cepted reference methods for the measurement of LDH activity (12). A comparison of the Amperometric and Wroblewski methods is shown in Table I for four sets of serum samples. The data for run I1 are shown in Figure 9. In conclusion, the differential amperometric method provides an inexpensive, easily automatable method for measuring serum LDH activity. The amperometric method has the advantages of the lactate-to-pyruvate methodthat is, reaction rates are linear over a wide range of activities, and the reagents are less expensive and more stable. For example, solutions of NADH, when stored, deteriorate spontaneously to form an inhibitor of the LDH reaction (12). The method has the additional advantage of cycling (12) E. Amador, L. E. Dorfman, and W. E. C. Wacker, Clin. Chem., 9, 391 (1963).
the NADH produced which prevents the reaction from quickly coming to equilibrium. The method has high sensitivity, is specific for total LDH when lactate is present as the substrate, and can be successfully applied to routine serum analysis for serum samples of 0.1 ml or less.
ACKNOWLEDGMENT We are grateful for the support and cooperation of The Ohio State University Hospital Clinical Chemistry Division in providing clinical samples. RECEIVEDfor review February 13, 1974. Accepted May 30, 1974. This study was supported in part by the National Institutes of Health Grant GM-15821. Marilyn Dix Smith was a Fellow of the American Foundation for Pharmaceutical Education and a Fellow of The Ohio State University.
New Procedure for Calibrating Glass Electrodes M. Filornena G. F. C . Carnoes and Arthur K. Covington Department of Physical Chemistry, University of Newcastle upon Tyne, €ngland
A new method for testing the performance of glass electrodes in the pH region 6.5-12.6 has been developed. Cells without liquid junction containing silver-silver chloride reference electrodes are used, and the test solutions consist of amine buffers and their hydrochlorides. With added soldium salts, it is now a simple matter to determine the sodium error at a given pH and pNa = 0, 1 , or 2 by transfer of a glass electrode between solutions and comparison of the potential difference obtained with that for a perfect glass electrode. These latter values were determined for the selected test buffer solutions using hydrogen gas electrodes, and the same cells permit standard pH ( S ) values to be calculated utilizing the BatesGuggenheim convention for the chloride ion activity coefficients. The advantages of the new procedure are discussed and results are given of its application to a range of commercially available glass electrodes.
It is current practice to use a glass electrode to measure the pH of a test solution ( X ) after the system: glass electrode
1
Solution I 3.5M or s a t KCl
I Hg,Cl, 1 Hg
(1) has been subjected to a two-point calibration ( I ) involving standard buffers SI and Sz of assigned pH(S), bracketing the pH of the test solution (X). A linear variation of emf with pH is assumed but the proportionality factor may not necessarily be (RTIF) In 10 = 59 mV/pH at 25 "C, as required theoretically. pH(S) values are established from hydrogen gas electrode measurements on cells without liquid junctions:
1 buffer
+
1 AgCl I Ag
(2 1 using the Bates-Guggenheim convention ( 2 ) to give the
Pt,H,
chloride
(1) R G Bates Determination of pH-Theory and Practtce 2nd ed , Wiley New York N Y 1973 p 87 ( 2 ) R G Bates and E A Guggenheim Pure Appl Chem 1, 163 (1960)
activity coefficient of chloride ion (ycl) and obtain pH(S) = -log m H y H from the experimental quantity -log mHyHyc1 given by cell ( 2 ) . Because of the liquid junction in cell 1, values of pH(X) determined using it (operational pH values) differ from pH(S) at high and low pH (1 < pH > 9) ( 3 ) . In the same regions of extreme pH, the glass electrode may show deviations from the (theoretical) hydrogen electrode function, the so-called acid and alkaline errors which can be regarded as a mixed response to hydrogen ions and to anions or alkali metal cations, respectively ( 4 ) . Poorly constructed electrodes may show deviations in the pH region 1-9. To test the performance of glass electrodes, cell 1 is commonly used with the standard NBS pH buffers ( 3 ) . Outside the regions covered by these, certain other solutions can be used, such as 0.1M HCl and 0.1M NaOH. All the solutions used contain varied amounts of Nai-, and this, coupled with the procedure of using cell 1 resulting in uncertainties due to irreproducibility of the liquid junction potential, makes the method of testing imprecise and unsatisfactory. For example, the British Standard Specification does not distinguish between errors of the glass electrode a t high pH in the presence of sodium ions and those due to other cations ( 5 ) . It specifies that the only metallic cations present should be Na+ and K+ in all solutions and solutions pH > 9 should contain 0.1 mol 1 . - l Na+ . Some manufacturers check all or representative samples of their glass electrodes by the two-point calibration method (sometimes three points 4, 7, and 9 pH) before dispatch. Sometimes nomograms or nomographs are included in the literature which accompanies the electrode indicating the likely errors in Na+ -containing solutions a t high pH. Opinions vary whether such data are a reliable or merely a rough guide to performance. (3) R . G. Bates, J . Res M a t . Bur. Stand.. Sect. A . 66, 179 (1962) ( 4 ) G. Eisenman, Ed., "Glass Electrodes for Hydrogen and Other Cations," Dekker, New York. N . Y . , 1967 (5) Specification for Glass Electrodes for measurement of pH, 882586 (1965), British Standards Institution, London.
