The Hydrogen Ion Concentration of Ferric Hydroxide Sols - The

THE HYDROGEN ION CONCENTRATION OF FERRIC HYDROXIDE SOLS

Downloaded by FLORIDA ATLANTIC UNIV on September 5, 2015 | http://pubs.acs.org Publication Date: January 1, 1931 | doi: 10.1021/j150337a014

BY WINIFRED L. MCCLATCHIE

Although a number of the more common sols of the suspensoid type have been extensively studied, the problem of accurately deducing their constitution is far from completely solved. A critical examination of the literature reveals many serious discrepancies, not the least of which is in the hydrogen ion concentrations determined by different methods with otherwise similar sols. For example, Browne,' using a hydrogen electrode directly in a ferric hydroxide sol, found a hydrogen ion concentration of 3 x 10-7 N for a sol comparable in the concentration of iron and chlorine and in electrical conductivity to one for which Wintgen and Biltz2 report a concentration of 3.8 x 10-4 N on the basis of hydrogen electrode measurements with the ultrafiltrate. Reliable values for the hydrogen ion concentrations of sols of metallic oxides and hydroxides are especially important in deducing the constitutions of these sols, for hydrogen ion is both the principal electrolytic cation and a part of the hydrolytic equilibrium, hydroxide acid basic salt water, that is presumably responsible for the stabilizing agent of many of these sols. Many methods have been used for evaluating these hydrogen ion concentrations, but no one has subjected the methods to a critical comparison, so that there is no reliable basis for evaluating the existing data, or of accounting for the discrepancies therein. The object of this investigation with ferric hydroxide sols was twofold. It was desired to compare the results obtained by several of the more common methods, in hopes that this might help to explain some of the present discrepancies. The principal object, however, was to establish a reliable method for such determinations, and to use the hydrogen ion concentrations thus obtained in conjunction with other physical and chemical data to deduce the constitution of these sols. Ferric hydroxide sols were considered to be most suitable for the investigation, for they have been the subject of extensive study, their chemical constitution is well established, they have been found to remain stable over a long period of time,a and their electrical properties are fairly constant with time.4 Experimental

+

e

+

Preparation of Sols. Sols for this work were prepared by neutralizing ferric chloride solutions with aqueous ammonia. The required amount of ferric chloride was dissolved in distilled water, the solution was cooled to IF.L. Browne: J. Am. Chem. Soc., 45, 297 (1923).

* R. Wintgen and M. Biltz: 2. physik. Chem., 107, 403 (1923). * J. Bohm: 2. anorg. Chem., 149, 203 (1925). 4

R.Wintgen and M. Biltz: 2. phyeik. Chem., 107, 403 (1923).


2088

WINIFRED L. McCLATCHIE

about IO’, and an approximately 1 2 per cent solution of ammonia was added dropwise, with mechanical stirring, until the amount added was equivalent t o 60 to 7 0 per cent of the iron. The ammonia solution was prepared by redistilling the stock reagent. Three sols were prspared by this method, ferric chloride solutions of the following concentrations being used: for sol 3, 600 g. of Sterling’s C. P. FeCls.6H20 in 4 . j liters of water; for sol 13, joo g. of Kahlbaum’s FeC13.6H20 “for analysis” in 2 liters of water; and for sol 20, 800 g. of h‘lerck’s FeC13.6Hz0“free from phosphorus” in 3. j liters of water. These sols were purified by dialysis for several weeks against continuously changing boiled-out distilled water with a specific conductivity of 1.5 to 2.5 X 100. The water was boiled in interconnected flasks, from which it passed into a large flask in which cellophane tubes containing t’he sol were suspended. All glass was Pyrex. Air entered the system only through wash bottles of 5 0 per cent’ sodium hydroxide and boiled-out distilled water. The removal of carbon dioxide was considered important in view of the work of Freundlich and Wosnessensky’ who showed that carbonate ion can serve as the stabilizing agent for ferric hydroxide sols, and it was thought possible that it might tend to replace chloride ion. Soft glass was used in the apparatus for t.he dialysis of sol 3, and the water showed a pH of 8.5 and a specific conductivity of about 1.5 X IO+. Experiments with this sol are therefore of only secondary value. All sols were stored in Pyrex containers equipped with inlet and outlet tubes, which were kept closed except for the removal of samples. Chemical Analysis of Sols. The sols were analyzed for iron and chlorine, both determinations being made on the same sample. The ferric hydroxide was dissolved by warming with concentrated sulfuric acid, and a double precipitation of iron was made, Iron was determined as ferric oxide, and the combined filtrates from each sample were used for the determination of chlorine as silver chloride. Each value in the second, third, and fourth columns of Table I represents the average of at least two determinations.

