REPORT FOR ANALYTICAL CHEMISTS
Standardization of Acidity Measurements Extension of the pH Concept to Mixed Solvents and Heavy Water
Roger G. Bates National Bureau of Standards Washington, D.C.
The modern concept of the operational pH value is examined in detail. Procedures by which the NBS pH scale (based on conventional hydrogen ion activity) was established are reviewed, and it is shown how these same methods and concepts can be extended to set up useful scales for acidity in nonaqueous and mixed solvent systems (using the unit pH*) and in heavy water (using the unit pD). The availability, in the form of standard reference materials, of two new pH standards and three new pD standards is announced.
τ ater this month the National -*-J Bureau of Standards will take a forward step in its continuing ef forts to broaden the base of practi cal acidity measurements. At this time, two new standards for the pH scale (at p H 3.8 and 10.0) will be made available. In addition, three standards defining a practical scale of pD in deuterium oxide (heavy water) will also take their places on the list of some 600 substances is sued as standard reference mate rials. These standards fix the prac tical pD scale in the range from pD 4.3 to pD 10.7 over the temperature range 5 to 50°C. They are in tended for adjusting commercial glass electrode pH meters which are then capable of yielding experimen tal pD values with an uncertainty less than 0.02 pD unit. In establishing these standards, the procedures on which a national pH scale was based some years ago have been followed, insofar as pos sible. The NBS pH scale is best described as a conventional scale of hydrogen ion activity. I t is the culmination of a considerable vol ume of experimental work, dating back to 1945, on acid-base behavior and acidity functions. I t owes its detailed character, however, to the adoption in 1959 of a single arbi trary ("conventional") numerical scale for the activity coefficient of a single ionic species. Standard ref erence materials for pH measure ment have been issued since 1945. It is appropriate at this time to re view the modern concept of pH and to consider means by which this concept can be usefully extended to apply to solvents such as alcoholwater mixtures and heavy water. Nature of the p H Scale
The dilemma of pH measurement is well known. I t has its origin in the essential incompatibility be tween the available experimental 28 A
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procedures for measuring acidity functions and the definitions of the degree of acidity that are most con sistent with the uses to which the numbers will be put. The hydrogen ion concentration is, of course, a quantity of unquestioned physical significance, yet there is no direct means of measuring this quantity accurately in most practical situa tions. The galvanic cell which is the heart of electrometric assem blies, on the other hand, responds to some combination of ionic activi ties, a combination in which the activity of the hydrogen ion is inex tricably linked with that of one or more counter ions. If pH is to be a function solely of hydrogen ion activity, a means must be found to separate the observed response into its components. It is appropriate, perhaps, to note here that there are persuasive argu ments against these attempts to separate the hydrogen ion activity from the activity of those anions with which hydrogen is associated in the solution whose acidity is of interest. These arguments recog nize quite rightly that no acid-base process of practical significance de pends solely on the activity of hy drogen ions, but that the complete expression of the equilibrium pro cess will always contain the activity of at least one other ion. The acid ity function p(aHyci), for example, possesses the thermodynamic valid ity that the po H or pH lacks. I t can often be measured quite conve niently with a glass electrode (or hydrogen electrode) and a silversilver chloride electrode, after the addition of a small known amount of a soluble salt to the "unknown" solution (1). There is considerable question, however, whether much has been gained by the incorpora tion of γοι- into the acidity func tion, when what is really needed in its place may be the activity coeffi cient of a wide variety of cations or
anions taking part in the equilibria or rate processes of interest. Consequently, the best approach seems to be one that ignores the plurality of counter ions and focuses attention solely on the activity of the protons or hydrogen ions to which the hydrogen electrode and glass electrode respond. Two facts of modern technical life have left their imprint on the definition of pH. The first is the very extensive, almost exclusive, use of the commercial glass electrode for the routine determination of the acidity of solutions and other media. The second is the acknowledged fact that the vast majority of practical acidity measurements need only be reproducible, inasmuch as a fundamental interpretation of these numbers in terms of hydrogen ion concentration or activity would be sheer delusion. These are the facts that have led to the adoption of an operational definition of PH: pH(X) = pHCS) +
j ^ i ï f o
tions, soft solids, moist soil, and slurries—in short, all media in which a reproducible emf can be measured. All who will take the trouble to examine the operational definition in terms of galvanic cell processes and the Nernst equation will easily convince themselves that pH(X) — pH( · fbf
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/ Laboratory
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37Î7 W Cortland Street CI cago, Illinois 60647 Loi al O't c « in N e * Yn r l' · C licago · Los Angeles Circle No. 83 an Readers' Seivice Card 34 A
