Practical Vacuum-Tube Potentiometer for pH Measurement with Glass Electrodes FREDROSEBURY, College of Physicians a n d Surgeons, Columbia University, New York, N. Y.
T
An easily operated vacuum-tube potentiometer a p o t e n t i o m e t e r on t h e described is the result for p~ measurements is described. is made vacuum-tube principle (2,9,10) of a series of trials and but inability to control gridof a low grid-current, low plate-voltage tube now e x p e r i m e n t s growing out of current f l u c t u a t i o n s arising available for this work. Galvanometer drift has a demand in this d e p a r t from leakage or e l e c t r o s t a t i c ment for a satisfactory means been reduced to a negligible minimum by autocharges, and those of the varimatic compensation for $lament voltage changes, ous battery voltages, has been of d e t e r m i n i n g the PH of inand ‘tfloating,, ofJilament battery on a charging a source of great a n n o y a n c e testinal c o n t e n t s . The glass electrode has been shown in the operation of these instrub y McInnes a n d Dole (5) source. Suggestions for a shielded thermostat merits. to develop potentials d i r e c tl Jr box and a high-insulation vacuum-tube switch A four-element, low gridproportionate t o p H even are given. One galvanometer is used f o r both c u r r e n t , low anode-potential in the presence of s u b s t a n c e s potentiometer standardization and as a platevacuum tube recently put on which i n t e r f e r e seriously the market1 (3, 6 , 6), and the current indicator, working as a null instrument with pH m e a s u r e m e n t s by incorporation of a number of in both functions. other eleo t rome t r i c means, new f e a t u r e s of construction and therefore t o promise a n d assembly, h a v e now greater accuracy than other methods. The high resistance eliminated this source of trouble. The present instrument of the glass membrane has, however, by necessitating the has the additional decided advantage of relative simplicity of use of a very sensitive electrostatic device for measuring the operation. The principles (7, 8, 112) upon which this instrument potential, proved a serious obstacle to the development of operates will be clear from a study of the vacuum-tube cira satisfactory instrument. Because of this high resistance, it is obviously not possible cuit, shown in Figure 1, For any given grid voltage, all to balance the potential against that of a potentiometer other conditions being maintained constant, the anode curdirectly. The Compton electrometer, as employed by rent, as indicated by a galvanometer, will assume a specific McInnes, has apparently been found unsatisfactory by other value. Insertion in the grid circuit of an additional voltage, workers. Several attempts have been made to construct that of the electrode system, causes the anode current to change, but upon opposing ~----------------r.- . -.-. -. -.-. -. an equal voltage from the 1 L k N . STUDENT POT# 7 6 5 8 potentiometer, t h e a n o d e VACUUM TUBE CIRCUIT current resumes its original value. T h e s y s t e m u s e d differs from that of Dubois (9) in t h a t no a t t e m p t is made to work the tube at its floating grid potential, where the g r i d c u r r e n t is z e r o . With s u i t a b l e precautions, the FP-54 tube can be made to have a grid current of 10-l6 amperes or less, which is low enough to preclude appreciable polarization effects. Anode current drifts due to filament- and grid-battery f l u c t u a t i o n s have been reduced to a negligible minim u m i n t w o w a y s . Part of the v o l t a g e drop across the filament is u t i l i z e d to supply a current which is p a s s e d through the galvanometer in opposition to the normal a n o d e c u r r e n t ( I ) . This method not only has the advantage of allowing the C .FAN: SLOW SPEED DfAN: FULLSPEED galvanometer to be operated E .HEATER: INTERMITTENT F -H€A.TETERCONTINUOUS from its true zero, but also G -GALV. LAMP HE i n s t r u m e n t to be
use
i
FIGURE1.
WIRING CIRCUITS FOR COMPLETE
GLASSELECTRODE SET-UP 398
1
General Electria FP-54.
October 15, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
399
automatically compensates for filament-battery fluctuations; a rise in filament voltage c ail s e s increased electron flow and hence a rise in anode c u r r e n t , but the counteracting c u r r e n t increases simultaneously, thus preventing a change in galv a n o m e t e r deflection. I n a d d i t i o n , l a r g e filamentb a t t e r y fluctuations h a v e been circumvented by floating this and the grid battery during use on a direct current charging source through a resistance of appropriate size so that t h e c h a r g i n g current will be slightly less than that of the load.
