Automatic Precision Glass Electrode p H Measurement with a Vibrating Reed Electrometer KURT i. KRAUS, ROBERT W. IIOLJIBERG, A N D C. J. BORKOWSKI Oak Ridge Nutional Laboratory, O a k Ridge, Tenn. Using a vibrating reed electrometer as detector, the potentials of glass-electrode assemblies were recorded on a strip-chart recorder. The precision of the measurements is approximately that of a Type K potentiometer and the reproducibility is sufficiently high to make it feasible to determine acidities of solutions to better than 0.1% under favorable conditions.
T
:Ites as a null t,ypc instrument where variations in the amplifier gain or in power supply voltages do not contribute to zero drift. iVith proper design the sensitivity is limited only by the noise produced by the thermal agitation in the input circuit. The zero drift, which is due t o the changes of contact potential of the dynamic eondenser surfaces, is usually about 0.1 mv. in 24 hours. The block diagram in Figure 1 shows a typical self-balancing GO-cycle vibrating reed. JVhen a voltage appears across the dynamic condenser input, an alternating current. voltage is generated and amplified by the alternating current amplifier. The output of the amplifier drives a motor, which in t’urn adjusts the potentiometer t o t,he null position. The indicated voltage on the self-balancing potentiometer is equal to the input voltage. In this way complet,ely automatic recording of the potentials is made. Where maximum precision is required, the recorder slide mire should balance only a small fraction of the total cell potential-e.g., 3 or 10 mv. full scale-while the major fraction of the cell potential is balanced manually by a high precision potentiometer--e.g., Rubicon Precision potentiometer or L. & N. Typc K-2 potentiometer. Because this potentiometer is on the low impedance side of the circuit, it need not be shielded. Calihralion of the potentiometer can readily be achieved with the same circuit. The potential of a standard cell is fed into the vibrating twd input and its potential is balanced against t,he output, of the potentiometer, whose dials are set for the nominal voltage of thc standard rcll. The Brown recorder is used as a null indicator. Figure I does not represent a unique circuit and certain varianis :we possible-for example, the Type I( potentiometer can he placed between the calomel cell and ground, or a 300-cycle reed with electronic feedback may be used. The construction of the dynamic condenser requires considerahle skill, especia,lly in the preparation of the condenser surfaces where it is important to obtain low and stable contact potentials. Both so-called “60-cycle” ( 7 ) and “300-cyclc” reeds (8) have been used. Because in the former case the reed is driven by stantiAPPARATUS ard 60-cycleJ 1le-volt alternating current, the Brown amplifier can 1e; used directly with only minor changes, such as removal of the Electronic Equipment. The vibrating reed electrometer is ’ chopper” and addition of a preamplifier, If a recorder is to be (wentially a high resistance direct current voltmeter which is used, the design of the 60-cycle reed is in general much simpler capable of measuring voltages in circuits of resistance up to than t’hat,of the 300-cycle reed, for Tvhich a special oscillator and 1015ohms. Because the direct current potential is converted to a separate amplifier antl det,ector are needed. The 300-cycle reed an alternating current voltage by the “chopper” or dynamic conrequires more maintenance in general than the 60-cycle reed. denser, a stable alternating current amplifier can be used in B Severtheless, most of the measurements reported here have been 100% feedback system. In this manner the vibrating reed opercarried out with the 300-cycle reed. A photograph of the coniplete assembly, using a 300-cycle reed, is shown in Figure 2. In all earlier measurements the Brown recorder was adjusted to give 10-mv. full-scale deflection, I r--;L;;--iL. MOTOR and thus a reading accuracv of 0.01 to 0.02 mv. - -. I1 0 More recently recorders h t h full-scale sensi1 ELECTRODE r -/ 1 ’ tivity of 3 mv. have been used (giving a reading 7I I e accuracy of 0.003 to 0.006 mv.). Although it apI CALOMEL I pears that from the point of view of noise level still greater sensitivity could be used--e.g., 1 mv. II I full scale-the reproducibility of the measurcMECHANICAL /I nients does not seem to warrant this at present, COUPLING y particularly in view of the fact that with this sensitivity the instrument would need considerably / BROWN more attention in kinetic studies where the changes of potential with time are of interest. Shielding. Aside from connecting cables, shieltling is needed only for the glass-electrode a>scmbly. This can be readily achieved by enclosing the cell and electrodes in an aluminum bo\. Temperature Control. Temperature control of the cell within the box was achieved by placing it Figure 1. Block Diagram of Vibrating Reed Assembly through a cover in a Dewar flask, fillcd with
