596
And. Chern. 1980, 52, 596-598
Plate-Shaped Silver/Silver Halide Reference Electrode for Determination of Fluoride Ion in Microliter Solution with Fluoride Ion Selective Electrode Koichi Chiba, Kin-ichi Tstrnoda, Yoshio Umezawa, * Hiroki Haraguchi, Shizuo Fujiwara, and Keiichiro Fuwa Oepartment of Chemistry, Fwct!tv of Science, The University of Tokyo, Hongo, Tokyo 113, Japan
for natural samples, the human urine from two persons was taken. For actual measurements, one to one mixtures of buffer solutions mentioned above and human urine or the standard solution were prepared. Procedures. (1) Place the plate reference electrode in a horizontal position. (2) Drop by micropipet a few microliters of sample solution onto a plate reference electrode. (3) Lower the ISE body in an upright position onto the plate reference electrode where the sample droplet exists. (4)Close the necessary electric circuit and measure the potential after a certain pre-determined time duration ( 5 min in this case to guarantee the equilibrium). (5) Pull up the ISE body and rinse both the reference and ion selective electrodes with deionized and distilled water for the following measurement. Measurements were performed at 18 f 0.5 "C. Of course, the sample solution was not stirred during the measurement.
Recently, the preqent aiithors developed a method for the determination of ultratrare fluoride by aluminum monofluoride (AIF) moleriilar 9hsnrption spectrometry ( I , 2). In the experiment, a carbon rod furnace was used as a formation cuvette of AIF and flrioride in a 5 ~ 1sample , could he analy7ed at the subnanogram level bv measuring the molecular absorption of the AlF hand near 227.46 nm. T h e method has been applied tn the analysis of fliioride, and the analytical data were consistent with thnqe analy7ed by an electrochemical method iitiliying a flrioride inn selective electrode (ISE). According t o the experiment, the 1SE method appears to be disadwntageorio over the AlF molecular absorption method In the sense that the conventional ISSE method requires a large volume sample, more than a few milliliters. Therefore, it is highly dwirahle to devise a simple system for the ISE method t o analyze ultratrace fluoride in a few microliters of sample soliitinn, particularly of clinical importance. Tt has been reported thet the measurement of fluoride in several ten microliters of solution with TSEs has been performed using qpecial tailor-made electrode assemblies ( 3 - 5 ) . However, in these cases, the design of the TSE itself, first of all, has to be modified and a l w the reference electrode including the salt bridge (junction) qhould be made verv small so that the contact of electrodes with the small volume of solution is well made. Based on these consideration$. even some commercial micro-ISF: systemq became availahle (6), although they were extremelv expensive. In the present paper, a novel and simplified approach for a microliter-ISE system was devised using commercially availahle conventional TSEs without any modification. An essential feature of the present system is the use of a homemade plate-shaped silver/qilver halide reference electrode. Containers such as a beaker are not needed; instead, a small thin laver space between a plate reference electrode and the flat bottom of the ME sensor is conveniently used for holding R few microliters of sample solution. Schematic representation of the principle for the present method is shown in Figure 1. A s an illustrative example. a fluoride ISE was tested to evaliiate the feasibility of the proposed method.
