Study of Errors in Determination of Hydrogen Ion Concentrations in Rainwater Samples Using Glass Electrode Method

ANALYTICAL SCIENCES AUGUST 1998, VOL. 14 749 1998 © The Japan Society for Analytical Chemistry

Toru OZEKI*†, Yasuo TSUBOSAKA*, Shizu NAKAYAMA*, Nobuaki OGAWA** and Takashi KIMOTO***

*Hyogo University of Teacher Education, Shimokume, Yashiro-cho, Kato-gun, Hyogo 673Ð1494, Japan **Faculty of Engineering and Resource Science, Akita University, Tegata Gakuencho, Akita 010Ð8502, Japan ***Research Institute of OceanoChemistry, Funahashi-cho 5-19, Tennoji-ku, Osaka 543Ð0024, Japan

The evaluation of the hydrogen ion concentration using the glass electrode method sometimes gives very serious errors when the method is applied to natural water samples of low ionic strength such as rainwater. The activity coefficient of the hydrogen ion and the liquid junction potential of the reference electrode are the sources of the error. In this report, the Ð4.5 Ð3 relations between the pH reading and the ionic strength of the sample solutions of 10 N H2SO4 (N=equiv dm ) were examined for eleven commercially available electrodes. Results were grouped into three typical patterns. Quantitative estimates of the inaccuracies of the determination of hydrogen ion concentration were given, based on the calculation of Donnan potentials over the liquid junction and DebyeÐHückel estimates of the activity coefficient. Recommendations for the choices of the reference electrode as well as the choices of the measurement protocols are also reported.

Keywords Measurement error, pH, glass electrode method, rainwater, activity coefficient, liquid junction potential, diffusion potential, Donnan potential

Acid rain is one of the serious environmental prob- phenomenological. Some studies have recommended lems. Even in Japan many trees and artificial construc- the ceramic type reference electrode8, while others have tions such as buildings have come to suffer from acid recommended the sleeve type.10 Furthermore, there rain. In order to learn the pollutant sources and mecha- was another conflict as to the stirring of the sample nisms of acid rain, we have developed a statistical tool solution.8,10,11 Brezinski has given some theoretical called “a constrained oblique rotational factor analy- considerations. He has interpreted the contribution of sis”.1,2 When the rainwater samples of 1991 to 1995 the liquid junction potential from the Donnan equilibri- collected by our group were analyzed by this method, um at the junction surface having the fixed charge.6 the under-estimation of the anion concentration for the But his study still has been limited in qualitative expla- samples of pH larger than 5.6, and under-estimation of nation; he has not determined what is the fixed charge. the cation concentration for the samples of pH less than Figure 1 shows the plots of the pH value measured by 5.6 were found. The reason of the former case was that three kinds of combination electrodes (A, B, C) for the Ð4.5 Ð3 bicarbonate ion was not measured. And the reason of solutions of 10 N H2SO4 (N=equiv dm ) with vari- the latter case is the measurement error of the pH. ous concentrations of KCl. Many environmental scien- Usually, concentrations of major ions in rainwater are tists using the glass electrode method may expect the quantified by ion chromatography; but the H+ ion con- pH 4.5 for these solutions; but the measured pH values centration is converted from the pH value measured deviated from the 4.5 line. Quantitative estimates of using the glass electrode. The error 0.1 in the pH these deviations were examined using a theoretical value, DpH, gives 25.9% error in the hydrogen ion con- model in this report. centration, D[H+]; and 0.2 of the DpH gives 58.5% error in D[H+]. In the 1980’s many papers were published about the Experimental performance tests of the glass electrode method. The problems on the evaluation of the hydrogen ion concen- The reference electrodes and the glass electrodes tration using the glass electrode for natural water sam- Eleven reference electrodes with various types of ple of low ionic strength have been pointed out.3Ð10 The junction were used to check their response patterns on liquid junction potential of the reference electrode has the pHÐlog(I) plot (I is the ionic strength of the sample been related to the errors. But most of the works were solution). The list of the electrodes is shown in Table 1. Nine were combined electrodes; two were single † To whom correspondence should be addressed. electrode types. Examination of the value of the pH 750 ANALYTICAL SCIENCES AUGUST 1998, VOL. 14