HIS paper describes the use of the vibrating reed (also called dynamic condenser or capacitative commutator j electromvter for high precision pH measurements with glass electrodes. The instrument may l x x used with a strip-chart recorder and the voltage sensitivitp attainable is determined by t,he accuracy of the auxiliary potentiometer, \vliic.h in this work has been a Leeds &Sorthrup Type I< potentiomcter. 1,eCaine and Waghornc havc described a vibrating reed electrometer for ionization current measurements (3). Scherbatskoy and co-workers used n similar instrument (5-7) for radioactivity measurementsin oil-well logging, \\.here :in instrument of high current sensitivity, great ruggedtirss, and adaptability for automatic operation \vas desired. A detailed study of this instrument has been made by Palevsky antl cw-workers ( d ) , who d s o matle cert.ain design changer arid improvemcants. -411 the aut,hors point out that the instrument can t)r used for potentid measurements in high resistance circuits. This instrument appeared to be ideally suited for voltage nieasureiiients of glass-rlectrodc~assemlilies and was expected to l)r: a considerable iniprovenient over the usual vacuum-tube elect rometers, inasmuch as it appeared to combine great ruggedness :uid freedom from zero drifts with high sensitivity, adaptability for iiutomatic. rec,odings, and complete 110-volt alternating c.nixnt opelntion. Since its first tests several years ago as a t,cicording prccision high impedance potentiometer, the instrutnent has been used routincly for precision potttntial nieasuremcnts i n this laboratory (1, e), and fov p o t ~ n t i a ni(wurenient~ l i n low rcsistance circuits as ~ v e l l .
I
--
ELEGTRT j
34 1
342
ANALYTICAL CHEMISTRY
water. The temperature within the Dewar was maintained a t the proper temperature by placing in it a copper coil through which thermostated water was pumped. This arrangement did not cause troubles in the detecting circuit, provided the coil was grounded to the case. The cells were a t first coated with paraffin on the outside. However, omission of such coating did not appear to influence the reSUlt5
appreciably to the drift. This c m he demonstrated by measurin a cell without liquid junction-i.e., consisting of two well-aged glass electrodes-where such drifts are negligible. Similarly, the prolonged drifts can often he eliminated by replacement of the calomel assembly. It appears t h a t these drifts become prominent if the flow rate of the velectrodes~~ through the asbestos wick is too small. For the measurements reported here the calomel electrodes were selected for low drift. The generally accepted technique of using glass electrodes involves their standadi.rdieation (preferably ttt two points) before measurement. Thus, glass electrodes are usually used to establish differences in acidities rather than absolute values. By strictly following this procedure the sensitivity of the instrument can be utilized fully without special precautions, as shown in Table I. The data for Table I were compiled by making a Series of consecutive measurements in an air-conditioned room whose temperature was 25" * 1" C . No other attempts were made to control temperature. The potential of the cell containing 0.01013 M hydrochloric acid was followed until it was constant, 0.4992 ml. of 0.1000 M hydrochloric acid was then added, and the resulting potential change was recorded. The cell was then cleaned and the measurements were repeated. It may be noticed from Table I that the potential changes ( A E ) thus attained show a considerably smaller standard deviation (c) than the various readings for the sfme acid. ( r was calculated accord-
ing
~
ing t~
-
Figure 2.
Vibrating Reed Eleotrometer Assembly for p H '"easurements 111
---
VIamC-Y..VI.W
yLIyII-.
,u-.uI-b-u
Electrode Assembly without Special Ten
25' * '1 C. potential (mv.) in 0.0 potential (mv.) after ml. of 0.01013 M H Brown recorder, fullaosle ee
T
E'
E,
Because cqreful temper'ature control ai the calomel electrode appeared of ~ m p o m n c e ,some calomel electrodes were sueciallv
.