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 11, SEPTEMBER 1974
1547
Table I. Details of Buffer Substances Mol W t
F.p. or B.p. " C
NC (CHtOH),
209.25
102-103 (solid)
Koch-Light
(CH,CH?OH), (CHXOH)3CNH2
121.05
170-171 (solid)
BDH BDH
CHZOH ,CH?.NH, CH?.(CH,)BCH?
Substance
2,2-Bis (hydroxymethyl) 2,2 ',2"-nitrilotriethanol (Bis-Tris) Tris(hydroxymethy1)aminomethane (Tris) Ethanolamine Piperidine Tetramethylammonium hydroxide (25% solution in water)
Aldrich (Ralph Emanuel Ltd.)
Pt,H, 1 Solution X 1 glass electrode
(3) The emf of the cell (3) is invariant of pH and composition of the solutions X if both the glass electrode and the hydrogen electrode are functioning perfectly as hydrogen ion-responsive electrodes. However, this is not a feasible testing procedure to employ under production and control laboratory conditions. It is, therefore, the basis of the new method that the cell without liquid junction glass electrode 1 Buffer + chloride 1 AgCl I Ag (4) should be used. As in cell 2, chloride is added in suitable form to enable the highly reproducible, and easily prepared, silver-silver chloride electrode to be used as reference electrode. Arising from the arbitrary choice of inner reference system and the variability of the asymmetry potential, the glass electrode always requires calibration; it, therefore, always yields the difference in pH between two solutions which is related to the emf of the double cell;
I AgCl
~
Buffer(B1)
+
chloride 1 glass 1 Buffer(B2) + chloride 1 AgCl I Ag The contributions from the inner electrode system and the asymmetry potential thus disappear. The expected response for the transfer of a perfect glass electrode from a buffer solution B1 to a buffer B2(3, , . .) can be established from measurement of the emf of cells: Ag 1 AgCl 1 Buffer(B1)
+
1 Buffer(B2) + chloride I AgCl 1 Ag The difference in pH between B1 and B2(3, . . .) can be calchloride 1 H,,Pt .
..
Pt,H,
culated from the emf of these cells, each of which yields -log ~ H ~ H Y C Iand , ycl is obtained from the BatesGuggenheim convention ( 2 ) , the procedure being the same as for the establishment of pH(S) values (6). It has, therefore, been the purpose of this work to develop a series of test buffer solutions and determine the cor(6) Reference 7 . pp 71-84.