Table 1 List of the reference electrodes examined

(A) (B) (C) (D) (E) (F) (G) (H) (I) Maker Product Junction Junction Slope Flow rate Response G-electrode name type material (mV/pH) (ml/h) type pH meter

OS-1 ORION 800300 sleeve sleeve-movable 58.6 50.0 A (*G1) (*M1) OS-2 ORION 900100 sleeve sleeve-movable 58.6 85.7 A (*G1) (*M1) HSM-1 HORIBA 6377-10D sleeve sleeve-movable 58.2 46.8 A (*G2) (*M2) HS-2 HORIBA 6377-10D sleeve sleeve-fixed 58.2 39.1 B (*G2) (*M2) HCA-1 HORIBA 6378-10D ceramic alumina 58.2 22.0 A (*G2) (*M2) HCA-2 HORIBA 6366-10D ceramic alumina 58.0 0.7 B (*G2) (*M2) HCAS-1 HORIBA 6328-10C ceramic alumina-silica 58.3 4.1 B (*G2) (*M3) HCAS-2 HORIBA 6328-10C ceramic alumina-silica 58.2 2.7 C (*G2) (*M3) HCAS-3 HORIBA 6328-10C ceramic alumina-silica 57.7 1.5 B (*G2) (*M3) TCAS-1 TOA GST-5311C ceramic alumina-silica 58.6 2.2 C (*G2) (*M4) TCAS-2 TOA GST-5311C ceramic alumina-silica 58.6 29.2 C (*G2) (*M4) (*G1) with a glass electrode ORION 9101-B; (*G2) combined type electrodes. The pH meters used are: (*M1) CORNING 125; (*M2) HORIBA F22; (*M3) HORIBA M8; (*M4) TOA HM-30S. reading and that of the emf indicated that all of the Procedure of pH measurement slopes of the potential to a unit pH were near Nernstian. The pH value was measured according to the follow- The term “flow rate” used in Table 1 indicates the rate ing protocol based on the recommendation in the ref.8. of the flow of the internal solution of the reference The electrodes and pH meter were standardized using a electrode through its junction. In order to determine 0.05 M (M=mol dmÐ3) potassium hydrogenphthalate the values, each electrode was dipped in pure water in a standard (pH 4.005 at 25ûC); and the Nernstian 50 ml beaker for one day, while we kept the inlet hole response was ascertained using a 0.025 M Na2HPO4+ 12,13 of the internal solution open. The value of the flow rate 0.025 M KH2PO4 standard (pH 6.865 at 25ûC). was estimated from the amount of the decrease of the A clean beaker containing a portion of the sample solu- internal solution and/or from the increase of the ClÐ tion was set in the water bath of 25.0±0.5ûC. After the concentration of the water in the beaker using an ion temperature of the solution came into a regulated region, chromatography system mentioned later. The value of the electrodes were rinsed first with distilled water of the flow rate gives a measure of the porosity of the high purity and then were rinsed with a portion of the junction. The most right column of Table 1 shows the sample solution. Then the electrodes were dipped into glass electrodes and pH meter used for the measure- the beaker. After stirring modestly for 1 min, the solu- ments. tion was allowed to settle to a quiescent state for 5 min. Both the value of the pH reading and that of Preparation of sample solutions the potential in mV units were recorded. The pH Sample solutions were 10Ð4.5 N sulfuric acid solutions value under stirring condition was also measured. The of several ionic strengths adjusted by potassium chlo- magnetic-stirrer used was the Top. Lab NR11 type. ride. The chemicals used here were analytical grade The rotation was about 500 rpm. regents. The potassium chloride salt was recrystallized so as to remove contamination of excess acid or base. In order to ascertain the hydrogen ion concentrations of Results and Discussion these sample solutions, a micro-titration based on the Gran plot method was applied.7Ð9 The titrations were Grouping of the electrode responses carried out by using an auto burette (Metrohm Multi- Measurements of pH from the lowest ionic strength Dosimat 665 system) and a pH meter (Horiba pH meter solution to the highest ionic strength solution were F-22, a glass electrode Horiba 6378-10D). The sulfate counted as one cycle; and five cycles of measurement ion concentrations were also determined by ion chro- were carried out. The response types (pHÐlog(I)) mea- matography (Dionex 2000i Model, AG4A guard col- sured for the eleven electrode couples can be grouped umn, AS4A separation column with ASRS suppressor). into three kinds, whose typical patterns are shown in The determined concentrations of the hydrogen ion Fig. 1. The circle points in this figure are the averaged were in accordance with those of a sulfate ion with neg- values of the five measurements; and the vertical error ligible errors, as expected from the chemical formula bars are their standard deviations. Most of the real H2SO4. The resultant concentrations of the hydrogen rainwaters have ionic strengths less than 0.001 M; thus ion in the sample solutions were ascertained to be the estimation of the hydrogen ion concentration by 10Ð4.5±0.02 N. using these electrodes tends to under-estimate. Table 1 shows that there are considerable variations in the flow ANALYTICAL SCIENCES AUGUST 1998, VOL. 14 751

