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667It can be said that it is erroneous to plot the points in this way, for each crc value in Table I11 corresponds not to a solution of constant total molality but to a, solution whose total molality varies (see Table I) between 4.02 and 4.51 31. This is true but, as Fig. 4 shows, the variation of ac with total molality is small and can be allowed for. At M = 4, crc is increasing by 0.0015 for unit change in M. It is possible, therefore, to make a small correction to the ten values of ac in Table I11 to give data corresponding to a constant total molality equal to the mean of the extremes, 4.25 M. The corrected valueis are plotted as the lower curve of Fig. 5; there is less scatter of the points but still a definite downward trend with slope -0.0019, givingpc = -0.0004.A similar correction applied to the CYB data (lower curve of Fig. 6) does not alter the constancy of O(B with changing x.The downward trend of the ac points in Fig. 5 with changing z at a constant total molality of 4.25 leading to Pc = -0.0004 is therefore fully consistent with the conclusion drawn from the variation of (ax1 + ac) with changing M (Fig. 4)The data a t M -4 have been used because they are the most complete and reliable. ,4t higher values of M , the limited solubility of potassium chloride restricts the range of x in which measurements cart be made. At lower values of M , the effect of experimental error is enhanced. Sever-theless, by plotting (YC against z (Fig. 7) the same downward trend with increasing x is observed in all cases. Pc is certainly independent of m within limits of experimental error and the assumption of an average value of pc = -0.0005 is sufficient to bring consistency into all the measurements made in this work. TABLE V SMOOTHED VALUES OF CYB AND oc 4 5 6 m 0 5 1 0 2 0 3 --c~B (0 013) 0 0095 0 0084 0 0084 0 0090 0.0098 . . . oc (0 025) 0 0235 0 0230 0 0235 0 0246 0 0260 0 0275 BB = 0, Bc = -0 0005 Table V gives smoothed value of CZB and ac a t round values of the total molality. Finally, Fig. 8 shows how YB and yc vary with composition of the mixture at a constant total molality of 4.With PB = 0, log 2B is linear in x, increasing from -0.2390 when 2 = 1 to -0.2030 a t the limit when x = 0. If the pc term is neglected, log yc decreases linearly from -0.1061 when x = 0 to -0.2045 a t x = 1; the introduction of the Pc term causes a slight downward curvature shown by the broken curve in Fig. 8, leading to a limiting value of -0.2125 at = 1. It should be noted that these limiting values of log YB when x = 0 and of log yc when x = 1 are almost but not exactly the same and are much closer to log YB when x = 1 than they are to log yc when x = 0, in other words, on mixing both electrolytes tend to behave more like potassium chloride than sodium chloride. The thermodynamic dissociation constant (Kbh) of the protonated form (BH+) of 2-amino-2-(hydroxymethyl)-l, 3 propanediol (tris-(hydroxymethy1)-aminomethane) has been determined at 11 temperatures from 0 to 50' by measurement of the electromotive force of hydrogen-silver ch...

The r ela tive stre ngths of ben zoic acid, 31 of its monosubstit ut ed derivatives, 7 disubstit uted d erivatives, and one trisubstituted derivative have been measured in t erms of the equilibrium constants (I(") for a ssociation with t he reference base 1,3-diphenylguanidine in benzene at 25° C. The measurements were performed spectrophotometrically, using the indicator a cid bromophthalein m a gen ta E (tetrabromophenolphthalein ethyl ester) as the reference acid, t he equilibrium con stant [(' for association of the indicat or with diphenylguauidine being known from pre vious work. Equilibrium con st ants for p-orsellinic (2,6-dihydroxy-p-toluic) acid and 2,4,6-trinitroben zoic a cid could not be m ea sured , a s t hese a cids appear t o associat e completely with diphenylguanidine under the condit ions of t h e experiments. Plotting log [(" values against t he corresponding p[(v a lues for aqueous solutions of t he a cids gives a n essentially linear relationship in the case of acids with m ela-a nd par asubst ituents only, but the line for pam-substituted acids has a slightly differen t slope from t he line for m-subst i tuted acids. Similar comparisons were made with published data on relative strengths in a lcohols or dioxane-wat er mixtures. orlho-Substit uted a cids show m arked solvent effects. Explanations of the solvent effeets h ave been suggested , a nd various t heoretical implications and possible applications of the r esults are discussed. The t h eor•etical discuss ion includes e valuation of constants of the H ammett equation.

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The stand a rd pH values or th e solutio n or potass ium hyd rogen phtha la te (mo la lil y 0.05 mol kg-I) have bee n red ele rmin ed ove r Ihe le mpe ralure range 0 to 60 °C, us ing S RM l 8Se1. Ext e ns ive meas ure me nts we re made or th e e mr or ce ll s or Ih e Iype Pd; H2(g, 1 a lm)J KH phlhal ale (rn = 0.05), KCI (ml JA gC I;Ag wh ere m.KCI was 0.015, O.OJ., or 0.005 mol kg-I, rrom whi c h va lues or th e acidil y run cti on p(a ,"Ycl) we re derived.The pH conve nti o n d efin es Yel in Ihe ra nge or ionic stre ngths 0 to 0 . 1 mo l kg-I a nd pe rmits co nvenlio nal va lu es or paH 10 be obl a in ed. According to NBS procedures, I)(lH ror se lected re re re nce so luli ons is idenlifi ed wilh Ih e s landard pH(S) in the ope ra ti onal de finiti o n or p H. The new va lues, give n in te rm s or Ihe the rm ody na mi c tempe ralure (7') b y 2073.44 pH(S) = --7'--13.3270 + 0.0451997' -3.4846 X 1O-' {' d irre r, on the average, by 0.003 unil rro m th e res ults based on th e 1944 data or Hamer and Acree .Key words: Acidil y; e mr; pH; phlh alate burrer; pOlass ium hydrogen p htha la te ; stand a rd ror pH.