TABLE I Iron and Chlorine Concentrations of the Sols and the Specific Electrical 0.02”C Conductivities a t 25‘ Sol

Gram atoms of Fe Gram atoms of C1 Gram atoms Fe Gram atoms C1 per 1000g. H10 per 1000g. H20

KX

IO4

O.IY97 0.3073

0.02242

8.02

3.287

I3

0.04450

20

0.274

0,0379

6.91 7.7

4.190

3

5.695

Electrical Conduclivity. The electrical conductivities of these sols were measured by a simple Wheatstone bridge method. The sols were allowed to stand for at least two months before final measurements were made, for it has been shown that their conductivity becomes practically const’ant within this times2 The electrical conductivities obtained are given in column 5 of Table I. 1 2

H. Freundlioh and S. Wosnessensky: Kolloid-Z., 33, 2 2 2 (1923). R. Wintgen and M. Biltz: 2. physik. Chem., 107, 403 (1923).

2089

FERRIC HYDROXIDE SOLS


Hydrogen Ion Concentrations The hydrogen ion concentrations of the sols were determined directly by means of the hydrogen, quinhydrone, and glass electrodes; and in the ultrafiltrate by means of the glass electrode. A . Measurements with the hydrogen electrode. Hydrogen electrode measurements were made by a method similar to that of Browne,' and of Pauli and M a t u h 2 Sol 3, with which the measurements were made, had a ratio of gram atoms of iron to gram atoms of chlorine of 8, and, according to Browne, a stable equilibrium can be obtained with all sols for which the ratio is greater than 5 . A cell of Pyrex with lightly platinized platinum wire electrodes was used. The electrodes were replatinised after each measurement with the sol. Hydrogen was supplied from a cylinder and was passed through solutions of mercuric chloride, silver nitrate, alkaline pyrogallol, alkaline permanganate, and distilled water. A dilute bridge solution containing simultaneously approximately 0.0039 N potassium chloride and IO-^ N hydrochloric acid was used. The cells measured may be written as follows, the arrow indicating the direction of positive current within the cell :

1

0.004NKCl

+ IO-~NHCI

Sat. KCl

~

N/1o,Hg,C12 KC1 solid

I

Hg

4

The difference in potential was measured by means of a type K Leeds and Northrup potentiometer with a high resistance, high sensitivity galvanometer. Electrodes were used with the sol only when they had given stable potentials with potassium acid phthalate solution. I n a number of cases erratic results were obtained, or there was no approach to equilibrium in 60 to 90 minutes. The determinations in which equilibrium appeared to have been attained are summarized in Table 11, where E is the observed electromotive force of the cell and Cn+ is the corresponding activity of hydrogen ion.

TABLE I1 Hydrogen Electrode Measurements Time in minutes to attain equilibrium

2

E

CH+ x

30 40

0.628 0.620

PH 5.03 4.92

40

0.639

5.22

0.60

IO

0.623

60

0.6.55

4.95 5~53

0.29

F. L. Browne: J. Am. Chem. Soo., 45,297 (1923). W.Pauli and J. Matula: Kolloid-Z., 21,49 (1917).

106

0.93 I .20

1.12



2090

These results lead to the same conclusions that have been drawn by other experimenters;' namely, that equilibrium is difficult to attain, and that the results are not reliable. This is not surprising, for a system containing even traces of ferric ion would show an oxidation reduction potential that would vary with the degree of reduction. There is also the possibility of flocculation of the colloid by the gas bubbles.2 B. Measurements wzth the glass electrode. Of all of the methods that have been used with ferric hydroxide sols, the glass electrode was considered to be the most promising for reliable results. I t eliminates most of the difficulties associated with the hydrogen electrode, such as oxidation reduction potentials and flocculation by gas bubbles. The only serious uncertainty is the liquid junction potential between the sol and bridge solution, which is necessarily present in any electrometric determination of a single ion activity. Two methods for measuring the electromotive force of the cells were uspd. The first was that described by K e ~ ~ i d gae ,Lindemann ~ electrometer being used as null point instrument in conjunction with a type K potentiometer. The second method was that of R o b e r t ~ o nfor , ~ which a high resistance, high sensitivity galvanometer was substituted for the electrometer of the previous method. It was sensitive to less than one millivolt with electrode resistances under ten megohms. Each electrode was standarized against M/2o potassium acid phthalate buffer, with an approximately neutral reference buffer, and a saturated potassium chloride bridge. The cells measured may be written as follows:

I

I

Hg Hg2Cl2,KCl Bridge solid sat. sol'n.