RESULTS AND DISCUSSION C h a r a c t e r i s t i c s of the P l a t e - S h a p e d Ag/AgX R e f e r e n c e Electrode. T h e reference electrode used here is not necessarily an ideal one b u t is a sort of pseudo reference electrode. Because of the geometrical requirement for the small sample volume, it is difficult to use the salt bridge. For this reason, the direct contact of the Ag/AgX plate with a sample solution results. Obviously, this causes some unwanted phenomena like the shift of the reference potential, which should be prevented in some way. This is particularly true for the natural samples where the level of the C1-, for instance, changes from one sample to another and this influences the potential of the plate-shaped Ag/AgCl reference electrode. Figure 2 shows the dependence of the observed potential of an Ag/AgCl plate-shaped reference electrode vs. a fluoride ion selective electrode on the added KC1 concentration in a sample solution. T h e sample volume used for each measurement was 20 wL. T h e shift of the potential with added C1- was found to be those expected from the Nernst equation for the Ag/AgCl electrode. For the Ag/AgBr reference electrode, the influence of the C1- became much smaller (Figure 3). In the case of an Ag/AgI plate reference electrode, the shift of the observed potential with increasing C1- concentration was negligible. These results can be interpreted on the basis of the fact t h a t the plate-shaped Ag/AgX reference electrode in this case behaves like a X- ion selective electrode: It is known t h a t iodide and bromide ion selective electrodes have the selectivity coefficient of 400(Br-/ClU and 106(I-/C1-) to chloride ions, respectively (6). Therefore, for the Ag/AgI plate reference electrode which has the largest selectivity coefficient to C1-, the influence of the chloride ion can mostly be eliminated if an appropriate amount of iodide ions is added t o the sample solution. Having these considerations in mind, one can still use these plate-shaped Ag/AgX electrodes for practical reference electrodes upon choosing appropriate experimental conditions. I t is noteworthy t h a t the Ag/AgX electrode as a practical reference electrode in this case should be used in sample solution with either a constant or the saturated C1- activity to prevent the shift of the reference potential following the fluctuating C1- activity. Because the 1- does not frequently exist in natural samples, the Ag/AgI reference electrode seems the best choice for preventing the C1 interference on the plate-shaped Ag/AgX reference electrode. T h e dependence of the observed potential of the Ag/AgI plate-shaped reference electrode vs. the fluoride ion
EXPERT M E N T A J, Reference Electrode. Plate-shaped silver/silver halide(Ag/ AgX) reference electrodes were made by anodic oxidation of silver plate (? cm x 2 c m , 0 5 m m thick) in 0.1 M KC1, KRr, or KI solution at +0 5 \' vs SCE for ca 5 min The Ag/AgCl, Ag/AgBr, and 4 p / A g l reference electrodes thus made were soldered with cilver wire for electrical contact w t h a potentiometer (TOA Model HM 5R) A fluoride TSE used wac from Denki Kagaku Keiki Co : A polvethylene layer surroundc the solid membrane (IAaF1in this race) 2nd prntriidec (ahout 0 2 m m ) from the electrode surface. Thic tvpe of ISE decign prevents direct electrical short circuit of the TaF, single crvstal with t h e siirface of the plate reference electrode Reagents. A F standard solution was prepared hy dissolving an analytical grade NaF in deioni7ed a n d distilled water. The tot81 ionir strength adjustment hiiffer (TTSAR) used has a composition of 1 M NaCI, 0 26 M CH,COOH, 0 75 M CH,COONa, and 0 001 M sodium citrate. "hen the AgIAgRr, and A g I A g I plate reference electrodes were used. 1 M K R r and KI, respectivelv. were fiirther added t o each hiiffer coliition For the Ag/ .4gC'l plate reference electrode, the buffer qoliitinn saturated with KC 1 waq also examined as shown in Table T 4*9 tvpical example 0003-270@/R0/0352-0596$010010
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1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL 52, NO. 3, MARCH 1980
__
- __
__
~
-
____
.
597
-
Table I. Determination of Fluoride Ion in Human Urine by t h e Fluoride Ion Selective Electrode plate-shaped A g i AgX reference electrode
._-
sample
convent io nal method, pg/mLb
-
--
~~~
AglAgI,
Ag/AgBr,
PdmL
clg/mL
AgIAgC1,
~
pglrnl,
IQ
0 44c
t
0.02
0 42
I
0 03
0 50
t
0 03
0 3 4 ( 0 33)'' + 0 02
I1
0.46c
2
0.02
0.46
i-
0 03
0 52
+
0 03
0
38 ( 0 15) + 0 02
'
A conven a A 2 0 - p L sample solution was used for each measurement except in the case of the conventional method tional Ag/AgCl reference electrode was used. Sample solution of 20 mL was used Stirring of the solution was done b y a magnetic stirrer. A 1 1 mixture of the TISAB solution and human urine ( i n volume) was used f o r each measurement ' Accurate t o 10.02 Values in parentheses were obtained when t h e sample solution was saturated with KC1 -
__
-
-
-
-
-
..._ -.