Fig. 1 The plots of the pH value measured for the solutions of Ð4.5 Fig. 2 Effect of stirring. The broken lines and the symbol ´ 10 N H2SO4 + xN KCl against their ionic strength I. The circle points are the averaged values of the five measure- denote the response patterns under unstirred condition; the ments; and the vertical error bars are their standard devia- solid lines and the symbol denote the response patterns tions. under stirred condition. rates and the response types of the electrodes even if differences arising at the following interfaces: they are of the same product name. It is likely to hap- pen due to different histories of electrodes such as the Ag /AgCl/ 0.1 M HCl /glass/ sample solution —————— ———— number and types of previous uses. The type-(A) elec- (10Ð4.5 N H SO , C M KCl) /junction/ 3.33 M 2 4 X ————— trode, to which most of the sleeve junction electrodes KCl/AgCl/Ag (1) —————— belong, shows almost constant pH shift independent with the ionic strength. The type-(B) electrode shows Many of the variables determining these potential dif- large positive pH shift at low ionic strength. The type- ferences are constant when only the sample solution is (C) electrode, to which many ceramic junction elec- varied. The dependence of the total emf upon the sam- trodes belong, shows positive pH shift at both the high ple solution is expressed as follows: and low ionic strength solutions. When the solution + was stirred, a negative shift of the reading of pH value DE=DEcont+(RT/F)ln[H ]+(RT/F)ln gH++DEjunc (2) was observed for the low ionic strength solution when + the type-(B) and type-(C) electrodes were used, as Here [H ] and gH+ are the concentration (molarity) and shown in Fig. 2. However, stirring causes little change the activity coefficient of the hydrogen ion of the sam- in the response of the type-(A) electrode. The broken ple solution. The term DEjunc is the liquid junction lines of the Fig. 2 denote the measurements for the potential of the reference electrode. The first term unstirred solution, and the solid lines denote the ones DEcont is constant and corresponds to the sum of the for the stirred solution. Davison et al. have reported a other potential differences at the interfaces constructing similar negative shift of the pH reading caused by stir- the circuit. The reading of the pH value using a pH ring.10 meter, pH*, corresponds to the DE.

Causes of the pH shifts pH*=(ÐDEF/2.303RT) + The glass electrode method is based on the Nernstian =(ÐDEcontF/2.303RT)Ðlog10[H ]Ðlog10gH+ response of the glass electrode to the activity of the Ð(F/2.303RT)DEjunc (3) hydrogen ion of the sample solution. The total electro- motive force (emf) between a glass electrode and a ref- When a calibration of the pH meter is carried out using erence electrode is given from the sum of the potential a pH 7 standard solution, the DEcont term is adjusted so 752 ANALYTICAL SCIENCES AUGUST 1998, VOL. 14 that the pH reading value pH* is expressed as: On the other hand, at both sides of the interface, the electric-neutrality condition has to hold; thus, + pH*=Ðlog10[H ]Ðlog10gH+Ð(F/2.303RT)DEjunc (4) + + Ð 2Ð [H ]d+[K ]d=[Cl ]d+2[SO4 ]d in solution + + Ð 2Ð The second term consisting of the activity coefficient of s+[H ]md+[K ]md=[Cl ]md+2[SO4 ]md in membrane the hydrogen ion gH+ can be calculated by the DebyeÐ (8) Hückel equation:12 where s is the fixed charge density in the membrane. Ðlog10g H+=AI / (1+Bå I ) (5) Using relation (7), Eq. (8) can be rewritten as

