex circuit. A change of five degrees after standardization alters the length of the entire scale 1.7%. Partridge, G . R., ibid., p. 112.
' DAY,R. A., A N D UNDERWOOD, A. L.,"Quantitative Analysis," Prentice-Hall, Ine., New Jersey, 1958, p. 295.
Electrometer circuit: RIR?, 2 2 meg; RIP 10; Rd, 200 K; S,, spdt; S1. dpst V1.CK5886; 81.2.7VHg; B?, 1.4 V Hg; Br,9.8 V Hg; 81, 1.4V.
Volume 40, Number 2, February 1963
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