'I'HE ELECTRICAL CONDUCTIVITY OF HYDROCHLORIC ACID AND POTASSIUM CHLORIDE IN PRESENCE OF SUCROSE.

BY ALFRED JOSEPH KIERAN.

( A Paper read before THE: FSRADAY SOCIETY, Afhday, &ne 26fh, 1922, PROFESSOR ALFRhD w. PORTER, F.R.S., PRESIDENT, the Chair.)

Received May I 8th, I 9 2 2.

The investigation briefly recorded below arose from an attempt to determine whether sucrose in aqueous solutions was capable of combining and forming a permanent union with hydrogen ion. For this purpose the equivalent conductivities of hydrochloric acid, through a wide range of dilution, have been determined in the presence of varying quantities of sucrose, the concentration of the latter being maintained constant in each series of measurements in order that the viscosity might also remain con- stant whilst the dilution of the acid varied. It was found that whilst the behaviour of potassium chloride in presence of sucrose is normal, in that the equivalent conductivity increases regularly with increasing dilution of the salt to an asymptotic limit, the behaviour of hydrochloric acid is abnormal, the equivalent conductivity passing through a maximum in the region N/300 - N/soo, depending on the concentration of the :sucrose solution employed as solvent and steadily falling thereafter with increasing dilution. ,4t first sight this might be taken as indicating replacement of the fast moving hydrogen ion by a slow moving complex. A more detailed examination of the system shows, however, that the abnormal behaviour is not due to this cause but is in fact connected with the relatively minute traces of electrolytic impurity present even in the purest samples of sucrose obtainable. The nature of the abnormality although traced to the some- what fortuitous cause of an impurity, is not without interest in view of the close association of minimal amounts of electrolytes with other organic substances of high molecular weight.

Experimntul.

Using the usual Wheatstone's bridge method, nieasurements of the electrical conductivity of hydrochloric acid were made at 2 5' C., in presence of 5 , 10, and 2 0 per cent. sucrose in turn. With each " solvent " the acid was examined over a range from 0-05N to o-ooogN.

The sucrose used was supplied by Messrs. Tate & Lyle, Liverpool, in a special form containing very little inorganic matter. The specific con- ductivity of a 5 per cent. aqueous solution was 2.2 x I O - ~ mhos., that of the water used was 0.7 x I O - ~ mhos.

The cell used for concentrations from o*o5N to o-ooogN was made of- boro-silicate glass and was fitted with vertical electrodes about I cm,

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I 2 0 ELECTRICAL CONDUCTIVITY OF HYDROCHLORJK ACID

square in area and 1.5 cm. apart. It was found that when using sucrose solutions in such a cell, heavily platinised electrodes produced a moving and uncertain minimum. This phenomenon was investigated in detail in this laboratory by Mr. R. Preston who eventually surmounted the difficulty by depositing only a small amount of platinum black on the electrodes.

The experimental data on the conductivity of mixtures of hydrochloric acid and sucrose at 25' C. are recorded below. In calculating the equi- valent conductivity from the specific conductivity the value of the specific conductivity of the sucrose was first subtracted from that of sucrose plus hydrochloric acid.

Hydrochloric Acid mid 5 Pcr Cent. Sticrose. Concentration of

Acid in Equivalents Per Litre.

Equivalent Con- ductivity in

Mhos.

0.05 . . . . . . . 367'5 0'02 . . . . . . . 375'4 0-01 . . . . . . . 380.3 0.005 . . . . . . . 382% 0-002 . . . . . . . 386.4 0-001 . . . . . . . 385'5 o.oo05 . . . . . . . 382.4

Hydrochloric Acid and 10 Per Cent. Sucrosc. 0.05 . . . . . . . 338.1 0'02 . . . . . . . 345'2 o*mg . . . . . . . 352.0 o*002 . . . . . . . 354'8 0'00 I . . . . . . . 3 52'4 0*0005 . . . . . . . 3 48.2

Hjidrocldoric Acid and 20 Per Cettt. Sitcrose. 0.05 . . . . . . . 282.5 0-oa . . . . . . . 287.5 O'OI . . . . . . . 290.3 0.005 . . . . . . . 291.8 O*OOI . . . . . . . 286.1 0-0005 . . . . . . . 276.0

From the tables it is evident that at considerable dilution the equivalent conductivity of hydrochloric acid in presence of sucrose falls after reaching a maximum, instead of rising slowly.

A similar experiment with potassium chloride and 10 per cent. sucrose was carried out with the following results :-

0'01 . . . . . . . 349.0

0'002 . . . . . . . 290'5

Potassium Chloride a:rd 10 Per Cetr f. Sucrosc. Concentration of

Salt in Equivalents Per Litre.

Equivalent Con- ductivity in Mhos.

0.01 . . . . . . . 116'0 0.005 . . . . . . . I 18.2 0'002 . . . . . . . 120'0 0'00 I . . . . . . . I2 1'2 0*0005 . . . . . . . 122'0

It will be observed that in the case of potassium chloride, the behaviour is normal, the equivalent conductivity increasing steadily with increasing dilution.

