N o r m a n
J.
K i n gA gricultural Laboratory, D ep artm ent of Agriculture and Stock, Brisbane, Queensland, A ustralia
T
H E urgent necessity for been investigated with m any series of buffer solutions. The buffer solutions of Clark and Lubs, critically with the hydrogen-eleclrode figures.
The salt error of the antim ony electrode is defined an d a new curve constructed with buffer solutions which are normal with respect to potassium chloride. The same range o f soils was measured with the antim ony electrode in normal potassium chloride suspension and the results compared with the hydrogen-eleclrode measurements.
it in h y d r o g c n - i o n exponent
determ inations in m any forms and w ith varying technic.
In all cases a series of buffer solutions has been measured slope experienced in the use of this electrode, and also why the same behavior of the electrode was not observed by all work
ers. T he first step was to discover if possible from the litera
ture the type of buffer solutions used by each worker. M any of the published articles stated the type used and from others certain conclusions m ay be drawn. Clark and Lubs buffers were used by Best (1), Oosting (14), and also by Roberts and Fenwick (16). Lava and Hemedes (12) used potassium di
hydrogen phosphate-sodium hydroxide mixtures. Franke and W illaman (4) do not sta te the type used, b u t as their range is from pH 1 to 12 they obviously could n ot have been Clark and Lubs. Harrison and Vridhachalam (5) calibrated their electrodes w ith Universal buffer solution. The m ajority of these investigators used rods of metallic antim ony as elec
trodes, though Roberts and Fenwick (16) used crystals of
nation, whereas others relied on the oxide present in th e m etal to bridge Instrum ent Com pany slide wire potentiom eter against a Veibel (22) half-cell. Quinhydrone for the standard was prepared according to the m ethod of Valeur (21). A mirror- type moving-coil galvanometer was used and all readings made in conjunction with a standard W eston cell w ith an N . P . L. certificate. Hydrogen for the hydrogen electrode was generated from electrolytic zinc and M erck’s G. R. sulfuric acid, and was purified according to Clark (3). T he bubbling type of hydrogen electrode was used for th e pH determ ina
tions.
The various buffer solutions used were as follows:
C lark a n d L ubs, p H 3.0 to p H 10.0
As mentioned previously, those investigators who have used stick antim ony electrodes in Clark and Lubs buffers
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is a straight line from pH 4.0 to about pH 7.0. A change in direction takes place between the la tte r figure and pH 8.0, from which point it is again a straight line to pH 10.0, though of different slope from th e first p a rt of th e curve. Eleven points were determined between pH 6.8 and pH 8.5 and the change is found to begin a t the point where the boric acid- potassium chloride-sodium hydroxide buffers begin to oper
ate. I t had previously been pointed out, and was the experi
ence of the author, th a t potassium chloride introduced a large salt error into antim ony electrode determinations. I t was decided as a m atter of interest to eliminate potassium chlo
ride from the series of buffers from pH 7.8 to 10.0 and to con
stru ct curves w ith the new solutions. Although the buffer value of the mixture would naturally be altered by this change, boric acid-sodium hydroxide mixtures are good buffers w ithout th e potassium chloride. The new deter
minations gave a line following on exactly from pH 7.6 where the potassium dihydrogen phosphate-sodium hy
droxide buffers had left off. The cause of the change in slope was therefore shown (at least with the Clark and Lubs buffers) to be due to the potassium chloride in the final series of solutions. I t has been known for some tim e th a t potas
sium chloride introduces a serious salt error, when pH deter
m inations on soils are being made, though Uhl and Kes- tranek (SO) stated th a t chlorides do n o t interfere in poten- tiom etric titrations with the antim ony electrode. I t is therefore logical to assume th a t the calibration of an elec
trode for use in systems where appreciable concentrations of chlorides do not usually exist (e. g., soils) should be carried out in the absence of potassium chloride.