1548
1
I
I
61.08 83.15
I
9.0-10.5 (liquid) 102-108 (liquid)
BDH
T h e New Method. It was the intention to develop a series of test solutions for the evaluation of glass electrode response with a particular view to the testing being simple, accurate, and rapid. A simple two-solution test to furnish, for example, the sodium error a t a particular pH for two or three different types of glass electrode in 10 minutes was the goal. The only method of testing glass electrodes which does not involve the assumption of constancy and reproducibility of liquid junction potentials or knowledge of activity coefficients is that of direct comparison with the hydrogen gas electrode in the cell
Ag
Formula
Supplier
rect emf changes for the transfer of a perfect glass electrode between any pair. By simple comparison of the values obtained experimentally for an actual glass electrode between any pair, the error of that glass electrode will be obtainable. For example, by transferring a glass electrode between a solution of pH 9 containing no alkali metal cations and a solution of the same pH containing 0.W Na+ , the sodium error of the glass electrode a t pH 9 and pNa 1can be quickly determined. Although in principle the method can be employed at any pH, it is the alkaline region which is of the greatest interest. Selection of suitable buffer solutions is by no means easy particularly for the pH region above 9. Substances used in the preparation of standard buffer solutions for the present purpose should satisfy the following criteria: a) be easy to obtain pure and, if drying is necessary, this can be done at 110-120 "C; b) be not hygroscopic, oxidized by air, nor readily absorb carbon dioxide; c) be readily soluble to a concentration that the buffer capacity is adequate; d) be not reduced by hydrogen gas, nor react with chloride ion; e) contain no alkali metal cations. The only range of substances that in any way meets these criteria for the basic region is organic nitrogen bases partially neutralized with HC1(7,8). The latter is a particular advantage that no additional chloride need be added to enable the silver-silver chloride electrode to be used. The substances chosen after screening trials, their sources and other relevant details are given in Table I. The buffer systems were chosen to cover the pH range 6.5 to 12.6 a t regular intervals, adjustment being made by varying the buffer ratio within the prescribed limits to achieve adequate buffer capacity. The addition of salts affects the pH of a buffer solution. It is possible to compensate for this effect by adjusting the buffer ratio to counter the change. It was found that the pH remained constant to within =t0.15 of the mean value in the presence of sodium salts. To have attempted to compensate for these small changes would have made preparation of solutions unreasonably complicated and tedious. Addition of Na+ was effected using soldium perchlorate in all cases except that of tetramethylammonium hydroxide where the perchlorate would precipitate, and sodium chloride had to be used. This has a more drastic effect on the emf's observed due to the response of the silver-silver chloride electrode to the additional chloride. An attempt has been made to keep the solution preparation as simple as possible so that it can be carried out with apparatus normally available in most analytical laboratories. Ultimately it is hoped that the standard test so(7) N E Good G D Winget W Winter T N Connolly S lzawa and R M M Singh B ~ o c h e r n ~ s t r5, y 467 (1966) (E) R G Bates A n n N Y Acad Scf 92, 357 (1961)
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 11, SEPTEMBER 1974
~~
~
Table 11. Preparation of Buffer Solutions
Table 111. Observed e m f Differences (mV) for a pH(S) a t 25 "C
Buffer composition
NO.
+
B1
0 . 0 2 0 M Bris-Tris" 0 . 0 2 0 M Bis-Tris 6 . 540 Hydrochloride B2 0 . 0 5 0 M Tris" 0 . lOOM T r i s H y d r o 7 . 901 chloride I33 0 . 0 2 0 M Ethanolamine 0 . 0 3 0 M Ethan9 ,385 olamine B4 0 , 0 7 5 M Piperidine 0.050M Piperidine 11, 3 3 6 Hydrochloride B6 0 , 0 5 0 M Tetramethylammonium hy12 , 6 3 1 droxidec$d 0 . 0 5 0 M T e t r a m e t h v l a m m o n i u m chloride Additional reagents required: 1M solution (e.g., BDH C V S ampoule) ; Sodium chloride (e.g., BDH Analar grade) ; Sodium perchlorate monohydratee (e.g., BDH Analar grade).
+
+
Perfect Glass Electrode Transferred between Na+-Free Solutions (25 "C) Bi
*
a Bis-Tris can be dried a t 80 " C . Tris can be dried a t 110 "C. Analysis showed a variation of 28 01,;; w/w + 0 . 2 on different batches which is negligible. A more dilute solution (0.01M) TMAH solution (B5) was tried but found to give unreliable results unless stringent precautions to keep out carbon dioxide were observed. e Analysis showed insignificant variation from monohydrate if taken from tightly stoppered bottles.
-
lutions will be made available commercially from the manufacturers in a way similar to the availability of standard pH buffer solutions and tablets. Likewise in devising the testing procedure, care has been taken not to make unfair demands on the equipment likely to be available or the operator.
EXPERIMENTAL Preparation of Solutions. Buffer solution compositions are shown in Table II. The primary buffer solutions (that is, without Na+) are prepared according to the following table, diluting each solution to 1 liter with distilled water. No.