CONSTRUCTION E L E C T R I C A LA. threepole double-throw s w i t c h , Xz in Figure 1, allows the galvanometer to be used in s t a n d a r d i z i n g the potent i o m e t e r as well as in the tube plate circuit. The contacts 4 and 7 p r e v e n t the interruption of plate current when the galvanometer is being used for standardization. RIGHT SECTION Switch 8 3 reverses the FIGURE 3. CONSTRUCTION OF THERMOSTAT Box potentiometer p o l a r i t y for measuring solutions of low pH. Resistance R1 provides a means of varying the grid ter, batteries, and associated apparatus, in an electrically voltage, which is indicated by a small panel voltmeter shielded box arranged to maintain constant temperature. of proper range. R7 is the galvanometer protective and A thermoregulator within the main chamber operates a heatdampiiig shunt, which consists of a series of resistors con- ing resistance (Electrad 200 ohms-75 watts) through a small nected to the studs of a rotary switch. The values of these relay whose coil is energized by one or two dry cells. A resistors for use with the L. & N. type 2500-e galvanometer thermometer placed in the test solution or in some other convenient place may be observed through the gauze-shielded window and illuminated with a small candelabra-base 110volt lamp operated by a switch on the front panel. The connections for this light and for the air-conditioning system are shown in Figure 1. Diagrams showing the box constructed by the author, which is a modification of one used by Dubois, are given as a suggestion (Figure 3). The vacuum tube and switch 81 are housed in a separate wooden box, lined inside with sheet copper and impregnated against air and moisture with hot wax or pitch; this box is built into a corner of the thermostat box. Its shield and that of the main chamber are to be thoroughly grounded. An arrangement of the apparatus with a good view of the tube box is shown in Figure 4. Details of the high-insulation switch, XI,bushings, and connections, are shown in Figure 5. Much depends upon the insulating qualities of this switch so that careful thought should be given to its construction and placeFIGURE2. INSIDEOF CHAMBER SHOWING ment. Three six-volt 100 ampere-hour storage batteries ELECTRODE SYSTEM are recommended for use with this instrument. Figure 6 shows a complete set-up. are 10, 100, 500, 1000, and 500 ohms, which give convenient steps of galvanometer sensitivity with good damping characOPERATION teristic. A view of the inside of the chamber showing the electrode system is given in Figure 2. The potentiometer is standardized in the usual way, makMECHANICAL. It is necessary to enclose the entire as- ing use of the galvanometer switch, 82. As balance is apPembly, with the exception of the potentiometer, galvanome- proached, the galvanometer shunt is moved toward the
ANALYTICAL EDITION
400
higher resistance studs, final adjustment of potentiometer .battery current being made with the shunt set on the last stud. After standardization is completed, the shunt is thrown back and the same step-by-step technic followed for all
FIGURE 4. VIEW
OF
TUBEBox
further operations. Switch SSis then thrown so as to connect the galvanometer to the anode circuit (e. m. f. position), and with the high-insulation switch, 81, on stud 2, which connects the grid of the tube directly to the working grid voltage, the galvanometer is brought to zero, by means of the resistances RS, Rs, Rd, and Ra. Switch X1 is then thrown to stud 1 so as to include the electrode system and the potentiometer measuring circuit, and the latter is adjusted until the galvanometer deflection returns to zero. If the deflection increases when the potentiometer setting is increased from zero volts, the potentiometer polarity should be reversed with switch Sa. At balance it is possible to throw switch 81 back and forth without causing a deflection of the galvanometer, and this provides a check on the accuracy of the potentiometer setting, which, a t zero galvanometer deflection, is the e. m. f. of the electrode system.