The Clark and Lubs buffer solutions were then made up as before, except th a t all th e solutions were made normal with respect to potassium chloride. A new curve was constructed a t 26° C. and was found to be a straig h t line from pH 4.0 to 10.0 and parallel to the line from which potassium chloride had been eliminated. The difference between the two curves was approximately 0.4 pH unit. This figure was then as
sumed to be th e salt error of the antim ony electrode with Clark and Lubs buffers.
I t remained to be proved whether a curve of similar slope was obtained with the other buffer solutions, and also whether the salt error was of the same magnitude.
Sorensen's mixtures of prim ary and secondary phosphates have only a small range from pH 5.3 to S.0. I t is of sufficient extent, however, to cover th e critical stage—viz., pH 7.5 to 8.0.
Here again the antimony electrode potential was a linear func
tion of the pH value, and the buffers norm al with respect to potassium chloride also gave a parallel straight line removed by approximately 0.4 pH from those in salt-free solution. In con
junction with Sorensen’s buffers Ringer’s mixtures of disodium phosphate and sodium hydroxide were used to extend the curve to the region of high pH value. The range pH 10.9 to 12 con
nected up accurately with th e Sorensen’s curve both in aqueous and normal potassium chloride solution.
Kolthoff’s (fl) buffer solutions made up from succinic acid, borax, and potassium dihydrogen phosphate gave a good range of standards from pH 3.0 to 9.2. These behaved similarly to the previous buffers, giving straight parallel lines about 0.4 pH a p art for the aqueous and potassium chloride solutions respec
tively.
M cllvane’s standards consisting of disodium phosphate and citric acid were found to give entirely erroneous and very erratic results w ith the antim ony electrode, particularly in the acid range of the buffer solutions. I t is assumed for w ant of a better explanation th a t complex antim onyl citrate compounds are formed analogous w ith the compounds formed between antim ony oxide and tartaric acid. Kolthoff and Furm an (10) state th a t
potentiom etric titratio n of tartaric acid with the antim ony elec
trode is inaccurate and results in measurable quantities of an ti
mony being in solution a t the end of the titration.
Sorensen's citrate-sodium hydroxide mixtures after W aibum also gave very erratic values.
Palitzsch’s borax-boric acid mixtures w ith a range from pH 6-8 to 9.2 gave a straight line of similar slope to th e preceding ones.
I t was considered th a t, for th e calibration of an electrode of this type, where salt errors are likely to occur as a result of th e introduction of some specific ion, it would be b etter to adopt a buffer series which contained similar compounds throughout its entire range. Such a series is given by the
September 15, 1933 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 325 ' Universal buffer m ixture of Prideaux and W ard (15) which,
on gradual neutralization with sodium hydroxide, would sub
ject th e electrode to changes of hydrogen-ion concentration ranging from pH 3 to 12. A series of such buffers, standard
ized w ith the hydrogen electrode and measured with the antim ony electrode, were found to give a straight-line graph of similar slope to th a t given by the previous buffer solutions.
When made up normal with respect to potassium chloride, however, a curve was obtained which is illustrated in Figure 3.
I t was noted when standardizing the more acid solutions of this series on th e hydrogen electrode th a t a perceptible and steady drift was occurring towards a lower potential, which seemed to imply chemical reaction in the system with conse
quent increase in hydrogen-ion concentration. Whatever th e explanation of this phenomenon, it appears th a t the Uni
versal buffer solution is unsuitable for use in normal potassium chloride. T he more alkaline portion of the curve is a straight line and removed by about 0.4 pH from th e normal curve.
Figures 1 and 2 show the curves obtained with the buffer solutions of Sorensen, Ringer, Palitzsch, and Kolthoff both in aqueous and norm al potassium chloride solution.