+ + Ð 2Ð 2 In this equation, the symbols A and B are quantities s+([H ]d+[K ]d)k=[Cl ]d/k+2[SO4 ]d/k (9) which vary with the temperature and dielectric constant of the solvent; for water medium, A=0.5115 and B Here =0.3291 at 25ûC. The symbol å is the “ion-size parameter” and a value 9 (Å in angstrom units) has been pro- k=exp(FDEDonn-S/RT) (10) posed for hydrogen ion.14 On the other hand, the liquid junction potential at the Thus junction part of the reference electrode may be modeled + + 3 2 Ð 2Ð as shown in Fig. 3. Generally speaking, the junction ([H ]d+[K ]d)k +sk Ð[Cl ]dkÐ2[SO4 ]d=0 (11) membrane can have a fixed charge s (mol dmÐ3) in it. + + Ð 2Ð Then the Donnan equilibrium occurs across the mem- As [H ]d, [K ]d, [Cl ]d, [SO4 ]d are known from the brane surface. In our case, there are expected two composition of the sample solution, the values of k, the + + Donnan equilibria at two surface parts of the mem- concentrations inside the membrane [H ]md, [K ]md, Ð 2Ð brane. Within the membrane a diffusion potential is [Cl ]md, [SO4 ]md, and DEDonn-S can be calculated with s also expected. Several treatments of such junction as a parameter. If the charge distribution is homoge- potentials have been reported.6,15Ð18 neous through the membrane, the concentrations + + Ð 2Ð The Donnan potential at the sample solution side is [H ]m0, [K ]m0, [Cl ]m0, [SO4 ]m0 at the other side of the presented as membrane and DEDonn-R at that interface can be calculated too, by using the composition of the internal solution + Ð DEDonn-S=EdÐEmd (6) of the reference electrode; namely, [K ]0=[Cl ]0=3.33 + 2Ð M; [H ]0=[SO4 ]0=0 M. where Ed denotes the potential of the sample solution; On the other hand, the potential difference within the and the Emd denotes the potential inside the membrane membrane is a diffusion potential. Several models to facing the solution. The sample solutions used for this estimate diffusion potential have been proposed. One + + Ð 2Ð experiment contain H , K , Cl , SO4 ions. These con- of them, the so-called Henderson’s equation based on a + + centrations (molarity) are denoted as [H ]d, [K ]d, constant-concentration-gradient assumption for each Ð 2Ð + + [Cl ]d, [SO4 ]d in solution, and as [H ]md, [K ]md, ion in the membrane, was applied in this work. This Ð 2Ð [Cl ]md, [SO4 ]md in the membrane. Then the following approach is adequate to our purpose, because the chem- relations are derived from the Donnan equilibria: ical composition of a low concentration side of the membrane can be well accounted in the estimation of + + 6,18 DEDonn-S=EdÐEmd=(RT/F)ln([H ]md/[H ]d) the potential: + + =(RT/F)ln([K ]md/[K ]d) Ð Ð =Ð(RT/F)ln([Cl ]md/[Cl ]d) DEDiff=EmdÐEm0 2Ð 2Ð 2 =Ð(RT/2F)ln([SO4 ]md/[SO4 ]d) (7) (RT·wiziDCi,m) (·wizi Ci,md) =Ð 2 ln 2 (12) (F·wizi DCi,m) (·wizi Ci,m0)