Vol. 4, No 4
CALCULATIOX OF PH
It has been observed that the asymmetry potential of the glass membrane, so-called by McInnes and Dole (5), often does not remain constant. This has been found to be especially true with a new glass cell; after a period of use it assumes a constant value. It is advisable, when first setting up the instrument, to test a batch of glass cells for potential drift, and to select one which shows relatively little. After some days readings obtained with this membrane will be found nearly constant for a single solution, showing no a p p r e ci a b l e drift from hour to hour, and only small c h a n g e s from day to day. During this period pH calc u l a t i o n s may be made satisfactorily by r e f e r e n c e to a standard of known pH, the relation being P H ~ PH, * E,
* Eg
0.0591
at 25" C.
where E represents FIGURE6. COMPLETESET-LP potential, g the unknown solution, and s the standard. After the glass membrane attains a constant asymmetry potential, the use of a standard is necessary only to check this fact from day to day; pH may then be calculated by the use of an empirical formula involving the combined constants, 5 in the following equation, for glass, silver, and calomel electrodes, as determined for a given set-up with solutions of known pH:
o:9;T
pH = -at 25" C.
Nard Rubber
FIGURE5. DETAILSOF HIGH ISSULATION SWITCHAND BUSHINGS In measuring grid current, a high resistance (100 megohms or more) of known value is substituted for the electrode system and the procedure as given above is followed. The voltage as read on the potentiometer and the known resistance are applied in Ohm's law.
(2) (3) (4) (5)
Precision in measurement will depend to some extent upon the g a l v a n o m e t e r a n d p o t e n tiometer employed. Numerous repeated tests have shown that the presence of the vacuum tube does not introduce errors of any kind in this set-up. Even before the asymmetry potential of the glass membrane becomes constant, m e a s u r e m e n t s a r e easily a t t a i n a b l e t o within A0.03 pH; with a constant and carefully calibrated i n s t r u m e n t the average error is reducible to the third decimal place. Besides it.s usefulness in pH measurement, many other applications involving determination of e. m. f. might be s u g g e s t e d for this instrument. LITERATURE CITED
(1) Berl, Herbert, and Wahlig, Chefn. Fabm'k, 1930, 35-5. Abstr. in Electronics, 2, 453 (1931). Dubois, D., J. id. Chem., 88, 729 (1930). Du Bridge, L. A., Phw. Rev., 37, 392-4130 (1931). Hill, s. E*,Science, [N* s.1 73 (1598)v 529 (1931). McInnes, D. A,, and Dole, M., J. Gen.Physiol., 12, 805 (1929); IND.ENQ. CREM.,Anal. Ed.. 1, 57 (1929); J . Am. Chem. SOC.,52, 29 (1930).
October 15, 1932
I N DU STR I AL AN D EN GI N EER IN G C H E MI STRY
(6) Metcalf and Thompson, Phys. Rev., 36,1489-94 (1930). (7) Morecroft, J. H., “Principles of Radio Communication,” 2nd ed., Chap. VI, Wiley, 1927. (8) Nottingham, W. B., J . Franklin Inst., 208, 469 (1929); 209, 287 (1930). (9) Stadie, W. C., J. Biol. Chem., 83, 477 (1929).
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(10) Stadie, W. C., O’Brien, H., and Laug, E. P., Ibid., 91, 243 (1931). (11) Van Der Biil, H. J., “The Thermionic Vacuum Tube,” 1st ed., McGraw-Hill, 1920. RECBIVEDApril 2 5 , 1932. Chemical Foundation.