Since the completion of this work there has come to hand the paper by B ritto n and Robinson (2) on the potentiometric titra tio n of Prideaux and W ard’s Universal buffer mixture w ith th e antim ony electrode, and the calibration of their elec
trodes w ith th a t solution. T he relation obtained by them is deviation from th e theoretical value, a single equation covers th e entire series. In no case was a deviation of more than 2
This equation attains more towards the theoretical value than th a t of other workers, the theoretical slope of which a t 24° C.
would be 58.9 millivolts per pH unit.
Me a s u r e m e n t s o f Aq u e o u s So i l Su s p e n s io n s
T he major portion of th e hydrogen-ion work carried out in this laboratory is in connection w ith soil reaction. As the application of th e antim ony electrode would proceed along these lines, it was decided to examine a number of soils vary
ing widely in distribution and reaction to te st fully th e applica
bility of the electrode. The soils used are principally of Queensland origin, though a few from New Guinea and India have been included as a m a tte r of interest. The pH deter
minations were made with th e bubbling hydrogen electrode, and the technic used was similar to th a t for th e buffer solu
tions. Readings were taken a t the half-minute period and in no case was a considerable drift experienced after this time.
Soils in the alkaline range were free of appreciable am ounts of carbonates. Difficulty was experienced in obtaining soils of pH greater than 8.5 with sufficiently low calcium carbonate to obviate disturbance of the carbonate equilibrium. Certain soils were therefore leached free of lime with 0.05 AT hydro
326
soils, these sufficed to te st the antim ony electrode a t a higher pH than was otherwise possible.
All determinations were carried out in triplicate, and repli
cates always agreed to within 1 millivolt. Antimony oxide was added to the system and the antim ony electrode allowed to stand in the soil suspension. The details of technic were similar to those used in the previous buffer solution work.
Fi g u r e 4 . Co r r e l a t i o n o f pH Va l u e s De t e r m i n e d b y t h e Hy d r o g e n a n d An t i m o n y Ox i d e El e c t r o d e i n No r m a l Po t a s s i u m Ch l o
r i d e Su s p e n s i o n
The results obtained are shown in Table I. The pH varied from 4.09 to 10.56 with th e antim ony electrode and the re
sults are considered to be highly satisfactory. The average difference between hydrogen and antim ony electrodes for 43 soils is less th a n 0.05 pH unit, the average positive devia
tion being 0.057 and the average negative deviation 0.049 pH unit. The maximum errors are + 0.10 and —0.10.
Similar results are recorded by Snyder (18), who used Franke and W illaman’s equation.
St i r r e d a n d Un s t i r r e d Su s p e n s i o n s
The above results were recorded with a stationary elec
trode. The technic of investigators has varied considerably.
Kolthoff and Furm an state th a t agitation is necessary;
Franke and Willaman and B ritton and Robinson used me
chanical stirrers; Roberts and Fenwick kept their solution flowing over antim ony crystals; Snyder used both a rocking electrode and a stationary one; Best, and Harrison and Vrid- hachalam worked ■with unstirred suspensions; and Oosting used both stirred and unstirred soil suspensions.
I t is the conclusion of the author th a t either m ethod is applicable, provided th a t the calibration of th e electrode is carried out w ith the same technic. F or instance, Snyder obtained b etter results w ith th e rocking electrode, b u t he was using an equation evolved by Franke and W illaman on stirred buffer solutions. The principal advantage of th e stationary method is th a t determ inations can be m ade in te st tubes in th e same way as for quinhydrone, and th a t the m ethod is suitable for field determinations w ith a portable poten
tiom eter of th e type recommended by Itano (8) or Harrison and Vridhachalam.