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Acidity Standards for Heavy Water The increasing use of deuterium oxide in nuclear technology, chemi cal synthesis, and research in the biomedical fields has created a need for improved methods for determin ing acidity in this solvent. Efforts to fill this need have now led to the issuance of standard reference materials defining three fixed points on a scale covering the range pD 4.3 to pD 10.7. The reference solu tions prepared from these materials
REPORT FOR ANALYTICAL CHEMISTS
Table IV Standard Reference Values pH*(S) for the Measurement of Acidity (pH*) in 50 Wt. Per Cent Methanol-Water Solution (molality)
t, °C
0.02 H Ac, 0.02 NaAc, 0.02 NaCI
0.02 NaHSuc, 0.02 NaCI
0.02 KH2P04, 0.02 Na 2 HP0 4 , 0.02 NaCI
10 15 20 25 30 35 40
5.560 5.549 5.543 5.540 5.540 5.543 5.550
5.806 5.786 5.770 5.757 5.748 5.743 5.741
7.937 7.916 7.898 7.884 7.872 7.863 7.858
Ac = acetate
Sue = succinate
50% MeOH
PHOSPHATE
H20
50% MeOH SUCCINATE ACETATE
H20
10
20
SUCCINATE ACETATE
30
40
50
t, °C Figure 1 . C o m p a r i s o n of p H in water w i t h p H * in 5 0 w t per c e n t m e t h a n o l water C o m p o s i t i o n s of t h e s o l u t i o n s : p h o s p h a t e — 0 . 0 2 m KH2PO4, 0 . 0 2 m Na z HP0 4 , 0 . 0 2 m NaCI; s u c c i n a t e — 0 . 0 2 m N a H s u c c i n a t e , 0 . 0 2 m NaCI; acetate — 0 . 0 2 m HAc, 0 . 0 2 m NaAc, 0 . 0 2 m NaCI
by solution in heavy water are intended for use in standardizing the pH meter with a glass electrode and an aqueous calomel reference electrode. A pH assembly adjusted in this way is capable of yielding pD values experimentally for unknown mixtures in heavy water. When the unknown solution matches the standards closely in composition, these values should have an uncertainty less than 0.02 unit. Although the procedures by which the standard reference values were assigned are again strictly analogous to those used for assigning standard pH values in ordinary water, there is one fundamental difference between pH and pD. The ultimate reference basis for a scale of deuterium ion activity is the deuterium gas electrode, whereas that for hydrogen ion activity is the hydrogen gas electrode. In heavy water the potential of the standard deuterium electrode is, by convention, taken to be zero at all temperatures, in exactly the same way as is the standard hydrogen electrode in ordinary water. Consequently, scales of pD and pH do not have the close interdependence one might wish. They are indeed separate scales and must be regarded as essentially unrelated. Routine measurements of pD by means of the pH meter are only possible if two conditions are met. First of all, the glass electrode must respond in Nernstian fashion to changes in the deuterium ion activity in heavy water, as it is known to do for the hydrogen ion activity in ordinary water in most instances. Secondly, this change in deuterium ion activity must not alter appreciably the liquid-junction potential between the heavy water solution and the aqueous KC1 solution of the salt bridge and calomel electrode. Happily, both of these conditions are satisfactorily met. The studies of Long and his co-workers (7, 8) showed some years ago that there appears to be a constant difference between the operational pH, determined with the glass electrode in heavy water solutions with reference to aqueous standards, and the VOL. 40, NO. 6, MAY 1968
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presumed or expected pD in these same solutions. Recent work (9) has confirmed this conclusion over a wider range of the pD scale than before. It has also been shown that the difference of pD obtained ex perimentally with the glass elec trode pH meter is the same regard less of whether the calomel elec trode and salt bridge are prepared with heavy water or with ordinary water. The investigations of Long and co-workers and those at the NBS suggest the feasibility of deriving pD values from measurements of the operational pH by applying a constant correction. Standards in heavy water are nonetheless to be preferred, in view of the likelihood that the asymmetry potential of the glass electrode is altered when the electrode is transferred from ordinary water to deuterium oxide. It is also to be presumed that a stable deuterium indicator glass electrode requires the complete re placement of H2O by D 2 0 in the outer gel layer of the glass mem brane where ion exchange occurs. While the replacement process is going on, some instability or drift of the potential may be expected. For these reasons, standards for pD in heavy water are desirable. Reference values of pD have been assigned to three selected buf fer solutions in heavy water in the same manner already outlined for pH(S) in ordinary water and for pH*(S) in nonaqueous and mixed solvents. The standard emf for the deuterium, silver-silver chloride cell must be determined by inde pendent measurements in deu terium chloride solutions, and this has already been done (10). With a knowledge of this constant, the acidity function p(a D yci) was derived from measurements of the same cell containing buffer and soluble chloride dissolved in heavy water. By extrapolation as a func tion of decreasing ratio of chloride to buffer, the acidity function for the buffer without added chloride was obtained. Finally, ραΏ was calculated from p(aDyci) by intro ducing the convention set forth in Equation 7, taking into account the dielectric constant and density of heavy water. 36 A
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Table V Standard Reference Values pD(S) for the Measurement of Acidity (pD) in Heavy Water
t, °c
0.05 KD2 citrate
5 10 15 20 25 30 35 40 45 50
Solution (molality) 0.025 KD 2 P0 4 -Ι Ο.025 Na2DPO