B1 B2 B3 B4 B6
W t of base,'g
8.364 18.157 3.051 10.648 36.5
Vol. of 1M HCl/ml
20 100 30 50 50
Buffer solutions in 250-ml amounts containing Na+ a t 10-1 and 10°M are prepared from these primary buffers as follows: For buffers B1, B2, B3, and B4, weigh out, respectively, 0.3511, 3.511, and 35.114 grams NaC104,Hz0 and make up to 250 ml with the appropriate primary buffer. We shall refer to these sodiurn-containing buffers as, for example B3N1, which means the ethanolamine buffer containing 10-IM Na ions ( i e . , pNa = 1). For buffer B6, the solutions are made using sodium chloride; the required quantities being 0.1461, 1.461, and 14.611 grams, respectively, again making up to 250 ml. Solutions may be stored in closed Pyrex flasks or bottles for periods up to one month. Sodium-free solutions can be stored for longer periods. All solutions will absorb carbon dioxide if left exposed to the atmosphere. The use of rubber bungs and tubing in dispensing vessels should be avoided. Apparatus. A pH meter with discrimination of 0.1 mV is desirable but a less accurate meter may be used if results of 1 mV accuracy only are required. Use of the mV scale is recommended to avoid complications which arise with slope factor variation and temperature compensat.ion. Open beakers are suitable for containing solutions while tests are being made, provided these are of a short-term nature. For extensive testing in all 24 solutions, a more elaborate closed vessel is suggested with necks to support the electrodes. In transfers between solutions, washing of an electrode with a little of the solution into which it is to be placed is the recommended (9) procedure. Swabbing with tissue or cotton wool is less satisfactory and may lead to carry over of sodium-containing solution into sodium-free solutions. ( 9 ) W H . Beck. A E. Bottom, and A. K. Covington, A n a / Chem.. 40, 501 (1968)
B2
B3
B4
B6
0
42.1 0
158.5 116.4 0
261.7 219.6 103.2 0
338.3 296 2 179.8 76.6 0
From: B1 B2 B3 B4
B6
+
+
To: B1
Table IV. Observed e m f Differences (mV) for a Perfect Glass Electrode Transferred between Solutions of Increasing Na Concentration at C o n s t a n t pH (25 "C) +
Bi
BiN2
BiNl
BiNO
B1 B2 B3
1.2 0.3 0.7 0.6 -4.5
8.6 3.2 6.8 5.7 -29.5
27.7 10.0 20.8 21.3 -81.5
B4 B6
Results to be presented for the buffer solutions refer to 25 "C. No serious inaccuracy is expected if tests are made in a draft-free atmosphere a t temperatures f 2 "C from this, provided all solutions are a t the same temperature. Ag/AgCl electrodes are available commercially. The results given here were obtained using the thermal-electrolytic type (10). From the nature of their preparation, the mass of silver-silver chloride tends to be porous and spongey and, hence, slower to come to equilibrium than the bi-electrolytic type. The bias potentials of the electrodes should be checked, and should not exceed *0.05 mV. A separate silver-silver chloride electrode was placed in each solution and left to reach temperature and electrochemical equilibrium, If only one reference electrode is to be used and transferred with the glass electrode, then it is preferable to use the bielectrolytic type. It is not possible to ascribe a slow response solely to the glass electrode using this double transfer technique, unless preliminary tests have been made on the time-response of the reference electrodes. Procedure. The buffer solutions may be used in either, or both, of two ways:- (A) testing the response of electrodes a t increasing pH in the absence of sodium ions (Bl, B2, B3, B4, B6). Analysis shows that buffer B6 may contain 5 X 10-4M Na+ impurity, in which case an error observed for transfer to B6 may arise for this reason. (B) Testing the response of electrodes to increasing Na+ concentrations at a constant pH (e.g., B3, B3N2, B3N1, B3NO). For either case, the error of the glass electrode on transferring it between two solutions can be obtained by subtracting the observed emf difference from the appropriate value for a perfect glass electrode given in Table I11 or IV. The error can only be ascribed to the second (higher pH) solution if the electrode is known (or assumed) to be error-free in the first solution. Examples. (A) A glass electrode was transferred between the buffers B1, B2, B3, B4, and B6. The observed emf values (E) were: B1
E/mV
-47.4
B2
B3
-89.5
B4
-205.7
B6
-306.9
-383.0
yielding differences (LE)with respect to solution B1: aE/mV
0
+42.1
+158.3
$259.5
+335.6
Comparing these with the first row of Table 111, we obtain for the error B1
B2
B3
B4
B6
error/mV 0 0.0 0.2 2.2 2.7 error/pH units 0 0.0 0.003 0.037 0.045 6 . 5 4 7.901 9 . 3 8 5 11.336 1 2 . 6 3 PH ( S ) (B) A glass electrode was transferred between the buffers B3, B3N2, B3N1 and B3NO. The observed emf values ( E )were: ( 1 0 ) D. J. G. lves and G J . Janz. "Reference Electrodes-Theory Practice," Academic Press, New York, N . Y . , 1961, p 203.
