Purification of sodium chloride and potassium chloride for use in electrochemical work, and the determination of small amounts of bromide

Self Cite

115

An accurate det.ermination by t he electro motive-force m ethod of t he di ssoc iat ion constant of an acid as strong as t he first ac idi c g roup of phospho ric acid r eq uires t hat the hydrogen-ion concen t rat ion be estab li s hed wi t h greater accuracy than is usually attainable. The un cer tain quantity is t he activ ity coe ffi cie nt of hydrochloric acid in the particular mixture und e r st ud y. It is shown th at the calculation of this coefficient by t he DebyeHi.ickel eq uat ion , which afford s a n adequate estimate of the sma ll hydroge n-ion co rrection in buffer solut io ns com posed of weak acids, fa il s to y ield the desired accuracy for mixtures in whi ch the acid is dissocia ted to an a pprec iable exte nt. In order to determine t he common logari thm of t he first di ssociation co ns tan t of phos phoric acid wi t h an acc uracy be lie ved to be ± 0 .005 uni t, the hydrogen-io n concentra tion of some of t he buffer mixtlll'es s tud ied was kept at lo w levels by use of a la rge excess of potassium dihydrogen phos phate. Ot her so lu tions contain ed formic acid in stead of hyd roc hlo ric acid , the reby lowerin g the hyd rogen-ion concentration st ill further. In addit ion , p a r t icu lar attention was g ive n to a cho ice of reaso nable valu es fo r t he activity coefficient used to es t im ate t he hyd roge n-io n cOl'J'ection .By t hese m eans t he first di ssociatio n con stant, Ie , of phosphoric acid was calculated from e mf measure ments of 7] phosphate buffer sol ut io ns, togeth er ",ith supplementary data from th e lit e rature foJ' 18 add itional so lu tio n . The result from 0° to 60° C is g iven by -log [(1 = 79 ; ; 31 __ 1.5535 + 0.013486 T , \\'h ere T is th e temperat ure in °IC Th e changes of heat content, e ntl'Op.v , a lld heat capac it y foJ' thc di ssociat ion of 1 mo le of aqueous phospho ri c acid in t he sta nd a rd state were fOlilid to be -7,650 j, -66.8 j deg-I , and -154 j c1 eg-l , res pective ly , at 25° C. Introdu ctionExperience has proved Lhat Lhe electromo tiveforce m ethod is well sui ted to the accura te determination of the dissociation constants of many weak acids and bases in aqueous solu tion . However, it is not gen erally r ecogniz ed that tbe accuracy obtainable by this method diminishes with increasing strength of the acid or base. I'Yh en approximately 10 percent or more of a weak acid is dissociated in solution, th e uncertain ty in establi shing th e tru e concen trations of the ionic and molecular species becom es so large that an exact determination of the dissociation con stant is no 'longer possible.Som e years ago th e author undertook to m easure the electromo tive force of a series of cells of the type Pt; H 2 (g, 1 atm) , KH2P04 (xm), H CI (m) AgCI; Ag (Cell 1) where m is molality, in an attemp t Lo extend the investigation of N ims [l jl Lo oLh er temperatures and buffer ratios. It was found to b e impossible to obtain an accura te value for th e first dissociation constant of phosphoric acid by analysis of t h e data for mixtur...

“…The potassium chloride was a fused sample free of bromide [5]. Reagent-grade gO-percent formic acid was distilled twice in an all-glass still.…”

Section: Experimental Procedures and Resultsmentioning

confidence: 99%

First dissociation constant of phosphoric acid from 0-degrees-C to 60-degrees-C; Limitations of the electromotive force method for moderately strong acids

Bates¹

1951

Self Cite

115

“…The sodium chloride used had been recrystallized from water. It was tcsted [7] and found to contain about 0.002 mole percent of bromide. The specific conductance of the water used to prepare the buffer solutions ranged from 0.4 to 0.7 X 10-6 mho.…”

Section: Experimental Procedures and Resultsmentioning

confidence: 99%

“…The cause of the drift is as yet unexplained. Traces of sulfide, such as might be dissolved from the rubber, have b een found not to have a large eff ec t on the po tential of the sil ver-sil ver-chloride electrode [7].…”

Section: Experimental Procedures and Resultsmentioning

confidence: 99%

See 1 more Smart Citation

Second dissociation constant of oxalic acid from 0 to 50 C, and the pH of certain oxalate buffer solutions

Pinching¹,

Bates²

1948

The second di ssociation constant of oxali c acid was determined at interval s of 5 d egrees from 0° to 50° C by measurement of the electromotive force of cells without liquid junction . Hydrogen a nd sil ver-sil ver-ch lorid e electrodes were employed . The so l u tions were composed of potassium binoxalate, sod ium oxalate, and sod ium ch loride. The valu e of the second dissoc iation con tant, 1(2, in t he tempe ratu re range st udi ed is given by the equation -log K 2= -T --6.5007 + 0.020095T,wh e re T is the abso lu te temperature. Th e thermodynamic quanti ties associated ,yilh the second di ssociation step were calculated. The paH values of 8 soLu t ions of potassium binoxalate and sodium oxalate in the ratio 1:5 were determined at 11 temperatures.-

“…This is close to t he inflection p oint. Th e po tassium chloride had b een purified by tr eatm ent wi th chlorine, precipi tation with gaseous hy drogen chloride, and fusion [17] . H ydrochloric acid was distilled and t he middle fraction collected and dilu ted t o abou t 0.1 M .…”

Section: Dissociation Constants Of Basesmentioning

confidence: 99%

Dissociation constants of weak bases from electromotive-force measurements of solutions of partially hydrolyzed salts

Bates¹,

Pinching²

1949

A method for the determination of dissociation constants of weak electrolytes by electromotive-force measuremen ts of solutions of partially hydrolyzed salts of a weak acid and a weak base is described. Although the precision is only half that of the conventional emf m ethod , this procedure has particular advantage in determinin g t he dissociation cons tants of certain bases, for som e of the experim e ntal difficul t ies encountered in adapting the usua l method to solutions containing these bases can be overcome or red uced . The hydrogen-ion concentration of a solution of a salt of this type depends upon the con stants of the weak acid, th e weak base, and water . If two of these are known, th e third can be evaluated by means of e mf m eas ure ments wi t hout the necessity of knowing the exact hydroge n-ion concentration, which is usually difficult to obtain. Hydroge n clectrod es and silver-silver chloride electrodes are used in the cells, and the solut ions are aqueou s mixtures containing the ions of the sa lt and an alkali chloride. The equations for t he calculation of di ssociation constants are deve loped. Th e method was tested by a determination of the basic dissociation constant of tris(hydroxymethyl)aminomcthane at 20 0 , 25 0 , and 30 0 C from three se ri es of meas u reme n ts : (a) by t he con ventio nal e m f m ethod ; (b) by emf studies of the hydrolysis of mixt ures of th e a mine and primary potassium phosphate ; and (c) by emf studies of the h yd rolysis of mixtures of the amine and potassium p-phenolsulfona te. The three determinations were in acceptable agreement. The negative logarithm of the hasic dissociation constant, Pf( b, was found to be 5.946 at 20 0 , 5.920 at 25 0 , and 5.896 at 30 0 C.