Va l u e o f An t i m o n y El e c t r o d e i n So i l Re a c t i o n Me a s u r e m e n t s
In agricultural and research laboratories where large numbers of soils are handled daily, and in soil survey opera
tions where reaction is an im portant factor in classification, a rapid method of pH determ ination becomes of primary importance. The tim e required for attainm ent of equilibrium with the hydrogen electrode, particularly w ith the Crowther electrode, usually outweighs the advantages accruing from
increased accuracy of th e determ ination. Biilman’s quin- • hydrone electrode has therefore been adopted in most labora
tories for routine work, and is em inently satisfactory with m ost soil types. W ith the growth of soil reaction work throughout th e world the lim itations of this electrode became apparent. Soils w ith a pH greater than 8.0 disturbed the quinone-hydroquinone equilibrium as a result of th e acid n ature of quinhydrone, and gave erroneous values. I t was also pointed out as early as 1929 b y McGeorge (IS) th a t cer
tain Hawaiian soils known to contain appreciable quantities of manganese dioxide gave higher pH values w ith the quin
hydrone th a n with th e hydrogen electrode. Heintze and Crowther (6) explain this as a reduction of the manganese dioxide by hydroquinone, th e resulting manganese hydroxide raising the pH of th e soil suspension.
Similar discrepancies have been noted with m any of the basaltic red loams of Queensland, and as this soil type con
stitu tes a fair percentage of the coastal agricultural areas, manganiferous soils a tta in considerable im portance in daily routine work. I t was w ith th e object of applying th e an ti
mony electrode to these soils and the strongly alkaline soils of the more arid districts th a t the above work was carried out.
A n t i m o n y E l e c t r o d e i n N KC1 S u s p e n s i o n s o p S o i l s
F or some years this laboratory has recorded reaction m easurem ents of soils in both aqueous and normal potassium chloride suspensions, as recently urged by the com mittee on soil reaction measurem ents for the International Society of Soil Science (19). The value of this la tte r figure is being gradually acknowledged by soil authorities. pH values in normal potassium chloride appear to be less influenced by changes in biological and meteorological conditions and thus
F i g u r e 5 . C o r r e l a t i o n o f p H V a l u e s D e t e r m i n e d b y t h e H y d r o g e n a n d A n t i m o n y O x i d e E l e c
t r o d e i n A q u e o u s S u s p e n s i o n
measure a more perm anent characteristic of th e soil. Wors- ley (S3) discussed the effect of neutral salt solutions on Egyp
tian soils, and European authorities use the m ethod widely in fertility studies. In Queensland the potassium chloride figure has been known to explain lack of fertility on sugar
cane lands when the pH of the aqueous suspension failed to throw any light on th e m atter. I t is not often w ith normal soils th a t the pH in normal potassium chloride suspension is greater th a n 8.5, and therefore th e pH of th e suspension docs not prohibit the use of th e quinhydrone electrode. B ut it is th e au th o r’s experience th a t manganiferous soils “ drift”
more frequently in potassium chloride than in aqueous sus
pension.
I t became im portant therefore th a t the antim ony electrode should be applicable in salt suspension, and consequently a stu d y of th e salt error of this electrode was considered neces
sary. In the previous work of B est and K ing (unpublished)
September 15, 1933 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 327 it was found th a t the e. m. f. of the antim ony electrode in a
normal potassium chloride suspension, if read off from the calibration curve, gave a figure approximately 0.4 pH unit higher th a n th e same suspension w ith the hydrogen elec
trode. This error was fairly constant for a large number of soils, though larger variations were occasionally experienced.
If this salt error were entirely due to a surface action on the electrode, it would be expected th a t the effect of normal potassium chloride on a buffer solution would be comparable with th a t on a soil suspension. I t was decided to measure the variation in pH of a number of buffer solutions made up normal w ith respect to potassium chloride, and compare these w ith th e pH of the buffers w ithout potassium chloride.
The results of this work are stated in the early part of this paper. E very series of buffers shows a difference of approxi
m ately 0.4 pH unit between the aqueous and the normal potassium chloride buffer solutions. As this figure agrees closely w ith th e salt error experienced in measuring soils, it was thought th a t this second curve could be utilized for giving the relation between e. m. f. and pH for the soils in normal potassium chloride suspension.