where DCi,m=Ci,mdÐCi,m0. These concentrations (molarity), DCi,m, Ci,md, Ci,m0 are obtained from the stage of calculation of the Donnan equilibria. The wi is the mobility of the i-th ion in the membrane, and is related to the equivalent conductivity li through an equation li= 2 wi|zi|F . Once the fixed charge in the membrane is known, the junction potential DEjunc=DEDonn-R+DEDiff +DEDonn-S can be calculated. Reversibly, the fixed charge density s can be optimized so that the response curve shown in Fig. 1 shall be reproduced. On the other hand, the mobility of the ion in membrane need not be the same as that in aqueous solution. Fig. 3 Model of the junction part of the reference electrode. The interaction of the chloride ion and AgCl precipitate ANALYTICAL SCIENCES AUGUST 1998, VOL. 14 753 clogged in the membrane can be expected. In this ceramic junction is a typical example of this type. The treatment, the equivalent conductivities of the other simulation result of this case is shown in Fig. 5. In this ions besides the chloride ion in the membrane lX(m) type, DEAct is almost same as that in Fig. 4, but the con- were assumed to be the same as those in aqueous solu- tribution of the DEDiff is small. The Donnan potential at tion lX(s); and that of the chloride ion in the membrane the membrane surface facing to the sample solution lClÐ(m) was optimized. DEDonn-S is very large at low ionic strength solution. The optimization parameters are s=5.66´10Ð5 M and + + 2Ð lX(m)=lX(s) X: H , K , SO4 fClÐ=0.968. The value of the fClÐ means that the mobility lClÐ(m)=fClÐlClÐ(s) (13) of chloride ion is a little bit slowed down (about 3.2%) as compared with that in aqueous solution. And the The fitting parameter fClÐ is the ratio of the lClÐ(m) in membrane has about 20 times the fixed positive charge the membrane to the lClÐ(s) in the solution. The equiv- of the type-(A) electrode. This charge causes the large alent conductivities of ions in infinite diluted aqueous shift of pH based on the DEDonn-S. solution are used for the l(s).18 Type-(C) electrode Type-(A) electrode The electrode HCAS-2 is a typical example of this The reference electrodes such as sleeve type gave the type. This electrode has been used for a long time simplest response of the pH reading to the ionic (over ten years) in our laboratory for the measurement strength, namely almost constant DpH shift. The calcu- of environmental water samples. This electrode does lated response curve using the optimized two param- not show any problem as long as it is used for the mea- eters is shown in panel-(I) of Fig. 4. The parameters surement of the solution of high ionic strength; but it Ð6 are s=3.0´10 M and fClÐ=0.990. These values mean gives large errors to the solution of low ionic strength. that there is only a small charge within the membrane; The simulation result of this case is shown in Fig. 6. and the ClÐ ion can move in the membrane at the same The optimizing parameters are s=4.83´10Ð5 M and mobility as in the aqueous solution. fClÐ=0.902. The Donnan potential DEDonn-S is very large The contributions to the total DpH are shown in the at the low ionic strength solutions, which is the same as panel-(II), where the calculated potentials are converted the type-(B) electrode. The difference to the type-(B) into pH units. The DEAct is the potential due to the electrode is that the type-(C) electrode has the DEDiff of activity coefficient of the hydrogen ion. DEDiff is the a right-up slope, as shown in panel-(II) of Fig. 6. This diffusion potential within the junction membrane. The trend in DEDiff comes from the fact that the mobility of sum of these two potentials, DE=DEAct+DEDiff, deter- the chloride ion in the membrane is slowed down about mines the major part of the total pH shift DpH. 9.8% as compared with that in solution.

Type-(B) electrode The electrode HCAS-1 having an alumina-silica

Fig. 4 Response of the type-(A) reference electrode to Fig. 5 Response of the type-(B) reference electrode to the ionic strength of the solution (HSM-1). the ionic strength of the solution (HCAS-1). 754 ANALYTICAL SCIENCES AUGUST 1998, VOL. 14