-
-
~~
3
4 Figure 1. Schematic diagram of the measurement system. (1) Fluoride ion selective electrode body. (2) Ag plate. (3) Ag/AgX. (4) Sample solution. (5) LaF, single crystal E/mv
1-
~
-_
5
_
- _. .
.
4
3
D F
CONCENTRATION OF FLUORIDE
Figure 3. Calibration curves for F- ion activities at various CI- conliO,
centration levels using Ag/AgBr plate reference electrode. Sample volume: 20 pL ( 0 )CI- not added, (D) CI- 1 M added, (A)CI 3 M added
100.
~
5oi
5 4 3 CONCENTRATION OF FLUORIDE
D F
Figure 2. Calibration curves for F- ion activities at various CI- concentration levels using Ag/AgCI plate reference electrode. Sample volume: 20 pL. ( 0 )CI- not added, (D) CI- 0.5 M added, (A)CI- 1 M added, ( * ) CI- 3 M added
selective electrode on t h e added KCl concentration is shown in Figure 4. The standard deciations of the observed potential values of an Ag/AgI plate-shaped reference electrode in 0.2, 2 , a n d 20 ppm standard F- solution for ten repeated measurements were ~k1.43,f 1 . 2 3 , and h1.51 mV, respectively, where for the conventional electrode assembly, standard deviations of f0.99, f1.06, and h0.52 mV for the same standard solutions were obtained, respectively. Also, their observed potential values vs. a F- I S E in standard F- solution were virtually the same a t the three different electrode specimens of the Ag/AgI plate-shaped reference electrodes. These two results indicate t h a t the stability of the plate-shaped Ag/ AgI reference electrode is extremely good. T h e influence of possible interfering ions such as SO,2-, NO3-, CN-, a n d S2were also examined. However, addition of 0.1 M SO4*-a n d NO3- and 100 ppm of CN- and S' , respectively, did not cause any interference. D e t e c t i o n Limit. Figure 4 also shows the F- activity vs. t h e potential relation of the F-I S E by using a Ag/AgT plate reference electrode, where t h e slope of t h e potential vs. t h e log activity curve is about 58 mV which is close to that expected from the Nernst equation. Fifty microliters of sample solution were used in this case. T h e concentration (activity) range where t h e calibration curve starts to bend ( l o 5M) is
6
-5
3
1
D F
CONCENTRATION OF FLUDRlnE
Figure 4. Calibration curves for F ion activities at various CI- concentration levels using the Ag/AgI plate reference electrode Sample volume 20 UL ( 0 )CI- not added, (D)CI 1 M added, (A)CI- 3 M added
about one order of magnitude higher than that of the ordinary measurement The same trends were found with other types of microliter-electrodes previouslc reported ( 3 ) A possible reason whv the microliter-electrode in general is less sensitive t h a n the conventional one is either the slower electrode response in diluted solution or the enhanced influence of dissolution of LaF, into a small volume solution. Limit of Sample Volume. Figure 5 indicates t h e dependence of the observed potential values of the present system on the volume of the sample solution. The obsened potentials were virtuallv constant even if the amount of the sample volume waq decreased down to 3 p L When the sample volume was decreased to 7 pL. the corresponding concentration observed was ~ l i g h t l vhigher than the actual one This is probably because the effect of sample evaporation is no longer negligible
598
Anal. Chem. 1980, 52, 598-600
t h a t human urine contains about 0.2~-0.3M of chloride ions. These chloride ions cause some shift of the potential for the reference electrode as mentioned earlier. Table I shows the analytical results of fluoride ion determination in human urine with the three different plate-shaped reference electrodes; the results by a conventional electrode assembly are also cited. As shown in this table, our results were t h e same as those obtained with a conventional cell assembly with the Ag/AgI electrode. In the case of the Ag/AgCl reference electrode, our results were somewhat different from those obtained with the conventional cell assembly. Even in this case, however, an accurate result was obtained when both the sample and standard solutions were equally saturated with KCl, as can be seen in the parentheses of Table I. This is simply due t o the fact that the chloride ion activity in standard solution was different from t h a t in the actual urine sample solution. However, in the case of the Ag/AgI reference electrode, t h e interference of the C1- is negligible as mentioned earlier. In addition, after continuous use for 3 months, the reliability of t h e plate-shaped Ag/AgX reference electrode has not changed. This means that the aging of the Ag/AgX reference electrode and the potential shift due to photochemical effects in diffuse light conditions are not significant. Further advantages of the present method are its simplicity and inexpensive fabrication.