This work was aided by a grant from The
Determination of Metals in Organic Combination D. L. TABERN AND E. F. SHELBERG, Abbott Laboratories, North Chicago, Ill. knowledge perhydrol-sulfuric S POINTED out in the A study has been made of the action of fuming acid decomposition has not been report of the Sub-Comsulfuric acid and 30 per cent hydrogen peroxide applied to the systematic demittee on Synthetic Orupon organo-metallic compounds containing composition of organo-metallic ganic Chemicals of the American mercury, arsenic, antimony, bismuth, gold, silver, compounds. Drug Manufacturers Associaand germanium. Decomposition of all organic tion, “organic c om p o u n d s of MERCURY mercury are assuming an ever matter is rapid and complete, leaving a solution increasing importance in mediSeveral methods were studied containing only the metal, usually as sulfate. f o r t h e c o m p l e t i o n of t h e cine as antiseptics, anti-syphiTo this solution, customary melhods of analysis analysis in each instance. In litics, and diuretics.” During the may be readily applied; precipitation as suljide the absence of halogen, dilution year 1930, ten selected methods has been found useful particularly in the cases with water and titrationof for the analysis of organic mermercury with potassium sulfocury compounds were studied in of mercury, antimony, bismuth, and germanium, cyanate in the presence of nitric five different laboratories under as has a modification of the Newberry method acid was found to be satisfactory the supervision of this committee. in the case of arsenic. and apparently accurate. Variations of surprising magniWhere speed is important, this tude were found to occur. Using the same method for a given compound, differences as great technic has many advantages, enabling the complete analysis as 2 or 3 per cent were noted. The results for a different to be carried out in 15 to 30 minutes. Precipitation by mercurial by different methods show even greater variation, Jamieson’s reagent and either titration with potassium being as much as 7 per cent in the cases of mercury salicylate iodate or weighing as mercury zinc sulfocyanate (3) were and mercurochrome. About this time, during the course of looked upon with favor until, in the case of mercury salicylate, a study of certain synthetic organo-metallic compounds in the titration gave low and inconsistent results; the reason for this laboratory, it became very desirable to secure a more this has not been found. repid method of metal analysis. Since the chief aim of this study was the elaboration of a Experience in the foregoing analyses suggested strongly method applicable with equal accuracy to all types of organic that greatest progress could be made by devising a simpler mercurials, with and without halogens, precipitation as sulfide and more satisfactory mode of decomposing the organic com- seemed to offer the greatest possibilities. pound than the sulfuric acid-permanganate, sulfuric-nitric It was-found that the oxidation mixture, after dilution acid, hydrochloric acid, perchloric acid, and persulfate mix- and cooling, could be precipitated directly by hydrogen sultures commonly employed in the past. This was found in the fide; in the absence of inorganic salts, the authors not only use of fuming sulfuric acid and 30 per cent hydrogen peroxide failed to observe the inaccuracies reported by Fenimore and (Superoxol). The fuming acid (I5 per cent SOa) dissolved the Wagner ( I ) , but found that precipitation was complete organic compounds either in the cold or on gentle warming, within 15 minutes. No formation of sulfur was observed. facilitating complete oxidation, and the excess hydrogen per- If the precipitate was washed successively with alcohol, oxide on spontaneous decomposition left no residue of in- carbon disulfide, and ether, drying was complete in 20 minutes organic salts. With nearly all of the compounds studied, at 105” C. oxidation was found to be complete within 3 to 5 minutes, PROCEDURE IN ABSENCE OF IODINE. The sample containgiving a water-white solution containing only sulfuric acid and ing approximately 0.1 gram of mercury is placed in a Kjeldahl the metal as sulfate. flask with 7 to 10 cc. of 15 per cent fuming sulfuric acid. The use of hydrogen peroxide for hastening decomposition (With easily decomposable substances, ordinary sulfuric during Kjeldahl digestions was first suggested by Kleeman (5) acid is satisfactory.) The substance is dissolved, if possible, in 1921, and has subsequently found application in biological by gentle warming. Thirty per cent hydrogen peroxide assays (6, 7, 12). Oakdale and Powers (9) have employed it (Superoxol) is added drop by drop, being allowed to flow as a secondary oxidizing agent in a new method of halogen down the side of the flask, which is agitated gently by hand. determination. Oxidation takes place a t once. Addition of hydrogen perSince the completion of most of the following work, the oxide is continued until the liquid is the most straw colored, authors have found that Graham (8) employed a sulfuric- when warming is increased until fumes of SO3 are abundant. peroxide mixture of the decomposition of mercurial insecticide Sometimes a little more hydrogen peroxide is required for samples, and Schulek and Villecz ( I O ) have utilized it in the complete decolorization; the total amount required will determination of arsenic. These methods do not, however, vary from 1 to 5 cc. It is essential that everything be in seem to be well known in this country, and to the best of our solution at this point. I n the absence of halogen, no loss of
A