charge of silica is about 2,19 and most of the pH values of rainwater are 4 to 7; thus such a ceramic membrane can not carry positive charge in it. Rather the participa- tion of KCl in the fixed charge is obvious, because the charge has decreased when the type-(C) electrode was immersed in 3 M KCl several days. Our hypothesis is illustrated in Fig. 7-(A). The internal solution of the reference electrode is concentrated 3.33 M KCl solution; thus some of the electrode AgCl 1Ðx is dissolved to make anionic [AgClx] ions. Such ions diffuse out to the sample solution through the junction membrane. As the rainwater solution usually does not contain a high concentration of chloride ion, the 1Ðx [AgClx] ions precipitate AgCl on the surface of the membrane. The clogging of AgCl has been warned about by several authors.5,20 The chloride ion in the membrane may interact with this AgCl, but the other ions such as potassium ion scarcely interact. This seems to be the reason why the mobility of the chloride ion is slowed down in the membrane. Fig. 6 Response of the type-(C) reference electrode to On the other hand, the junction part of the reference the ionic strength of the solution (HCAS-2). electrode giving the fixed positive charge looks black. When a reference electrode is used in the bright room for a long time, its junction membrane becomes black The origin of the anomalous response of pH vs. log(I) because the above mentioned AgCl is photo-decom- The anomalous response is caused by the fixed posi- posed by light to give rise to silver metal. Thus the tive charge and the slow-down of the mobility of chlo- mixture of silver metal and AgCl precipitate is clogging ride ion in the membrane. Brezinski has also proposed the membrane. Suppose there is some silver metal the fixed positive charge in the membrane.6 But he has upon the insulating substratum, as shown at the right not concluded what is acting as positive charge. In our part of Fig. 7 (B). The PZC, potential of zero charge, experiments, junctions giving the anomalous response of a silver metal is +0.05 V vs. NHE.21 However, the were ceramic type made of silica. But the pH of zero silver metal contacting to AgCl precipitate has the fol-

Fig. 7 (A) Effect of fixed positive charge and AgCl precipitate in the junction membrane on ion transfer. (B) The origin of the positive charge on silver metal in contact with AgCl precipitate. ANALYTICAL SCIENCES AUGUST 1998, VOL. 14 755 lowing potential: trodes such as the one with sleeve type junction give almost constant shift in the reading of the pH from the Ð + E=+0.2222Ð0.059log10[Cl ] (14) value expected from Ðlog10[H ]. Thus the estimation of + the true value of Ðlog10[H ] is possible by subtracting where the first term is the standard redox potential of this pH shift from the value of the pH measured. In 0 the Ag/AgCl, EAg/AgCl. Equation (14) means that the Fig. 8, the panel (II) shows the distribution of the ionic potential of the silver contacting to AgCl becomes balance ratios (total anion/total cation) calculated for more positive as the chloride ion concentration is 30 rainwater samples collected during 1996, whose pH decreased. The reason is that the precipitate AgCl values are less than 5.6; this distribution plot obviously tends to dissociate so as to supply chloride ion into shows the central position of the ionic balance value solution; but the counter silver ion is not a free ion in larger than 1, meaning an excess anion concentration the solution; rather, only the positive charge distributes and less cation concentration. Now, DpH=0.1 was cor- over the silver metal. With decrease of the chloride ion rected; then the new distribution plot shown in panel-(I) concentration, the more positive charge distributes on was obtained. Apparently this distribution plot has a the silver metal. This positively charged silver metal maximum at the center position. The measurement seems to act as a fixed positive charge in the mem- using the type-(A) reference electrodes giving a con- brane. stant shift of pH is advantageous because the correction However, at the part of the membrane close to the ref- of the DpH is easily carried out. erence electrode where the chloride ion concentration is We’d like to recommend the use of the type-(A) ref- high, some adsorption of chloride ion to the AgCl is erence electrode, and the calibration using a quality expected. Such part works as the fixed negative control sample (QCS) solution of low ionic strength, in charge. It means that charge distribution varies with addition to the nominal pH 4 and 7 buffered solutions. the distance from the sample solution. The parameters A measurement procedure of the QCS solution of veri- obtained in this work correspond to the ones at the part fied pH such as pH 4.00, and the correction of the bias of the membrane close to the sample solution. The DpH have been proposed by the IUPAC, too.22 The compositional change of the sample solution seems to sleeve type junction, however, is easily contaminated be converted to the change of liquid junction potential by acid, base, or pH-buffering substance. Thus, the through the properties of such part of the membrane. internal solution of the reference electrode should be renewed frequently. The effect of stirring of solution on the response Some researchers have reported the advantages of the stirring;11 but others have warned about the error intro- duced by the stirring.8,10 As is shown in Fig. 2, the pH values read for the solutions of high ionic strength are almost the same, regardless of whether the procedure was with stirring or without; but those read for the solutions of very low ionic strength tend to shift to lower values. Especially, the type-(C) electrode shows a very anomalous change of the response. The reading on the pH-meter using the type-(C) electrode, HCAS-2 of Fig. + 2, gives a value of about 4.15, although Ðlog10[H ] is 4.5. The positive increase of DpH at low ionic strength region under unstirred condition is the contribution of the DEDonn-S term. This Donnan potential generates when a compositional equilibrium holds between the surface of the membrane and the bulk of the sample solution. If the stirring breaks the equilibrium and brings the composition of the surface close to that of the bulk solution, the potential DEDonn-S term will decrease. Then the sum of the potentials may give the response curve shown by the solid lines of Fig. 2. If the junction of the electrode does not have the fixed charge and the contribution from DEDonn-S is negligible, the stirring does not affect the pH response. The type- (A) electrode is such a case. Fig. 8 Distribution plots of the ionic balance (åanion/ Choice of the electrodes and correction after measure- åcation) for real rainwater samples of pH less than 5.6, mea- ment sured using the type-(A) electrode (HSM-1); (I) after correc- As mentioned above, the type-(A) reference elec- tion and (II) before correction of DpH shift 0.1. 756 ANALYTICAL SCIENCES AUGUST 1998, VOL. 14