T he curve obtained for the buffers in normal potassium chloride was parallel to th e original curve and removed from it by approxim ately 0.4 pH unit. The slope of the curve re
mains constant, the equation for potassium chloride suspen
sions reading
E = 0.041 + 0.0575 pH or
E - 0.041 p H 0.0575
(10) Kolthoff and Furman, "Potentiometrie Titrations,” Wiley, 1926.
(11) Kolthoff, I . M., and Hartong, B. D., Rec. Iran, chim., 44, 113 (1925).
(12) Lava, V. G., and Hemedes, E. D., Philippine Agr., 27, 337 (1928).
(13) McGeorge, W. T., Soil Sei., 27, 83 (1929).
(14) Oosting, W. A. J-, Mededeel. Landbou. Wageningcn, 1921, 3.
(15) Prideaux, E. B. R., and Ward, A. T., J . Chem. Soc., 125, 426 (1924).
(16) Roberts, E. J., and Fenwick, F. J., J. Am. Chem. Soc., 50, 2125 (1928).
(17) Shukov and Awsejewitch, Z. Elektrochem., 35, 349 (1929).
(18) Snyder, E. F., Soil Sei., 26, 107 (1928).
(19) Soil Research, Soil Reaction Com., 2, 144 (1930).
(20) Uhl, A., and Kestranek, W., Monatsh., 44, 29 (1923).
(21) Valeur, A., Ann. chim. phys., 21, 547 (1900).
(22) Veibel, S., J. Clicm. Soc., 123, 2203 (1923).
(23) Worsley, R. R. le G., Ministry Agr. Egypt, Bull. 83, 16 (1929).
Re c e i v e d N o v e m b e r 15, 1932.
In Figures 4 and 5 are shown the values for the hydrogen and antim ony electrodes on the 43 soils plotted against each other, and also the theoretical line. Figure 4 refers to the aqueous suspensions and Figure 5 to th e suspensions in nor
mal potassium chloride.
The same series of 43 soils measured previously in aqueous suspension were used for this wrork, and the la tte r columns of Table I show th e results obtained. The average error as compared with the hydrogen electrode is 0.058 pH unit, which is slightly larger th a n th e error in aqueous suspension. The average positive deviation is 0.06 pH and the average nega
tive deviation is 0.055 pH unit. These figures are consid
ered fairly satisfactory for agricultural advisory or soil sur
vey work where th e sampling error is likely to exceed the error of the determ ination. F or such operations the anti
mony electrode is strongly recommended.
I t is robust in construction and does not appear to be affected by “ poisons” of th e type which affect the hydrogen electrode. Its range of applicability is considerable and is only lim ited b y the pH a t which antim ony trioxide is soluble in the system under examination. I t is proved to be a fairly reliable indicator of hydrogen-ion concentration and is accu
rate enough for certain avenues of soil work where extreme precision is n o t required. I t is ideally suited for field work, particularly in the form illustrated by Harrison and A'rid- hachalam (5).
Li t e r a t u r e Ci t e d
(1) Best, R. J., J . Agr. Sci., 21, 337 (1931).
(2) Britton, H. T., and Robinson, R. A ., J. Chem. Soc., 1931, 458.
(3) Clark, W. M., "Determination of Hydrogen Ions,” Williams &
Wilkins, 1923.
(4) Franke, K. W., and WiUaman, J. J., I n d . E .v g . C h e m . , 20 , Si (192S).
(5) Harrison, W. H., and Vridhachalam, P. X., Mem. Dept. Agr.
India, 10, 157 (1929).
(6 ) Heintze, S . G., and Crowther, E . M., Trans. 2nd. Comm. In tern. Soc. Soil Sci., 1929A, 102.
(7) Itano, A., Ber. Ohara. Inst. land. Forsch. Japan, 4, 273 (1929).
(8) Ibid., 4, 19(1929).
(9) Kolthoff, I. M„ J. Biol. Chem., 63, 135 (1925).