T H E REACTION BETWEEN POTASSIUM PERSULPHATE AND POTASSIUM IODIDE I N GELATIN SOLS* BY SIDNEY OWEN RAWLING AND JOHN WILLIAX GLASSETT.
In aqueous solutions, persulphates and iodides react according to the equation K2S2Og 2KI = 2K2S04 1,
+
+
The reaction is comparatively slow and its velocity under various conditions has been investigated quantitatively by Slater Price1. Friend and Vallance2record that with S / 1 6 solutions of potassium iodide and ammonium persulphate no change of reaction velocity is caused by the addition of gelatin up to a concentration amounting to 0.1 per cent. The method adopted by Price was to titrate the iodine set free from the reaction mixture after various intervals of time. This when used for a system containing gelatin does not give a true record of the main reaction because the iodine liberated is slowly removed by the gelatin. I n spite of this, it has been shown by the experiments which will now be described that the initial rate at which the iodine is liberated is greater in the presence of gelatin than in pure 20 d0 60 80 I00 aqueous solutions. This rate diminishes quickly and the general type of curve FIG.I representing the velocity of liberation of iodine is shown in Fig. I, in which-the volume of this sulphate solution (N/Ioo) required by 2 j cc of the reaction mixture is plotted against time.
Experimental Gelatin sols were made up by dispersing j grams of gelatin in 2 5 0 ccs. of distilled water. In carrying out the experiments the sol to be tested was brought to 2 joand IOO ccs. of it were mixed with j o ccs. of a solution of potassium iodide (N/20) at 25'. At a noted time j o ccs. of potassium persulphate solution (N/2o) were quickly added. 2 j ccs. of the mixture were withdrawn from time to time and titrated-in a large volume of cold water--with a solution of sodium thiosulphate N/IOO. In preliminary experiments it was found that the addition of acid caused a very marked increase in the apparent rate of *Communication KO. 38 from the British Photographic Research Association Laboratories. 1 J. Chem. Soc. 121, 473 (1922). 22.physik. Chem. 27, 474-512 (1898).
R E A C T 1 0 6 B E T W E E N PERSULPHATE AND IODIDE
415
reaction, though Price found (loc. cit., 490) that in the absence of catalyscrs, acids have very little effect. On account of this the experiment was repeated several times with sols containing varying amounts of acid. This was done with two kinds of gelatin and for one of these both hydrochloric and sulphuric acids were tried. The results of the experiments are recorded in Tahle I as titration values after allowing the reaction t o run for 40 minutes.
-
FIG.z
2 /77//7.
I m//7
FIG.3
TABLE I Gelatin A. Acid present per litre of sol. milli. eqts. 0.0
Hydrochloric Acid. Acid present per litre of sol. milli. eqts.
3.45
47.7
11 .65
53 . O 58.2
11.70
31.8 37.2
10.9
10.8
Gelatin A.
Sulphurjc Acid.
7 15.9 21.3 26.8
Gelatin €3. 0.0
5.6 11.2
16.8
5"
Titration value for 25 ccs. mixture. ccs.
5.3 10.6 26.5 IO.
2
5.2
7.25
8.0 9.9
Titration value per 2 j ccs. mixture. ccs.
11.50
11.1
11.0 11.1
Hydrochloric Acid. 2.3 4.0
5.6 7.3
These results are plotted in Fig.
2
22.4
7,7
28.0 33.6
8.3
8.5
416
0.5 4.2
03
$5-0
SIDNEY OWEN RAWLING AXD JOHN WILLIAM GLASSETT
to remove the inorganic or “ash” constituents. Most of the gelatins contained sulphite and it was at first thought that the length of the inter-
/
TABLE I1 Gelatin A.. Concentration of gelatin per cent.
0.85 0.49
5.0 2.2
I .o
0.28
0.83 0.00
Apparent Relative Initial Reaction T’elocity. Given by slope of line A B.
0.22
(water)
0.08
R E A C T I O S B E T W E E N PERSULPHATE AND IODIDE
417
According to Davies and Oakesl, C. R. Smith2,and others, there is no tendency to gel formation in gelatin sols a t about 40’. It was found, however, that at 39’ the rate of reartion with gelatin present to the extent of I per cent increased the rate of reaction about 4 or 5 times a t this temperature, so that the presence of the gel form is not responsible for the accelerating effect. It was also shown repeatedly that the velocity of reaction measured in a sol which had been freshly cooled to 25’ was the same as that in a sol which had been allowed to stand for two hours a t this temperature. Thus the accelerating effect seems to be independent of the age of the sol.