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 11, SEPTEMBER 1974
and
1549
B3
Table V. Errors of Glass Electrodes Expressed in pH Units for Transfers between B1 and B2, , . . 6 B2
B3
7.90
9.39
B4 11.34
0.007 0.007 0.007 0.005 0.003 0.002 0,039 0.036 0.014 0.020 0,019 0,019 0.027 0.025 0,029 0.024 0.025 0,027 0.047 0.046 0.047 0.047 0.051 0.046 0.044 0.049 0.057 0.049 0.051 0.051
0.042 0,042 0.044 0.042 0.037 0,036 0.066 0.064 0,034 0,037 0.039 0.037 0.046 0,044 0.049 0.042 0.044 0,046 0.062 0.061 0.062 0.062 0,066 0.064 0,064 0.069 0.076 0,066 0.062 0.062
0.000 0.000 0.002 0.000 0.000 0.000 0,019 0.017 0,003 0,008 0.007 0.007 0,012 0.008 0.012 0.010 0.010 0.014 ' 0.012 0.012 0,010 0.010 0.015 0.014 0.008 0.017 0,010 0.005 0.012 0.012
Manufacturers' recommended pH range or special use
B6 12 63
0,057, 0.061 0.057) 0.046) 0.046 0.115 0.0511 0.0561 0,110 0.098) 0,064 0,061 0.069 0.061 0.079 0,090 0.081 0.105 0.101 0.098 0.083 0,090 0.179 0.157 0.076, 0.076)
B3N2
B3N1
B3NO
E/mV -201.8 -201.8 -206.8 yielding differences ( L E )with respect to solution B3 AE,!mV
0
0
-219.8
5.0
18.0
Comparing these with the third row of Table IV, we obtain for the error at a constant pH of 9.38 f 0.11:
0-14
error/mV error/pH units
0-14
0-14
0 0
1.8 0.030
0.7 0.012
2.8 0.048
RESULTS AND DISCUSSION Advantages of the New Testing Procedure. The accuracy of the new testing procedure is limited only by the accuracy of preparation of solutions, the effect of carbon dioxide absorption, and the accuracy of measurement of the emf difference. It is not subject to vagaries of reproducibility of liquid junction potentials. To determine the sodium error of an electrode a t a single sodium concentration and pH is a very simple procedure, and may lead to an improvement in electrode specification from the manufacturers if the new test procedure is adopted. A simple statement that the sodium error did not exceed x mV or x/59 pH units with a universally adopted procedure for determining this, would be welcomed by users. It should, however, be pointed out that the sodium error may vary with the age and treatment of the electrode. Although it was not the intention that the new test buffer solutions should be used as pH standard buffers, there is no reason why they should not be useful as secondary standards since the pH(S) values have been assigned by the same method as used by NBS. The buffers containing 1M Na+ should be excluded from this statement
0-12
high alkaline
low temperature 0-14 0-14 0-12 0-14 0-11
0-11 0-14 0-11 0-14
Table VI. Errors of Glass Electrodes, Expressed in p H Units, for Transfers between Buffer Solutions Bi and BiN B1
B2
PH(S) [Na 1
6.55
6.63
6.86
M
10-2
10-1
loo
0.000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0.000 0,000
0,000 0.000 0.000 0,000 0,000 0,000 0.010 0.008 0.000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0,000 0.002 0.002 0,000 0,000 0,000
0.000 0,000 0,000 0,000 0,000 0,000 0.020 0.020 0.000 0,000 0,000 0,000 0,000 0.000 0.002 0,000 0.002 0.005 0,000 0,000 0.007 0.005 0,000 0,000 0,000 0.000 0.025 0.022 0,000 0.000
B3
7.90
7.93
7.97
10-2
10-1
lo3
-
9.45
9.60
11.34
B4 11.39
11.58
12.63
12.60
12.46
10-2
10-1
10'
lo-!
lo-'
10'
lo-?