I t was at first thought that the abnormality of the equivalent conductivity

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AND POTASSIUM CHLORIDE I N PRESENCE OF SUCROSE 121

in the case of hydrochloric acid and sucrose was evidence of a union between sucrose and hydrogen ion, but, as is shown below, the behaviour can be completely accounted for by the presence of electrolytic impurity in the sucrose.

If the abnormality in the values of the equivalent conductivity of hydro- chloric acid and sucrose be due to chemical or physicaI action between sucrose and hydrogen ion then the position of the maximum in the conductivity curves should not alter sensibly when one uses samples of sucrose of different specific conductivity. If, on the other hand, the disturbance of the conductivity curves be clue to the action of the hydro- chloric acid on the impurities in the sucrose then the less impurity there exists, the less pronounced will be the deviation from the normal. More- over, the position of the maximum will vary with different varieties of sucrose.

Accordingly measurements were made of the equivalent conductivity of

IOO 200 400 500 I000 Dilution in Litres.

2000

hydrochloric acid in presence of different varieties of sucrose. Four different varieties were used having a specific conductivity in 10 per cent. solution of 2.9, 3.3, 4.2, and 12.4 gemmhos. The accompanying graph was drawn from the results obtained. I t will be noticed that as the conductivity of the sucrose falls, the drop in the curves becomes less pronounced and the position of the maximum occurs at greater dilution. One is led from these results to the conclusion that the shape of the curves is due rather to the impurity in, than to the specific chemical or physical nature of the sucrose.

This conclusion was further strengthened when very accurate measure- ments revealed the fact that the equivalent conductivities of aqueous sohtions of hydrochloric acid went through a maximurn even in absence of sucrose. This phenomenon has already been pointed out by Whetham,l by Goodwin and Haskell,2 and by Kendall.3

IZ. phys . CJze;n., 55, 204, rgo6. Phys. Review. 19, 380, 1904. -7. Anzeu. CJ!cm. Soc., 3, 7, 19x7.

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I 2 2 ELECTRICAL CONDUCTIVITY OF HYDROCHLORIC ACID

Viewed in the light of this fact it seems almost certain that in the case of the hydrochloric acid and sucrose the abnormality is due to the presence of electrolytic impurity in the sucrose.

Although examination of the arc spectrum failed to show the presence of any metal in the sucrose, the assumption that the impurity consists mainly of an organic salt of calcium, that is, the salt of a weak acid, would be inferred from the technical mode of purification. Such a salt would react with a certain amount of hydrochloric acid producing a corresponding amount of practically unionised organic acid.

Assuming that an organic salt of calcium is accounting for the specific conductivity of a 10 per cent. sucrose solution, namely 2.9 x I O - ~ mhos., it can be shown that the concentration of such a salt is 2 -I x 10 - 5 normal. This would react with an equal amount of hydrochloric acid having a specific conductivity (allowing for the viscosity of the sucrose solution) of 7 -5 x I o - mhos. The organic acid formed would be practically unionised in presence of the remainder of hydrochloric acid whilst the original calcium salt would be replaced by calcium chloride having practically the same specific conductivity as the organic salt. Hence the observed specific con- ductivity of hydrochloric acid in sucrose solution is 7-5 x I O - ~ minus 2.9 x I O - ~ , or 4-6 x I O - ~ mhos. less than it would be if no disturbing effects were present. In other words to obtain the true specific conduc- tivity of hydrochloric acid in 10 per cent. sucrose solution an addition of 4-6 x I O - ~ mhos. instead of a deduction of 2-9 x I O - ~ mhos. must be made to the observed values. When this correction is applied the values given below are obtained.

Hydrochloric At id aird 10 Per C w t . Szicrosc. Concentration of

Acid in Equivalents

Per Litre.

Corrected Equivalent

Conductivity.

0'01 . . . . . . . 349'8 0.002 . . . . . . . 358-6 0.05 . . . . . . . 353'5 0'001 . . . . . . . 3 59'9 o 0005 . . . . . . . 363'2

I t will be seen that the maximum has vanished and that the assumption that the observed specific conductivity of the sucrose is due to dissolved organic salts of calcium, is sufficient to account for the abnormality in the concentration curves.

Relation between Ionic JfubiZity a d Viscosity.

Green1 and Martin and Masson have investigated the electrical con- ductivity of hydrochloric acid and potassium chloride in presence of sucrose and have suggested the applicability of the equation :-

A = h'qm

where h = equivalent conductivity in medium of unit viscosity. A' = equivalent conductivity in a medium of viscosity -q. m = a constant for the particular system.

Green finds that for the case of hydrochloric acid and sucrose in = 0.55,. whilst for potassium chloride and sucrose m = 0.7.

I'jf. Cl~cnr. SOC., 93, 2023, 190s. 2 Ibid. , 79, 707, 1901.