Discussion The effect of the presence of gelatin upon the salt reaction here described may be considered in relation to four factors: ( I ) the reaction between the iodine liberated, and the gelatin, ( 2 ) the influence of catalysts present as impurities in the gelatin ( 3 ) the formation of gelatin salts of persulphuric and hydriodic acids and (4) the adsorption of the reacting ions by gelatin. ( I ) The first factor tames the apparent velocity curve Fig. I as determined by the rate of liberation of iodine to be very different from what the real velocity curve would be. The actual amount of iodine measured at any time is never as great as it would be if there were no reaction between gelatin and iodine. A further coiiiplication occurs on this account if gelatin itself is the accelerator, for the actual amount of gelatin is diminished as more and more iodogelatin is formed. ( 2 ) The accelerating effect might be ascribed t o the presence of catalysers contained as impurities in the gelatin. The most likely impurities of this kind are to be looked for in the inorganic of “ash” content. Analysis showed that the ash in each case consisted of the following ingredients present in varying amounts :Fe203,CaO, MgO, A1203,Cl’, sO”4 and P O’”4 The one constituent of the ash which is known to produce acceleration of the reaction under consideration is iron. This was shown by Slater Price (loc. cit.) who also showed that the reaction velocity in pure aqueous solutions is almost independent of hydrion concentration unless iron or some other such catalyser be present. It seemed therefore that a possible explanation of the phenomena described here could be found by taking the iron content of the gelatins into consideration. The B gelatin was a powdered sample and contained 0.013 per cent of iron, which is present to some extent as finely divided particles of metal. The addition of acid would cause this to dissolve and so to become active. The iron content of the A gelatin was only 0 . 0 0 2 3 per cent. Thus the reaction mixture in the case of the B gelatin would contain 0.000023 gram atoms of iron per litre. Whilst in the case of the A gelatin the concentration would be 0.000004 gram atoms of iron per litre. J. Am. Chem. SOC.41, 135-150 (1919).
* J. Am. Chem.
SOC.44, 464-479
(1922).
418
S1DP;EY OWEK RATVLING A S D JOHN WILLIA3I GLASSETT
Slater Price found that with S/90persulphate and XI40 iodide in N / z o o sulphuric acid the acceleration due to a concentration of 0.000024 of iron was about 81 per cent. In the particular experiment described here the maximum reaction velocity att)ainedwith the B gelatin was about 400 per cent above that obtained without gelatin present. This indicated that the iron in the gelatin could not be responsible for the whole of the acceleration. This conclusion was confirmed when it was found that the A gelatin containing only one sixth as much iron as the B sample gave the higher maximum reaction velocity. When the second experimental method described here was applied to gelatins which had been treated so as to remove almost all the inorganic constituents, it was found that the accelerating effect was still present, in fact one such gelatin, kindly supplied by Dr. Mees and Sheppard of the Kodak laboratories, containing only 0.035 per cent of ash gave one of the highest values for initial reaction velocity yet recorded for any of the samples of gelatin examined. From these results it was concluded that if the acceleration is due to catalysts present as impurities, these impurities are of an organic nature and must be present in all gelatins, for none of the twenty or thirty samples of gelatin examined failed to give the effect. (3) As already pointed out the velocity of reaction in the asbence of gelatin is almost independent of the hydrion concentration and since the considerable effect of acids cannot readily be explained as being due to iron, etc., it seemed that the modern theories of colloidal behavior of gelatin might provide an explanation. From the work of C. R. Smith1, Davies and Oakes2, and BogueS, it seems that at 40’ a gelatin sol is far simpler than a t temperature below 30’. J. Loeb4puts forward a theory of the conditions prevailing in protein sols. He supposes that “although gelatin solutions may be, and probably are, as a rule true solutions, consisting of isolated protein ions and molecules distributed equally through the water, they contain under certain conditions submicroscopic, solid particles of gelatin .” The solid particles referred to are presumably formed in the neighbourhood of the “transition temperature” of Davies and Oakes (Zoc. cit.), and have the properties of small particles of jelly. They must therefore be subject to the laws of membrane equilibria enunciated by Donnan, and applied by Procter5 to the explanation of the swelling of gelatin jellies when surrounded by aqueous solutions of acids. If we accept Loeb’s theory we must consider a gelatin sol at 2 5 ’ as consisting of two phases: the continuous phase being a solution of gelatin ions and molecules, and the disperse phase being particles of jelly. In an acid sol there will be a distribution of hydrogen ions and the acid anions between the two phases which may be expressed by the equation x1 y1 = x2 yz J. Am. Chem. SOC. 41, 135-1 j o (1919).
