pH standards of high acidity and high alkalinity and the practical scale of pH

Abstract

The pra ct ical scale of pH is defin ed in te rms of t he electromotive f or ce of t h e galvani c cell P t; H z (g), solu t ion I satd . KC I I stand a rd , H z (g); Pt. This p oten t ial is us uall y ob t ained as t h e differ en ce of t wo electromo t ive for ce values fo r a cell with gla ss and calomel electrodes, one of which is a calibration wi t h a standa rd of kn own pH . Appro priate correctio ns must be applied if t he glass electr ode d oes n ot res po nd to changes in hydrogen-ion activi ty in ex a ctly t he sam e ma nne r a s th e hy droge n electr ode. However , t here is no simp le m ean s of correc ting pH m eas urem en ts fo r t he p o te n tial differen ces a t t he junctio ns of t he solu t ion a nd t he stand a rd wit h t he solu t io n of p otassium chlo rid e. These errors a re suffi ciently lar ge in hi g hl y ac id a nd hi ghl y alka lin e solu t ions to r ender un certain t he in te rpretation of m easured pH in t hese r egio ns, i n spi te of t he fact t hat r easona bly acc urate standards of h y drogen-i on activi ty a re available at in term edi ate p H valu es.The purpose of t his stud y was twofold: (a) to determin e the exten t of a berration of t he practi cal pH scale n eal' i ts e nd s, a nd (h ) to select new stan da rd s that mig ht improve t he a cclll'acy of pH m eas ureme n ts a nd fa cili t a te t he ir in te rp retation over t he ent ire practical sca le, w it h particular atte nt ion to t he regions of hi gh acidi ty and high alkalini ty . The r esults indi cate t ha t pH o b tain ed by ad justme nt of t h e m eter wi t h t he p resen t stand a rd s (pht halate, pH 4. 01 at 25° C; phosphate, p H 6.86; bo rax, pH 9. 1S) will usua ll y be low by at least 0.02 to 0 .05 uni t a bo ve pH 11, whil e e rrors as great as 0 .0 3 uni t, eit h er positive or nega ti ve, a r e no t u n co mm o n below pH 2.5. The fo ll o wing a ddit.io na l standa rds we re selected to s upplem en t t he t hree presen t ly a vailable: (1) O.Ol-M p ota ssium tetr oxa latep H 2.15 at 25° C; (2) p otassium h ydrogen t artrate (saturated at room te mperat ure)-pH (5) O.OI-M t ri sodium phosphate-pH 11. 72 . The ch oice was based o n a comparison of p H deri ved from cell s wi t h a nd wi t ho ut liquid j un ction in a study of 4 1 promising standard solu t io ns . The p H o n t he practical sca le was d etermin ed at 25° C, and electro motive fo rce m easurem e nts of h y droge n-s ilver chloride cell s wit hou t liquid jun ction were made a t 0°, 10°, 25°, a nd3So C.

Acidity function p( a H γ Cl ) as a step to pH assessment

“…(13), (14), and (15), always produce a straight line of negative slope, experimental studies by Bates [6,7] resulted in both negative and positive slopes, Fig. 4; this has led to questioning of the theoretical interpretation involved.…”

“…An experimental study by Bates et al [6,7] of 0.01 mol Kg -1 H 3 Cit solutions, and application of the equation:…”

A simple procedure for the estimation of very small amounts of nitrogen in lipids

“…is prepared from the calculated amounts of Na2HPO4,12H20 and NaOH (cf. Bates, Pinching & Smith, 1950), and stored at room temp. to avoid crystallization.…”

Evaluation of the residual liquid junction potential contribution to the uncertainty in pH measurement: A case study on low ionic strength natural waters

“…As was suggested by Bates [18], the RLJP errors for standard pH buffers can be evaluated by comparing the pH value measured "operationally", using the cell containing two liquid junctions (of the free-diffusion type) separated with a salt bridge, with the corresponding p˛H value derived from the measurements of the cell without liquid junction. The results of such measurements, typically plotted against pH values of the buffers (see Fig.…”