✍ Scribed by Helmut Sigel; Andreas D. Zuberbühler; Osamu Yamauchi
No coin nor oath required. For personal study only.
It 1s emphasized and shown that the concept of pH 1s more complicated than nught be thought (and to some extent also unsatisfactory), there are (at least) three pH scales m general use and it IS the am to make this fact recogmzed These scales arc (1) an actlvlty scale, where the hydrogen ion actMty IS measured based on NBS or smular standards by carefully ehmmatmg the hquid-junction potentials of the electrode system via experImenta determmatlons, (2) a practical scale, wluch has unmtentlonally developed by convemence over the past ca 30 years, 1s based on now generally available combined glass electrodes together with NBS (or related) buffers used for cahbratlon, and (3) a concentration scale which uses strong acids and/or bases for calibration and defines the pa-meter readmg m terms of -log[H+] Scale (2) 1s clearly the one least well defined, yet d 1s also the one most widely used If a 'pH' 1s measured for a given constant Hf concentration m the three scales, its value decreases m the order (1) > ( 2) > ( 3) Scales ( 1) and (3) may be converted mto each other by usmg the smgle ion actiwty coefficients of H', e g , at 25 o C and at lomc strengths of 0 1 and 0 5 M the differences correspond to 0 11 and 0 15 log unit, respectively The conversion term from scale (2) to (3) corresponds at 25 o C and an lomc strength between 0 1 and 0 5 h4 to about 0 03 log unit It IS evident that any acldlty constant, 1 e pK, value, determmed for a given system (HA + A-+ H') IS affected to the same extent, hence, the mentloned conversion factors have to be apphed d pK, values determmed m different scales are to be compared or used It may be added that many workers beheve that combmed glass electrodes measure the hydrogen Ion actlvlty and that they are workmg m scale (11, yet tius 1s llot the case, they are actually workmg m scale (2) Moreover it 1s also barely (or not at all) recogmzed that the values m scale (2) are m fact closer to those of scale (3) and not to those of scale (11, as is often assumed Some general comments regardmg potentiometnc pH titrations and the determmatlon of eqmhbrmm constants (1 e , pK, values and stab&y constants of metal ion complexes) are also made, and the advantages of different tltratlon procedures are discussed and pItfalls are pomted out
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