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AND POTASSIUM CHLORIDE I N PRESENCE OF SUCROSE 123

The above equation has been applied to data obtained in this laboratory and the value of m has been computed for solutions of various electrolytes in presence of various non-electrolytes. The viscosities used are those given by Powe1l.l

Electrolyte. Non-Electrolyte. In.

HCl . . . Sucrose . . . 0'57 HC1 . . . Dextrose . . . 0.68 KC1 . . . Sucrose . . . 0.73 KC1 . . . Dextrose . . . 0'77 KCI . . . Glycerol . . . 0.92

Viscosity Corrcctim f o r Uydroge?en Ion it8 Presence of Sucrose.

The index PZ in the equation :- A = A' n' 'I

has been shown to be equal to 0-57 for the case of hydrochloric acid in presence of sucrose. Although this value refers to the molecule of hydro- chloric acid as a whole it is capable of distribution between the two ions, hydrogen and chlorine.

For the case of an infinitely dilute solution of hydrochloric acid in presence of sucrose we have

[ ~ H ' ( s ) f- vcl'(s)]yo'57 = 422'4 . (1)

where UH-(~) = mobility of hydrogen ion in presence of sucrose. VCI'(,) = mobility of chlorine ion in presence of sucrose.

MacInnes2 has found that in the case of several chlorides including hydrochloric acid

TCl'Ago7 = K where Tcl' = transport number of chlorine ion.

A = equivalent conductivity of electrolyte. 7 .= viscosity produced by the presence of the electrolyte alone.

Using lithium chloride in a series of sucrose solutions Green has shown that A0g0'7 = k, where A. is the equivalent conductivity of lithium chloride at infinite dilution in presence of sucrose. Macfnnes concludes that the exponent 0-7 represents the effect of viscosity on the conductance of potassium, lithium, and chloride ions, but from the table given above we are lead to the value 0.73 as a more exact figure for the exponent. We can hence write

V c 1 ' ( ~ ) ~ ~ ' ~ 3 = k . - (2)

VCl' = f?? = 75 '2

When the concentration of sucrose is zero and 7 = I we obtain

Substituting in equation ( 2 ) we find

Combining equations ( I ) and (3) we have

'J. Cheni. SOL, 105, I, 1914. 3 7. Anzer. Clitnz. SOC., 45, 1217, 1921.

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124 ELECTRICAL CONDUCTIVITY OF HYDROCHLORIC ACID

Putting

where UH* = mobility of hydrogen ion in water we obtain

The following table gives the values of m obtained from equation ( 5 ) for solutions containing different amounts of sucrose and therefore possess- ing different vaIues of p :-

Grams of Sucrose

Solution. per 100 C.C. of 4. In.

I0 1.313 0-54 20 r.794 0'54 30 2.616 0'54 40 4'073 0.5 4 50 6.825 0'54 60 12.701 0'54

The value 0.54 represents, then, the viscosity correction to be applied 'to hydrogen ion in sucrose solution. By a calculation similar to the above it was found that the best value for the exponent in the case of hydrogen ion in presence of dextrose was 0.67.

Relation between Viscosi& Cowection and Nature of the Non-Elec fro+?.

For the case of the movement of an ion through a solution of a non- electrolyte it has been pointed out by Kraus 1 that the smaller the molecules of non-electrolyte and the larger the ions, the more nearly does the value of m approach unity. This is in harmony with the results obtained in this investigation, as evidenced by the table given above in which values of m have been computed for several electrolytes in presence of various non- electrolytes. The case of potassium chloride and glycerol is interesting in view of the fact that the concentration of glycerol used produced the same viscosity as a 10 per cent. sucrose solution. In the case of potassium chloride and 10 per cent. sucrose solution m was found to be 0.73, whilst for potassium chloride and glycerol m equals 0.92. This is evidence of the fact that the resistance offered by a medium to the passage of an ion is not a mere function of the viscosity of the medium but depends also on the relative dimensions of the molecules of the medium, and the moving ion.

Summary.

I . The equivalent conductivities of hydrochloric acid and potassium chloride over a wide range of concentration and in presence of varying .amounts of sucrose have been determined.

2. I t has been shown that minute amounts of electrolytes, such as are present even in very pure sucrose, are sufficient to cause a decrease in the equivalent conductivity of hydrochloric acid at great dilutions.

3. The best value of m in the equation A = h'qm has been computed €or various systems and the results shown to be in accordance with the

Iy. Amer. Chem. SOC., 36, 35, 1914.

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-4ND POTASSIUM CHLORIDE I N PRESENCE OF SUCROSE 1 2 5

principle that the smaller the molecules of the medium and the larger the ions, the more nearly will the value of m approach unity.

I desire to express my thanks to Professor W. C. McC. Lewis under whose direction this work was carried out, and to Messrs. Tate & Lyle for the generous supply of sucrose.

Musjratt Laboratory of PhysicuC and EZectvo-Chcmisfvy, University of Liveq5ooZ.

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