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Walter Lund University of Oslo 1033 Biindern, OSIO3, Norway
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The Standardization of pH Meters A comparison of procedures for different types of instruments
When a nH meter is standardized aeainst a single standard buffer, i t is implicirlv assumed that the metw rrading for an eventual unknown solution will wrrrspmd to the value ~
~~
~
~
where pH(x) and pH(s) refer to the unknown and standard solutions, respectively, and E(x) and E(s) are the corresponding potentials. Equation (1) corresponds to the operational definition of pH; the pH(s) value refers to the conventional activity scale. T o obtain a correct value for pH(x) the nH electrode must exhibit an ideal resDonse of RTlnlOIF mV per pH unit. As glass electrodes sometimes have a slightly less-than-theoretical response, a second standard buffer is often employed in the standardization of pH meters, to check the behavior of the electrode in auestion. If an incorrect reading is obtained for the second buffer, one is faced with the problem of correcting for the non-ideal electrode response. The procedure for doing this depends on the type of pH meter used. The different approaches which may be employed are discussed below, and the limitations of the various procedures are pointed out. In some cases the standardization of the pH meter must be carried out at a temperature different from that of the actual measurement. In these cases the change in temperature is compensated instrumentally; however, as will he shown below, an exact temperature compensation is rarely possible. Measurements at Constant Temperature For accurate pH measurements it is highly recommended that the standard buffers and unknown solutions are all at the same temperature, and that the glass and reference electrodes are also allowed to reach this temperature before measurements are made. In the following discussion it is assumed that this condition is fulfilled. In routine pH measurements the p H meter is normally standardized against a single standard huffer, assuming an electrode response of RTlnlOlF mV per p H unit. From an
Variation in the cell potential with pH.The numbers refer toeither differentp* sitions of the slope control or to different temperatures. instrumental point of view the standardization implies simply that the offset control of the DH meter be adiusted until the correct pH rending is obtained for this buffer; the ofiset conrnd is labelled 'calil,ration." "stnndardi~ation." "huffer," ~~i~symn~etry"etc. hy different maout'act~~rers. The magnitude of the offset d e ~ e n d on s the "standard potential" E, uf 111e electrochemicai cell according to the equation ~~~
~
~~~
As already mentioned a second standard buffer is usually employed for checking the response of the glass electrode. If a low response is observed, it is possible to compensate for this instrumentally by means of the temperature compensator of the pH meter' (a low response corresponds to a low temperLBates, R. G., "Determination of pH," 2nd Ed., John Wiley & Sons, New York, 1973, p. 102.
Volume 56. Number 2,February 1979 / 129
ature, in terms of millivolt per pH unit). In many modern instruments the adjustment is done with a separate knoh, laheled "slope," "sensitivity," "mV1pH" etc., to avoid confusion with temperature compensation as such. The slope control, which covers at least the range 98-100% of the theoretical value RTlnlOIF mV ~ eDH r unit functions in exactlv the same way as the temperature compensator; what is said below about the slope control is therefore eauallv to the tem. . apolicahle .. perature compensator. The effect of the slope control is illustrated in the figure; the straight EIpH lines (see eqn. (2)) obtained for different settings of the slope control have a single intersection point, which is here called the slope point. Again there is no general agreement as to terminology; the slope point is also referred to as the "isoootential noint.", "isothermal intersection ooint." , "zero point," "electrical zero" etc. By definition, the slope point is that point on the nH meter scale which is unaffected by variations in the slope'control. Most direct-readinr! DH meters have a fixed slooe .. . . point. . which ctmt.slamds toa 1,117.0 reading. For analog instr1unent5 the slooe DOlnt will usually coincide with the mechanical iero of the meter, i.e. the meter reading indicated when the instrument is switched off. When in doubt the position of the slope point can he checked by setting the instrument (analog or digital) to the expected value with the offset (standardization) control, and then moving the slope control. If the instrument reading is unaffected by variations in the slope control, this reading corresponds to the slope point of the instrument. The position of the slope point is of particular concern in a two-buffer standardizing procedure. When the slope control is adiusted, the orieinal calibration of the pH meter will normall; he affecrrd,~~nless the instrumental slope point coincides with t h pH ~ \ ~ 1 u of e the first standard t)uffrr. In thr latter case an eventual low electrode response can he cornpensated simply hy adjusting the slope control until the instrument shows a correct reading also for the second standard buffer. If the slooe ooint does not coincide with the oH of the first huffer, the brigina~calibration must always h;checked once the slooe control has been moved. I t mav he necessarv to repwt thr sequenn. of srnndardizatiun~(offseradjustment aaainst buffer no. 1) and rcwunse nmrction (slo~t!control adjustment against huffer no. 2) many times before the correct positions of hoth the offset and the slope control have been established, resulting in accurate readings for both standard huffers. Unfortunatelv. none of the nrimarv standard buffers recommended for Gndardization'of p ~ h e t e r have s a pH value of exactlv. 7.0.. eorresoondine to the slope . .ooint of most DH meters. For less acc;rate work one might calibrate the;nstrument against one of the commercially available mixed buffers, which has a pH value of 7.0. When accurate measurements are needed, it is recommended that the pH meter Ile standardized against the primary stnntlanl h f f r r nearest phosphate buffer. This i~uffer to pH 7.0, which means the 1:I . . has a pH value of 6.88 a t 20°C. The procedure for electrode response correction discussed above. is ereatlv.. simnlified if the uH meter is eouinned with a varihdislope point. The slope'poiat of the p ' m;rr ~ can then he set to the exact \,slue ufthe first standard bulfer, and the rt:sl~onsecorrrcted with the slope nmrrol, without affectine the urieinal calibration. reeilrdless o i the DH valur of the first buffe;. ~ n f o r t u n a t e l yo z y the more expensive pH meters are equipped with a variable slope point.