~

M/zoKHP m1

I

1

Glass ~

1

I

Ref. KC1 HgZC12 Hg buffer sat. solid ~

, ~

The results obtained are summarized in Table 111. Eso, is the total electromotive force of the cell with the sol, and EKHPis that with M/zo potassium acid phthalate. The KCl, HCl bridge was the same as that used with the hydrogen electrode. Glass electrode measurements were also made with several samples of ultrafiltrate. The sol was filtered through a cellophane membrane by means of a high pressure apparatus, the details of which will be described in a subsequent communication. The hydrogen ion concentrations depend upon the conditions of ultrafiltration, as will be explained in discussing the significance of the results of ultrafiltration. This method is therefore an uncertain means of finding such values for the sols. The range of results is given in Table V. ' R . Wint,gen and M. Biltz: Z. physik. Chem., 107, 403 (1923); A. W. Thomas and A. Frieden: J. Am. Chem. Soc., 45, 2522 ( 1 9 2 3 ) . 2H.&I. Stark: J. Am. Chem. Soc., 52, 2730 (1930). 3P. T. Kerridge: Biochem. J., 19, 611 (1925). 4 G. R . Robertson: Ind. Eng. Chem., Anal. Ed., 3, 5 (1931).

2091


TABLEI11


Sol

3 3 3 3 3 3

Glass Electrode Measurements T" EKHP

Bridge

0.I54 0.151

N/IO KCl

0.146 0.146

KC1H C1

0 .I407

0.1402

I3 13 13 I3 I3

N/IO KC1

I3 I3

KCl, HC1

/ 0.1327

20

KClHCl

0,1550

20

0.146 i 0.144 0.140 0.142

0.171 0.171 0.163 0.163

21

0.1310 0 . I310

21

0.164 0.164 0.163 0.163 0.163

20

21

I8 I8

4.9

5.5 5.5 7.2

5 .o

I7 I7 17

4.30 4.28 4.30 4.37 4.33

4.3 4.7

20

0.1310 0.1300

0 . ISLO

CH+X 106 5.5

4 I4 4.14

21

0.1310 0.1310

I o 1325

pH

4.26 4.31 4.26 4.26 I

7.2

5.2

5 .O

21

4.00

10.0

21

4.00

10.0

21

4.38 4.35

4.2 4.5

20

TABLEIV Quinhydrone Electrode Measurements Sol

Bridge

E

CH+X

T

pH

0.1270*

20

0.1271

20

0.1145

23

4.16 4.16 4.18

6.9 6.9 6.6

0.1182 0.1170

23 23 23

4.12 4.13 4.05 4.06 4 .os 4.43 4.44 4.30 4.30 4.29 4.07 4.19

7.6 7.4 8.9 8.7 8.9 3.7 3.6

3

KCI, HCl

3 3

Sat. KCl N/IOOKCl

3 I3

N/goo KCl KC1, HCl

0. I 2 2 0

20

Sat. KCl

20

N/IOOKCl

20

N/soo KCl

0.1329* 0.1335 0.0980 0.0975 0.1061 0.1062 0.1064 0.1193 0.1173

20 20

24 24 24 24 24 24 24

* This calomel half celi showed an E.M.F. of 0.3345 V.

10)

5 .o 5.0 5.1 8.5 8 .o

In all others it was 0.3410 V.


2092


C. Measurements with the quinhydrone electrode. The quinhydrone electrode, in conjunction with various bridge solutions, was used with the same potentiometric set up as was the hydrogen electrode. With saturated potassium chloride, which was more dense than the sol, the siphon leading from the electrode compartment was filled with sol, which prevented mechanical mixing, and also coagulation in the region of the electrode, the latter occurrence having been found radically to alter the readings. The calomel half cell was checked against an M / 2 0 potassium acid phthalate buffer. The cells measured may be written as follows:

Hg

I

Hg2CI2,N/IO KC1 solid

I

Bridge sol'n.

~

Sol, quinhydrone solid 3

Discussion of Results The following is a comparison of the values that were considered to be the most' reliable by the various methods used.