Tab& sium Chloride)
of
this occurred, t6e temperature the assemblv drouued raDidl< prest jecth temr the; y r s i o n in B thermostat when not in use circum;ented this complication reasonably. well. Electrodes. Most experiments were carried out with standard Beckman electrodes-calomel electrode No. 270and glasselectrode No. 290. Because measurements of uotentials in circuits havine resistances as great as 10" ohms can-readily be made, glass ele; trodes of resistance very much higher than the usual approximately 108 ohms could probably he U I RESULTS
In a cell of the type wed in pH measurements, the precision and reproducibility are determined by a number of factors aside from the characteristics of the electronic circuit. These are primarily the response of the measuring electrode, reproducibility of the liquid junction potentials, and drift in the reference potential. It was found that these factors are considerably more important than the characteristics of the measuring circuit, which is very fast, reproducible, and free of drift as indicated by measurement of the potential of a standard cell. The potentials of glw-electrode assemblies often show prolonged drifts which may last from a few minutes to several hours and may be as high as several millivolts. It is believed that these drifts are due to changes in the liquid junction potentials with time, or changes in the characteristics of the glass electrodes. If the glass electrodes have been aged at not too high an acldityi.e., in 0.01 M hydrochloric acid-they usually do not contribute
A".
*
332.41
0.10
338.86 0.10
--_--
Table 11. Potential of Glass-Calome. -.-l~ sium Chloride) Eleotrode Assembly with T e m p e r a t u r e Control Alternating measurements of two soids. A . 0.01003 M HCI. M HCi Solution temperature 25.10° * 0.05" C. Brown recorder. full-soak sensitivity 3 m V . Date A B 5/16
5/17 5/18 5/25 5/26 5/27
4".
331.220 331.205 331.205 331.168 331.153 331.166 331.159 331.159 331.144 331.138 331.068 331.263 331.278 331.281 331.296 331.424 331.378 331.083
331.209 0.096
310.034 309.997 510.037 310.003 309.970 309.961 809.973
300.967 309.961 309.958 309.891 310.104 310.083 310.086 310.107 310.241 310.205 309.893 310.026
0.096
B. 0.00439
4R 21.186 21.208 21.168 21.162 21.181 21.195 21.186 21.198 21.184 21,180 21.167 21.159
81.196 21.195 21.189 21.183
21.174 21.189
21.188 0.013
343
V O L U M E 22, NO. 2, F E B R U A R Y 1 9 5 0 observations, 5 the arithmetrical average, and x the observed value.) Because u for AE was only slightly larger than the estimated reading error (10 mv. full scale), it appeared that recording a t a smaller value for full-scale sensitivity would further lower the standard deviation of the readings. A similar set of data, determined a t a full-scale sensitivity of 3 mv., is shown in Table 11. For these data the cell was thermostated as described above. The recorded measurements represent the alternate measurements (to equilibrium) of 0.01003 M and 0.00439 M hydrochloric acid solutions over a period of 2 weeks using the same electrodes. The “absolute” readings showed a standard deviation u = ea. 0.1 mv., while the differences showed the very low value u = 0.013.
Table 111. Potential of Glass-Calomel (Saturated Potassium Chloride) Electrode Assembly with Special Temperature Control of Calomel Cell AlteJsti.ng measurements of two acids.
Av. a
?+TO
PARAFFIN S E A L
DEWAR FLASK
SATURATED KCI H g - H g 2 C 1 2 MIXTURE
MERCURY POOL
Figure 3.
Calomel Half-Cell in Dewar Flask
I t is believed that the rather large values of u for the absolute measurements are mainly due to inadequate thermostating of the calomel electrode. For this reason a calomel half-cell, built in a small Dewar flask as described above, was used for a series of similar measurements (Table 111)using a recorder of 10-mv. fullscale sensitivity. For this series of measurements the solutions again were alternated and the series was completed in one 8-hour period. The standard deviations for the absolute readings were only 0.025 mv., in close agreement with the error for the differences ( A E ) of the measurements in this series as well as the series in Table I. Inasmuch as the standard deviation for the measurement of potential differences is rather low (under favorable circumstances *0.015 mv.), it appears feasible to determine the concentration of acids by glass-electrode p1-I measurements directly without need for titration, provided all other variables are carefully controlled and standardized. This may be of particular interest for solutions containing hydrolyzable ions. If the permissible analytical error is O . l % , a total error in the potential measurements of ca. 26 pv. can be tolerated. As can be seen from Table 11,this is almost twice the standard deviation which can be attained. For proper use of the glass electrode for measurement of the acidity of solutions, it is necessary to determine the Nernst slopes (potential change of assembly for a tenfold change in oxonium ion activity) in the acid systems under consideration. These slopes
17.29 17.33 17.32 17.29 17.35 17.35
,...
