No abstract

A study was made of the reproducibility of the electrolytic, thermal-electrolytic, and thermal types of the silver-silver chloride electrode in 0.05 N solution of potassium chloride. The presence or absence of light, and of air dissolved in the solution, have no significant effect on their potentials. A marked aging effect was noted, the origin of which was traced to concentration-polarization occurring during the preparation of the electrodes. The attainment of concentration equilibrium between freshly prepared electrodes and solution is slow and may require from 1 to 20 days, depending on porosity of the electrodes and stirring of the solution. For the same reason, freshly prepared electrodes behave as cathodes towards aged electrodes, the initial difference in potential often exceeding a millivolt. When sufficient time is allowed for the establishment of concentration equilibrium, these types of silver-silver chloride electrodes assume the same potential, within abo ut 0.02 mv.

The National Bureau of Standards conventional activity scale of pH is defined in terms of a series of standard buffer solutions prepared from certified materials issued as NBS Standard Samples. In order to increase the accuracy of measurements at low and high pH, standards of high acidity and high alkalinity are needed to supplement the four now available, which cover adequately the intermediate range from 3.5 to 9.2. Potassium t.etroxalate dihydrate, chosen as the fifth pH standard, is a stable, crystalline substance composed, in addition to water, of equal molar amounts of oxalic acid and potassium acid oxalate. Its solutions are t.herefore well buffered in the concentration range 0.01 to 0.1 M and are highly acidic, with pH ranging from 1.5 to 2.2. Electromotive-force data were obtained for 12 mixt.ures of potassium tetroxalata and potassium chloride at 13 temperat,ures from 0° to 60° C, and pH values were assigned to 0.1-, 0.05-, 0.025-, and O.OI-M solutions of potassium tetroxalate without. added chloride. The 0.05-M solution, which has a pH value of 1.68 from 15° to 35° C, is recommended as a standard for measurem ents at low pH .

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.

Tests of the reproducibility in potential of the electrolytic, thermal-electrolytic, and thermal types of silver-silver chloride, silver-silver bromide, and silver-silver iodide electrodes, in both acid and neutral solutions, are reported. All of these silver-silver halide electrodes show an aging effect, such that freshly prepared electrodes behave as cathodes towards electrodes previously aged in the solution.They ar e not affected in potential by exposure to light, but the presence of oxygen disturbs the potentials of the silver-silver chloride and silver-silver bromide electrodes in acid solutions, and of the silver-silver iodide electrodes in both acid and neutral solutions. Except in the case of the silver-silver iodide electrodes, of which the thermal-electrolytic type seems more reliable than the electrolytic or the thermal type, the equilibrium potential is independent of the type, within about 0.02 mv.