11. Y. Asano, S. Ito and F. Kobayashi, Nippon Kagaku Kaishi, We would like to thank Dr. Toshiyuki Osakai of Kobe 1980, 1516. University for his helpful suggestions. This work was support- 12. R. G. Bates, “Determination of pH: Theory and Practice”, ed by Grant-in-Aid (No.07640806) for Scientific Research from Wiley, New York, 1973. the Ministry of Education, Science, Sports and Culture of Japan. 13. A. K. Covington, R. G. Bates and R. A. Durst, Pure Appl. Chem., 55, 1467 (1983). 14. J. Kielland, J. Am. Chem. Soc., 59, 1675 (1937). References 15. K. H. Meyer and J. F. Sievers, Helv. Chim. Acta, 19, 649 (1936). 1. T. Ozeki, K. Koide and T. Kimoto, Environ. Sci. Technol., 16. T. Teorell, Proc. Soc. Exp. Biol. Med., 33, 282 (1935). 29, 1638 (1995). 17. T. Teorell, Prog. Biophys. Biophys. Chem., 3, 305 (1953). 2. T. Ozeki, K. Koide, N. Ogawa, T. Adzuhata, M. Kajikawa 18. T. Hanai, “Membrane and Ions” (in Japanese), Kagaku- and T. Kimoto, Anal. Sci., 13, 169 (1997). Dojin, Tokyo, 1978. 3. J. N. Galloway, B. J. Cosby and G. E. Likens, Limnol. 19. W. Stumm, “Chemistry of the Solid-Water Interface”, Oceanogr., 24, 1161 (1979). Table 2.2, John Wiley & Sons, Inc., New York, 1992. 4. S. Y. Jr. Tyree, Atmos. Environ., 5, 57 (1981). 20. S. Ito, H. Hachiya, K. Baba, Y. Asano and H. Wada, 5. D. P. Brezinski, Anal. Chim. Acta, 134, 247 (1982). Talanta, 42, 1685 (1995). 6. D. P. Brezinski, Analyst [London], 108, 425 (1983). 21. R. Tamamushi, “Electrochemistry” (in Japanese), Table 4.2 7. N. R. McQuaker, P. D. Kluckner and D. K. Sandberg, and Appendix Table 7, Tokyo-Kagaku-Dojin, Tokyo, 1974. Environ. Sci. Technol., 17, 431 (1983). 22. R. A. Durst, W. Davison and W. F. Koch, Pure Appl. 8. W. F. Koch and G. Marinenko, ASTM Spec. Tech. Publ., Chem., 66, 649 (1994). 823, 10 (1983). 9. C. A. Johnson and L. Sigg, Chimia, 39, 59 (1985). (Received January 19, 1998) 10. W. Davison and C. Woof, Anal. Chem., 57, 2567 (1985). (Accepted April 23, 1998)