TABLEV Summary of Results of Hydrogen Ion Det,erminations Electrode

Bridge

Age in weeks

pH

CH+x

105

Sol 3 Glass JJ

Quinhydrone J, 1J

1)

Hydrogen

N/IO KC1 KC1, HC1 Sat. KC1 N/IOOKC1 N/goo KC1 KCl, HC1 KCl, HCl

28

4.26

5.5

42

61

4 .f4 4.18

6.6

61 61

4.05

7.2

7.5 8.9

4.12

33 4.16 30 5.66 to 4 . 9 2

6.9 0.22

to

1.20

Sol 13 Glass 1,

Quinhydrone

X/IO KCl KC1, HC1 KCl, HCl Sol

Glass Quinhydrone >J

JJ

Glass

KCl, HC1 Sat. KCl N/ioo KCI N/5oo KC1

5.0

4.30 4.00

10.0

4.05

8.9

7

4.37

4.3

28 28 28

4.43

3.7

4.30 4.08

5 .O 8.2

21

35 26 20

Ultrafiltrate from Sol 13 N/IO KC1

2.8 to 8 . 4

2093



An examination of these results shows that the glass and quinhydrone electrodes agree fairly well when used with the same bridge solution and with sols of the same age. The hydrogen electrode, however, although used with the same bridge solution, indicated a concentration of only from one-fifth to one-thirtieth of that shown by the other methods. It may, therefore, be concluded that the glass and quinhydrone electrodes give a true measure of the hydrogen ion concentration of ferric hydroxide sols, and that the hydrogen electrode gives values that are erroneous and much too low. The effect of different concentrations of bridge solution is shown by the quinhydrone electrode determinations. The differences in hydrogen ion concentration may be seen from Table V and in terms of electromotive forces are shown by the following values: Sol

East. xcl

- EN/IOO Kcl

Esat. KCI

- EN/SOO Kcl

3

0.0030

0.0075

20

0.0085

0.0205

I n general, an increase in the concentration of the potassium chloride bridge solution decreases the apparent concentration of hydrogen ion; and there is no discontinuity in this effect when flocculation of the sol occurs so long as the sol in the region of the electrode is unaffected. It, therefore, seems probable that concentrated potassium chloride bridge solutions give the more nearly correct results, as is the case with simple electrolytes, for they more nearly eliminate diffusion potential, and there is no evidence of a membrane potential due to flocculated colloid. It has already been pointed out that the hydrogen ion concentrations previously recorded for ferric hydroxide sols, determined by various methods, show wide and inconsistent differences. Several investigators have used these values as a basis for deducing the constitution of the sols studied, and a critical examination of their methods and results is essential in evaluating their conclusions. This is especially true of much of the work of Pauli and his associates, who use such determinations as part of their evidence for classing ferric hydroxide sols as polyvalent strong electrolytes. Pauli and Matula,' by use of the hydrogen electrode in the sol, obtained about 8 X IO-^ N ; Pauli and Walter: by means of the hydrogen electrode in the flocculation filtrate, found it to be 3 X IO+ N; and Kuhnl and Pauli3 used a microtitration method to obtain 3.6 X IO-^ N. It has already been shown that the first method gives low results, and there is no reason to assume that the flocculation filtrate would be identical with the dispersion medium of a sol, and results in this paper show that trustworthy measurements could not be made if coagulation occurred near the electrodes. It is also to be expected that titration would alter the hydrolytic equilibrium of a sol, and results by this method were higher than would W. Pauli and J. Matula: Kolloid-Z., 21, 49 (1917).

* W. Pauli and G. Walter: Kolloidchem. Beihefte, 17, 256 (1923), 3

N. Kiihnl and W. Pauli: Kolloidchem. Beihefte, 20,319 (1925).

2094

WINIFRED L . MaCLATCHIE


be predicted by comparison with sols r3 and 2 0 of this paper. I t must, therefore] be concluded that the hydrogen ion determinations used by Pauli and his collaborators are not to be depended upon, but must be replaced by the much higher acidities here shown to be found with glass and quinhydrone electrodes. summary Apparent hydrogen ion concentrations as given by hydrogen, glass, and quinhydrone electrodes have been compared, using sols of ferric hydroxide prepared with special precautions to avoid impurities and of known chemical composition and electrical conductivity. Results with glass and quinhydrone electrodes agree, but hydrogen electrodes give very low and erroneous results. From experiments with various concentrations and acidities of the bridge solutions it was concluded that concentrated solutions of potassium chloride give the most nearly correct results, especially since flocculation of the sol has no effect so long as it does not occur in the neighborhood of the electrode. My cordial thanks are due to Professor James W. McBain a t whose suggestion this work was carried out. Department o j Chmzstry, Stanjord Unmerszty, Calzjornza.

The Hydrogen Ion Concentration of Ferric Hydroxide Sols - The

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