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Effectof Temperature
In this discussion so far it has been assumed that the samples, standards, and electrodes are all at the same temperature, which is kept constant throughout the measurements. However, pH measurements are often carried out under
130 1 Journal of Chemical Education
conditions where this assumption is no longer valid. Thus, in field measurements one cannot normally wait for a complete temperature equilibration of solutions and electrodes. In industrial applications it is often very difficult t o control the temperature of the electrodes. In research work it may he of interest to studv s~ecificallvthe variation of DH with temperature. In most dfthese cases the standardizaiion of the pH meter must be carried out at a temperature different from that of the actual measurement. One then has to rely on the temperature compensation provided by the p H meter; unfortunately, this compensation is sometimes inadequate, as will he shown below. Normally, a variation in temperature will affect the true pH of the sample as well as the electrode system. For a given solution the variation in pH with temperature is not easily predicted; generally the pH-temperature coefficient varies from one solution to another. Strictly speaking, there is no quantitative relationship between pH values ohtained a t different temperatures. Therefore, the temperature compensat ion pnwided by most pH meters rurrecls mly fur the effect of 1rmper;lture on the elrctmde system. This efiect, which amounis to -0.2 mV or 0.003 p H unit per degree, normally predominates over the variation in pH with temperature for acidic samples, whereas alkaline solutions often have much larger temperature coefficients. Referring to eqn. (2) the temperature affects hoth Ex0and the slope factor RTlnlOIF. Provided the temperature variations are kept within certain limits (-20' according to Mattockz) the straight EIpH lines ohtained for a given electrode system at different temperatures will have a single intersection point, as illustrated in the figure. The intersection point, which corresponds to the pH a t which the emf of the pH cell is invariant with remln~aturc,is usually called the isr~pf,tmr~ol r ~ ~ . r nit/ (lt:~~cn(liootherypeofglassi~n(l ; referenceelectrodes commercial glass electrodes often have an iso-point around pH 7 when used in conjunction with a saturated calomel external reference electrode. The isopotential point is an important parameter when temperature comoensation is discussed. I t should not be confused with the slope point of the pH meter; as already explained the latter is a . purely. instrumental parameter, which d&ds only on the rltr tronic design of the instrument. Mcst man~lfactuwrsof pH meters do not distinguish hetween the two concepts, as demonstrated for instance by their choice of terminology. As indicated in the figure an adequate temperature compensation is possible only when the slope point of the pH meter coincides with the iso~otentialDH of the electrode system, and even then the compensatio~willbe valid only for a limited temperature range (-20'-see above), because of a slight variation in the &point with temperature. This variation results from the non-linear dependence of Ezo with temperature. Because of the difficulties encountered in carrying out a n exact t r n ~ p r r , ~ t ucompensation, rr one should whenever possiltle, and always in [he most accurate work, standardize the pH meter a t the temperature at which the unknowns are to he measured; under these conditions the standard potential EV0cancels out. When pH is tr, he measured at some given temperature, one should illso he anmareof any ttxnperature gradienrs within the pH cell. Freq~~entlv ~mlythe lower partsol thrglassand ref. rrenrr elrztrodes are immersed in the sample sdution; in these cases the calomel and silver/silver chloride elements of the external and internal reference electrodes may well he nearly at room temperature, regardless of the temperature of the
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%Mattock, G., "pH Measurement and Titration," Heywoad & Co. Ltd., London, 1961,~.190.