17.28 17.29 17,33 17.36 17.319 0.030
Average calciilated excluding 1 3 .
______
JUNCTION
OE
+
313.427 0.025
330.751: 0.021
U
41
B. 0.005025
0.01004 M HC1.
+
R
PLATINUM CONTACT
A.
M H U
Ionic strength. 0.5 (KC1) Temperature. 24.67’ * 0.03’, C. Brown recorder, full-scale sensitiviti- 10 mv. 0,01004 .If HsO 0.005025 .If Ha0 2 313.45 1 330.74 1 313.43 330.76 3 313.43 330.75 3 6 313.46 330.75 8 7 313.45 10 330.80 9 313.40 12 330.75 11 313.38 14 (330.29) 13 313.45 16 330.73 15 313.43 18 330.72 17 313.42 20 330.75 19 313.40 22 330.76 21
~
~
(which a t 25” C. should be 59.15 niv.) were found to be close to this value (at constant ionic strength) for most electrodes a t low acidity, as shown in Table IV, which represents data simultaneously obtained in a cell containing three glass electrodes and a calomel reference electrode. Because the ionic strength of the solutions remained constant and the concentration of oxonium ions was small compared with the ionic strength, the slopes were calculated assuming that the activity coefficients of the oxonium ions and the liquid junction potentials are the same for the two solutions. Some data on Eimer and Amend standard buffers of nominal pH 4.00 ==I 0.01 and 7.00 =t0.02 have been included. The Nernst slopes are somewhat high, the deviation from the theoretical value being outside the experimental error. The close agreement between the various glass electrodes which is described in Table IV is somewhat misleading, because, a t least a t higher acidity and high ionic strength, definite deviations of the electrodes from each other become apparent. Measurements of potential differences for different acids using the same glass electrode and different calomel electrodes are shown in Table V. The data were obtained again simultaneously for the three combinations of electrodes and it can be seen that the potential differences found are independent of the particular calomel electrode used.
Table IV.
Nernst Slopes for 7;arious Glass Electrodes
Brown recorder, full-scale sensitivity 3 m r . T = 25.000 f 0.050 c. Glass E . Mv. Eleo0.010029 .W HC1 0.004391 HC1 trode 1.00 M C1- (KCI) 1.00 .If C1- IKCI) 1 342.735 321.542 340.214 2 361.332 368,570 3 389,677 E & A Buffers p H 4.00 IIFr 7.00 1 232.38 49 84 2 231.82 69 20 280.14 3 97 5 2
Table V. Calomel Eiectrode 1 2 3
AE, A l v .
21.193 21.118 21.107
182 54 182.62 182.62
Slope, hIv./pH 59.08 08.87 58.84
GO 81 60 87 60.87
Influence of Calomel Cell on Nernst Slopes
.
[Ionic strength 0.5 (KCl)]
E , Mv.
o o i 0 0 3 .w HCI 342.927 342.848 342.604
0.004391 M HCI 321.839 321.732 321.528
E , 1Iv. 21.09 21.12 21.08
Slope, 1lv./pH 58.96 89.04 58.93
Recorder Full Scale,
Ilr. 3 3 3
344
ANALYTICAL CHEMISTRY ACKYOWLEDG3IEYT
(3) LeCaine, H., and Waghwne, J. G., Can. J . Research, 19, 21 (1941).
It is a pleasure to acknowledge the assistance of J. East and 0 . C. Cole in assembling and maintaining the instruments. The
(4)
authors are indebted to the Instrument Divisions of the MetalIurgical Laboratory of the I’niversitv of Chicago (now Argonne Kational Laboratory) arid of the Oak IlidKe National Laboratorv for the reeds antl amplifirrs.