lo-'
10'
-
'
+
1550
0.000 0.007 0.003 0,000 0,000
Manufacturers'
B6
9.39
-
pH range or
special use
0 . 0 0 2 0 . 0 0 5 0 . 0 0 7 0 . 0 2 4 0.035 0 . 0 5 7 0 . 0 3 0 0 . 0 5 1 0 . 0 6 9 0 . 0 3 5 0 . 0 5 7 0 . 0 7 6 ) o-14 0 , 0 0 0 0 . 0 0 2 0 . 0 0 3 0.014 0 . 0 2 4 0 . 0 3 0 0 . 0 2 7 0 . 0 4 9 0 . 0 6 2 0 . 0 3 7 0 . 0 6 2 0 , 0 9 1
0,000 0.000 0,000 0,000 0.017 0.015 0,000 0.000 0,002 0.000 0,000 0,000 0.000 0,000 0,000 0.000 0,000 0.000 0.002 0.002 0.002 0.002 0.002 0,000 0.012 0,010 0,000 0,000
0,000 0,000 0.000 0,000 0.025 0.025 0.002 0.002 0.003 0.002 0,000 0,000 0,000 0.000 0.002 0.003 0,000 0,000 0.007 0.003 0.007 0.005 0.002 0.002 0.014 0.017 0,000 0.000
0.002 0.000 0.002 0.002 0.035 0.037 0.002 0.002 0,014 0.010 0,000 0,000 0.003 0.000 0.005 0.010 0.002 0.002 0.017 0.014 0.010 0.010 0.005 0.003 0.057 0.044 0.005 0.005
0.012 0.014 0.012 0.012 0.069 0.073 0.012 0.012 0,064 0.074 0,000 0,000 0.005 0.003 0.025 0.030 0.003 0,002 0.015 0.010 0.017 0.015 0.007 0.007 0.078 0.078 0.014 0.014
0.020 0.022 0.030 0.032 0,100 0.103 0.030 0.032 0,095 0.093 0,000 0,000 0.008 0.008 0.035 0.046 0.008 0,012 0.020 0.012 0.054 0.054 0.017 0.015 0.125 0.108 0.019 0.017
0.027 0.030 0.047 0.047 0.110 0.120 0.049 0.051 0.125 0.127 0.003 0.002 0.029 0.022 0.054 0.090 0.014 0.019 0.049 0.037 0.095 0.095 0.020 0.019 0.259 0.213 0.027 0.027
0.020 0.027 0.020 0.019 0.091 0.098 0.019 0.019 0.100 0.091 0.005 0.003 0.007 0.005 0.046 0.074 0.007 0.007 0.047 0.037 0.069 0.064 0.010 0.010 0.193 0.156 0.032 0.034
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 11, SEPTEMBER 1974
0.034 0.042 0.042 0.044 0.159 0.172 0.041 0.041 0.204 0.182 0.010 0.007 0.010 0.008 0.122 0.193 0.015 0.014 0.127 0.112 0.159 0.147 0.019 0.019 0.424 0.346 0.039 0.049
0.044 0.064 0.066 0.071 0.304 0.333 0.058 0.059 0.473 0.443 0.014 0.012 0.039 0.030 0.301 0.455 0.019 0.024 0.363 0.340 0.394 0.379 0.044 0.035 0.720 0.757 0.049 0.052
0.034 0.035 0.027 0.029 0.095 0.113 0.022 0.022 0.147 0.135 0.015 0.014 0.022 0.014 0.083 0.134 0.014 0.014 0.113 0.096 0.118 0.096 0.014 0.012 0.324 0.262 0.035 0.039
0.054 0.061 0.049 0.054 0.220 0.255 0.047 0.046 0.411 0.367 0.024 0.020 0.044 0.017 0.225 0.350 0.022 0.024 0.289 0.262 0.298 0.291 0.020 0.020 0,678 0.593 0,044 0.054
0 . 0 7 4 ' o-14 0.088! 0.069'1 o-14 0.0781 0.472)) op12 0.524> 0.073'[ high alkaline 0.069 0 . 8 7 2 ( low 0.8081 temperature 0.042\ 0.034/ 0.095) 0.068/ 0.495 0.691) 0.034 0.0301 0.622) 0 . 5 9 2 ) 0-11 0.6491 0 . 6 3 9 ' 0-11 0.051\k 0.0521 1.204' 1.087i 0.0541 0.0611
for two reasons: a ) the Bates-Guggenheim convention has been assumed valid at a n ionic strength (ten times greater than originally intended; b) if used in cell 1, the liquid junction potential effect will be large. Neither of these have any bearing on the new method of testing. Accurate knowledge of the pH(S) values of the test solutions is not essential. Application of the New Method of Testing to Commercially Available pH Glass Electrodes. All the principal U.K.manufacturers kindly donated samples of their current ranges of pH responsive glass electrodes for testing by the new method. To these were added a few other electrodes available in the U.K. Duplicate electrodes were conditioned according to instructions given in the manufacturer’s literature. These referred to 0 . M HCl or distilled water for various specified times. In the absence of a recommendation, electrodes were conditioned in distilled water overnight. Results are given in Table V of tests of 15 commercial electrodes in the buffers B1 B6 which contain no Na+ ions, and in Table VI for the tests a t constant pH and increasing Na+ concentration. Also given in the tables are the manufacturers’ recommended range of use for their electrodes, but the type and source of the electrodes has been otherwise concealed by the allocation of code letters. Duplicate electrodes (denoted by a subscript) usually showed identical performance from the first time of use after the recommended conditioning treatment. In a few cases, one of a pair showed higher errors associated with drifting potentials, but the situation improved on further conditioning. Electrode Sp was not a new electrode, but had been in routine use for over a year. Electrodes recommended for the pH range 0-11 or 12 (e.g., N, S, U, V, X) often showed large potential-time variations a t the highest pH and Na+ concentrations. Even at pH 9.5 and pNa 0, electrodes N, P, and V show errors of about 0.1 pH and electrode X is even worse.