* J. Am. Chem. SOC.44, 464-479
(1922). J. Am. Chem. SOC.44, 1313-1322 (1922). “Proteins and the Theory of Colloidal Behaviour” (1922). J. Chem. POC. 105, 313 (1914).
REACTION BETWEElU PERSULPHATE AND IODIDE
410
where XI is the concentration of hydrogen ions in the continuous phase, y l i s the concentration of acid anions in the continuous phase, x2 and y2are the corresponding concentrations in the jelly phase. 1,oeb (op. cit. Chapter 1 2 ) found that when hydrochloric acid is added to a sol of gelatin a t 25' the viscosity increases. This was explained as being due to an increase in concentration of diffusible ions within the jelly particles over that in the continuous phase resulting in the swelling of the particles. The hydrion concentration itself is higher in the continuous phase so that the effect is mainly due to an increase of chloride ion concentration in the disperse phase. According t o this idea, in the case of the reaction mixture of persulphate and iodide in acid gelatin sols there is a higher concentration of persulphate and iodide ions in the jelly phase which would result in an increase in the reaction velocity in that phase, and a diminution of velocity in the continuous phase. The reaction velocity actually measured would then lie between the velocities in the two phases and would depend upon the difference between the concentration of the reacting ions in the two phases and upon the relative volume occupied by the disperse phase. Suppose now that a gelatin sol containing persulphate and iodide may he considered as corresponding with the two-phased system discussed. According to Loeb there is no chemical combination between gelatin and acid radicles in this condition and thus according to this theory we should expect no catalytic action at the isoelectric pcjint. Experiment showed however that even a t the isoelectric point there is still a fairly marked effect and the addition of alkali appears to make this less. Work with alkali is rendered difficult, however, owing to the reaction between iodine and free alkalies. It may further be noted that the maximum concentration of diffusible ions within the jelly phase occurs-according to Loeb's experiments on viscosityat a Sorensen value of 3.0. We should therefore expect a maximum reaction velocity t o occur about pH 3.0 and that further addition of acid would cause a decreased velocity. The experiments showed however that although the maximum acceleration occurred about pH 2.8, further addition of acid did not produce any diminution in the effect. Thus the theory of colloidal behaviour of gelatin as developed by Loeb cannot explain all the facts, and this is emphasized by the observations that gelatins giving practically isoelectric sols exert a considerable accelerating effect, and that above the temperature of gel formation the effect still occurs. (4) From what has been said already we are left to consider the possibility of adsorption of the reacting ions on the surface micelles of gelatin. These simpler particles may he supposed to exist a t temperatures a t which the gel form cannot exist. By assuming adsorption of the ions upon these the accelerating effect of gelatin a t 39' may be explained. It may be pointed out that much work has been done concerning the laws governing the combination of acids with gelatin in which the amount of acid combined has been calmlated from the difference between the hydrogen ion concentration in an aqueous solution of the acid that and in a corresponding
420
SIDKEY OWEN RAWLING AND J O H N WILLIAM GLASSETT
solution of the acid containing a definite quantity of gelatin. These measurements when made at temperatures at which the gel form exists may be erroneous since they do not take into account the different hydrogen ion concentrations in the different phases of the sol, nor do they consider a higher concentration-or perhaps a different chemical condition of gelatin-in the gel particles from that which exists in the surrounding liquid, a state of affairs which must necessarily exist if membrane equilibria ale to exeit any influence at all. Indeed, for quantitative purposes it seems as if the expression “hydrogen ion concentration of a gelatin sol” has little meaning except a t temperatures at which the gelatin is entirely in the sol form or the gel foim, for in other cases the pH value must be different in the different phases. Luther’ describes a method for finding the copper ion concentration in dilute solutions. This depends on the observation of reaction velocity between persulphate and thiosulphate and iodide in the presence of iron and copper salts as catalysts. It may be pointed out that the presence of gelatin would interfere with this method.
Summary The reaction in solution between iodides and persulphates is accelerated by the presence of gelatin. The acceleration is increased up to a maximum by the addition of acids. Alkalies reduce the effect. Even with sols a t the isoelectric point considerable acceleration occurs. The presence of the “gel” form of gelatin is not necessary for the effect to occur since the same phenomena may be observed above the so-called “transition temperature” of gelatin. The presence of inorganic catalysts such as iron have been shown not to be the cause of the acceleration, nor does Loeb’s theory offer a satisfactory explanation of the facts. The effect may be due either t o an organic catalyst in the gelatin or to adsorption of the reacting ions on gelatin particles which are different from the “submicroscopic particles of jelly” postulated by Loeb. Trans. Faraday SOC. 19, 394 (1923).