(6) Scherbatskoy, S. A . , and Fearson, R. E., Phys. Rev., 70, 96
LITER4TIJRE CITED
(1) Kraus, K. A , , Holmberg. 11. IT-., : ~ n d Kelson. F.. unpublished
results. (2) Kraus, K. A., and Nelson, F., J . A m . Chrm. Soc., 71, 2517 (1949)
Palevsky, H., Swank, It. K., and Grenchik, R., Rev. Sci. Znstru-
ments, 18, 298 (1947). (5) Scherbatskoy, S. A., et al., U.S.Patents 2,349,225 (May 16, 1944), 2,301,389 (Oct. 31, 1944).
(1946). ( 7 ) Srhcibstskov, S. A., Gilmartin, T. H., and Swift, G., Rev. Sci. Instruments, 18, 415 (1947). ( 8 ) Swank, It. K., and Forstat. H., Atomic Energy Commission, Rept. CP-3595 (August 1946). RECEIVED Septemher 14, 1949. Based o n work performed under Contravt W-7405 eng. 26 for the Atomic EnPrgy Project at Oak Ridge National Laboratory.
SURFACE CHROMATOGRAPHY Refinements in Apparatus and Technique JARIES E. MEINHARD
AND
NONItIS F. I I \ L L
C’nicersity of Wisconsin, Madison 6 , Wis.
By means of an extended technique and improved designs in pipet construction for surface chromatography a useful variable, the ratio of developer volume to sample volume, is automatically controlled. By fixing this ratio at appropriate values and relating it to the extent of migration of a zone the chromatographic behavior of a given solute may be accurately appraised. Details of a superior ripening technique and of pipet construction and calibration are given.
T
HE sample pipc>t previously described for usc i n surface chromatography (31, and shown in Figure l , a , has the disadvantage of heing fragile arid easily broken. A more rugged pipet patterned after that of ilndrrson ( 1 ) was preparcd (Figure 1,b) to eliminate this difficulty but was found to have certain ot,lier drawbacks. The \vrltl a t point li must be carefully formed and thoroughly anncded in order to prevrnt the occurrence of excessive st,rains. Furthermorc, \vhen the delivery tip is d r a m out to satisfactory outsitk tlimensions the bore suffers reduction in size beyond practical limits. The const>ructionshown in Figuw 1 ,c, however, resolves thesc problems and is readily drawn from i-mm. tubing of either lient-r(v4stant or soft, glass. I t s prcparation ma!. he visualized hy rcferencc to F’igurc 2.
ccntwed whilc the glass is worked. Thc weld is made a t y until complete fusion has taken place and a is considerably thickened. The glass is then drawn out in the form, e , and cut a t f n h e r e the outside diameter is 1.0 t,o 1.3 mm. and the inside diameter, 0.15 t o 0.40 nini. It is highly desirable to have the delivery bore slightly smallcr t,han the bore a t d, so that the net capillary force of the filled pipet is in the direct>ionof delivery. Otherwise drainage of the pipet into the adsorbent surface must, be started with a geiitlc puff of air. The scratch a t f m a y be made rvit,ha tungsten carbidc crystal nr a chip of porcelain, or by pressing the tube with a knifc Iilatle against a piece of silicon carbide paper and rotating. The inside diameter at,e is 0.4 to 0.8 nim. and the length from inner til) to delivery tip is 25 to 40 mni. The over-all length is 130 to 150 mm. The delivery tip is beveled on emery polishing paper (Behr-llanning, Type 0),so that the actual contact surface has a tliamcter of 0.5 to 0.6 mni. A plug of cotton batting is pushed into the shank of the pipet to protect the inner capillary tip from dust.
A pipct so made is completely automatic both in filling and in tlclivery iuto the chromatographic surface; neither suction nor
m Y
C Figure 1. Sample Pipets
,
The bulb, a, is dran-n from the same tubing as 0. In drawing the inside capillary, d, the glass is first thickened so that the final outside diameter at the tip will be 0.8 to 1.3 mm. and the inside diameter, 0.20 to 0.50 mni. Thr holder, c, is employed to keep d
e
Figure 2.
Construction of Sample Pipet