4001
350
i
I
300
123
5
10
20
VOLUME OF SAMPLES
50
PL
Figure 5. Effect of sample volumes on the potential of F- ISE using Ag/AgI plate reference electrode
T h e necessary solution volume for t h e present method is essentially dependent on t h e structure of the bottom of t h e ISE: The relative height of the polyethylene layer surrounding t h e solid membrane (LaF, single crystal in this case) with respect to the electrode surface is critical, because a small thin space structured by a polyethylene "wall" and the flat parallel layers of working a n d reference electrodes actually act as a sample holder for the present method (Figure 1). If this space is not small enough, i t cannot be filled u p with a few microliters of sample solution, and therefore the electrical contact of t h e ISE with t h e solution is not well performed. Because of this reason, we should choose the ISE which has a relatively low polyethylene layer surrounding t h e solid membrane. F l u o r i d e Ion in H u m a n Urine. As an illustrative example of natural samples such as body fluids, the F- concentration in human urine was chosen. The reason for this is that human urine is a mixture of a variety of substances and this fact seems most suitable for evaluating the effect of interference of foreign substances on the present experimental assembly. I t is known
LITERATURE CITED (1) K. Tsunoda, K . Fujiwara. and K . Fuwa, Anal. Chem., 45, 2035 (1977). (2) K. Tsunoda. K. Chiba, H. Harawchi, and K . Fuwa. Anal. Chem.. 51. 2059 (1979) (3) R A Durst, and K Taylor, Anal Chem , 39, 1483 (1967) (4) A S Hallswoth. J A Weatherell. and D Deutsch, Anal Chem , 48, 1660 (1976) ( 5 ) P. Venkaleswarlu. Anal. Chem., 46, 874 (1974). (6) G. J. Moody, and J. D. R. Thomas, "Selective Ion Sensitive Electrodes", Merrow Publishing Co. Ltd., Watford, Hentfordshire, England, 197 1.
RECEIVED for review August 6, 1979.
Accepted December 5,
1979.
Determination of Oxygen-I 8 Content of Water by Hydrolysis of Phosphorus Pentachloride and Measurement by Gas Chromatography and Mass Spectrometry with Selected Ion Monitoring Thomas R. Sharp and Robert D. Minard" D e p a ~ m e n of t Chemistry, The Pennsylvania State University, University Park, Pennsylvania
the accuracy of measurement of both the amounts of H,O and CO, used for t h e equilibration. Furthermore, the '*O of t h e H 2 0 is diluted during equilibration by t h e l6O of the C 0 2 sample. An innovation, which eliminates these problems, is the pyrolysis of an H 2 0 sample with guanidine hydrochloride (2). The CO, produced from pyrolysis contains oxygen with the same l80content as that of the original H 2 0 sample. The method, however, still requires t h e purification and manipulation of gaseous samples. Both of these methods have been widely used. A significant investment of time, expertise, and specialized equipment, however, is necessary for doing these kinds of manipulations. The method which we describe here does not require any gas handling a t all. Quantitation is necessary only for the measurement of ion intensities, and the calculations then involve only normalization of the results. These aspects make this
There is no question that "0 has been an extremely useful isotopic probe in elaborating a large a n d diverse number of chemical mechanisms. One of the most convenient sources of commercially available l80is [180]H,0. The irony therein lies, however, t h a t H 2 0 is one of t h e most difficult chemicals in which t o measure l80 content. Desorption of residual [ 1 6 0 ] H 2 0in the inlet systems of most mass spectrometers obviates direct measurement. A substantial literature describing various manipulations which have been developed t o indirectly measure t h e l80content of H20 has grown out of this difficulty. T h e classical method, developed by Cohn and Urey ( I ) , consisted of equilibrating a sample of H 2 0 with a known amount of COz, followed by determination of the "0 content of t h e COz,and back-calculating via the known equilibrium constant for this reaction. This method is dependent upon 0003-2700/80/0352-0598$01 OO/O
16802
I
1980 American Chemical Society