-
Of the electrodes recommended for the full pH range (014), Q and T are clearly the best, with K, L, M, R, W close behind. The electrode 0 recommended as “high alkaline” was no better than the full-range electrodes. It may be pointed out that some electrodes (0, Q, T) show apparently larger errors in solution B6 (Table V) than in the corresponding sodium ion-containing buffers (Table VI). One possible explanation for this is that the transfers in sodium-free solutions were done some weeks after the other tests, and it is well known that errors increase with the age of the electrode It should be recalled that B6 does contain a small amount of alkali metal ions (5 x 10-4M). The effect of carbon dioxide contamination also leads to an apparently increased error of the glass electrode, but this was not the reason here. We conclude that the new method of testing presented here, and appraised by studies on 30 commercial samples of the currently available U.K.range of glass electrodes, satisfactorily provides the analytical chemist with a simple means for evaluating glass electrode performance in alkaline buffers with and without sodium ions.
ACKNOWLEDGMENT This work was carried out with the encouragement of the British Standards Institution Committee LBC/16 concerned with Glass Electrodes and pH meters. We are indebted to the various past and present members of the committee for their helpful suggestions and in particular to its former Chairman, the late J. E. Prue for his advice. We acknowledge the support of the U.K.Manufacturers of glass electrodes with their gifts of glass electrodes.
RECEIVED for review December 31, 1973. Accepted February 19, 1974. We are grateful to the Instituta de Alta Cultura, Portugal, for granting financial support, and to the University of Lisbon for study leave, to one of us (MFGFCC).
Alternating Current Polarographic Determination of Uranium in Complex Minerals Characterized by Electron Probe Analysis A. M. Bond Department of Inorganic Chemistry, University of Melbourne, Parkville, 3052, Victoria, Australia
V. S. Biskupsky and D. A. Wark Department of Geology, University of Melbourne, Parkville, 3052, Victoria, Australia
Polarographic methods generally do not have the required specificity for undertaking the direct determination of a particular element in complex systems such as those encountered in geochemical analysis. In this work, it is shown that phase-selective high frequency ac polarography enables the determination of uranium in extremely complex minerals such as euxenite, samarskite, thorite, thorianite, and monazite and in glasses without the usually required separation procedures. The minerals are completely characterized by electron probe data, to show the wide range of other elements tolerable. The attempted determination of uranium by atomic absorption spectrometry, X-ray fluorescence, and
fission track analysis is also reported. The dlfflculties encountered in these methods because of the large matrix corrections required, or other reasons, demonstrate the usefulness of the proposed polarographic method. The specificity, not usually attributed to polarography, results from the ability of the high frequency ac method to discriminate against nonreversible electrode processes and species reduced at more positive potentials than uranium. However, high quality polarographic instrumentation is needed as the use of the conventional two-electrode cell arrangement can give rise to undesirable Instrumental artefacts, possibly explaining previously reported interferences.
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, NO. 11, SEPTEMBER 1974
1551
