T H E STARCH-IODINE REACTION
BY N. R. DHAR
Two distinct views have been put forward in order to explain the mechanism of the starch iodine reaction. Several authors, notably Rouvierl, Euler and Myrback2 and others have formulated the view that on the addition of iodine t o starch paste, definite chemical compounde are formed. The number of formulae denoting the compound starch-iodine has come up to thirteen, the iodine content varying from .3 to more than 20 per cent. In opposition to the above view, Kuster3 has shown that the blue colour of starch iodide can be easily explained by the phenomenon of the adsorption of iodine by starch. Padoa and Savare4, Katayama5, Berczeller6, Bancroft’ and Lottermosers are supporters of the adsorption view. It is well known that starch forms a colloid when mixed with warm water and on the addition of iodine to this colloidal starch the well known blue substance which is certainly colloidal in nature is obtained. It appears that colloidal starch is negatively charged and on the addition of iodine the amount of charge on each particle is increased. Like most colloidal substance starch has a good adsorptive power, as has been shown by Robieong, LloydlO, Rakovski” and others. Evidently there is hardly any doubt that adsorption plays a very important part in the formation of the blue substance. It is apparent that the blue substance, starch iodide, is colloidal in its nature. I n order to throw light on the nature and on the constitution of the blue substance, electric conductivity measurements were carried on. Several years ago Duc1aux12,found positively charged ferric hydroxide sol is fairly conducting. I n a recent paper Wintgrenla has determined the conductivity of stannic acid peptised by alkali. Very recently Sen, Ganguly and Dhar have found that ‘negatively charged ferric hydroxide sol is also fairly conducting. Fifteen grams of Lintners’ soluble starch (British Drug Houses Ltd: London) were made up to 1000cc. using warm water a t the beginning. The conductivity of this colloidal solution was determined. When N/IO alcoholic solution of iodine, the conductivity of which was determined before, was added to the starch solution, it was found that the conductivity of the blue Compt. rend. 117,461. 2Ann. 428, I (1922);Arkiv. Kem. Min. Geol. 8, No. 9, I (1922). Ann. 283 364 (1891). Atti, R.Accad. Lincei, 14, I, 467 (1905). Z.anorg. Chem. 56,209 (1907). Biochem. Z.84,106(1917). “Applied Colloid Chemistry”, 104 (1921). * Z. angew. Chem. 34,427 (1921). Proc. Camb. Phil. SOC.15, 548 (1910). lo J. Am. Chem. SOC. 33, 1213(1911). I1 J. Russ. Phys. Chem. SOC.45, 7, 13 (1913); 46,24 (1914). l2Cornpt. rend. 140, 1468 (1905);Kolloid-Z. 3, 126 (1908). lS Z.Phys. Chem. 103, 238 (1922).
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N. R. DHAR
substance thus obtained is greater than the sum of the conductivities of the two substances. The conductivity measurements were carried out a t 25’ and the following results were obkained:-
TABLE I Substance Conductivity Absolute alcohol 4 . 8 6 X IO-^ N/ IO alcoholic iodine I .36X10-~ :.N/Io iodine (in alcohol) 8 . 7 6 X IO-^ Aqueous iodine (about N / ~ o o o ) 4.32 x IO-‘ Water 3 .37xIO-6 .*.Iodine (in water) 0 .g g x 10-6 Starch (colloidal) I 5 grams per litre 2.7 XIO-~ Starch (alone) 2.36X IO-^ 20cc. starch+. I cc N / I O alcoholic iodine 2 . ~ ~ X I O - ~ 11 11 11 . 2 cc l 1 ” 2.63 X IO-^ 11 11 2 . 7 4 x IO+ l1 l’ .3 cc l 1 l‘ .4cc1’ 11 ” 3.01x10-5 7) l1 3.85xIo-5 l1 l1 .5 cc l 1 11 l1 l1 + .6 cc l 1 ” 3 .93 x 17 ” ” . 8 cc ” ” 4 . 8 5 X IO-^ 11 5.96X10-~ l1 l1 . g cc l 1
+ + + + + +
))
T.4BLE
Substance 2 0 cc. starch sol 11
l1
11
l1
11
l1
11
l1
lJ
l1
’l
l1
l1
l1
l1
l1
l1
11 11
11
2 01 1cc
starch sol
’)
l1
11
l1
l1
11 11
l1
l1
l1
l1
)l
l1
l1
11 11
11 Conductivity
+ IO-^ + cccc N/IO alcoholic iodine 7 . 7 4xx IO-^ 9.52 x + 3 cc 8.86 X IO-^ ++ 45 cccc 8 .28 X IO-^ I 2
’l
11
11
11
”
I O . 01
11
11
++ 67 cccc + 8 cc
++
+ + ++ +
11
11
11
11
11
l1
11
TABLE I11 cc alcohol cc l1 3 cc 4 cc 5 cc l1 6 cc l1 7 cc l’ I 2
:;
” ”
”
’)
7.63X10-~
7 . I 5x
6.77 X IO-^
2.28X10-~’ I.99x10-5 I . 83 X IO-^ I .66X10-~ I . 5 I X I O -5 I . 3 9 x IO+ 1.28X10-~
From the foregoing experimental results it is quite clear that on addition of alcoholic iodine to a sol of starch, the conductivity is greatly increased. It will be seen from Tables I and I1 that, as the amount of iodine goes on increasing the conductivity of the mixture also goes on increasing to a maximum and then it falls off on further addition of iodine. This decrease in the conductivity is certainly due to the falling off of the conductivity of starch on the addition of absolute alcohol. The experimental results in Tables I and I11 conclusively prove that on addition of absolute alcohol to starch, the conductivity gradually falls off as we go on adding more of alcohol to a definite volume of the starclt sol.
T H E STARCH-IODINE REACTION
127
On comparing Tables I1 and I11 we find that the ratios of conductivities when corresponding amounts of the substances are added are 3.4, 5 , 5.2, 5.35, 5.45, 5 . 5 , 5.6 and 5.8. These numbers are in increasing order of magnitude. I n other words, these ratios conclusively prove that the greater the amount of iodine added to a definite volume of starch ~ o lthe , greater is the conductivity of the mixture. Consequently, the foregoing experimental results prove that on the addition of alcoholic iodine to starch, the conductivity of the blue substance produced is rnuch greater than the sum of the conductivities of the reacting substances (viz starch and iodine) It seems,therefore, that the blue compound is apprecially conducting and gives out some ions probably of the micellar type. From my Conductivity experiments I am of the opinion that this blue substance formed by the adsorption of iodine by starch is something like an unstable chemical compoiind and behaves like an unstable iodide. I n order to explain the conductivity of colloids and substances like soap, the micelle theory of ions has been advanced.1 The same conception of ionic micelle has been applied to a sol of stanriic acid peptised by alkali2. It is supposed by these authors that one of the ions given out by colloids and substances like soap, which is micellar in nature, contains many atoms and is very heavy3. It seems, probable, that with starch iodide we get a heavy micellar ion containing starch and the other ion may be the simple iodide ion. I n order to see whether other colloids would show greater conductivity as in the case of starch iodide several conductivity experiments were made with weak electrolytes like acetic acid, boric acid, arsenious acid, etc., when treated with freshly precipitated and well washed ferric hydroxide. Starting with a sample of glacial acetic acid which is 16.5 N. I find it is practically non-conducting. When freshly precipitated and well washed ferric hydroxide, which is dried in air, is added to glacial acetic acid and shaken the conductivity of the mixture is highly increased. Similarly the conductivity of 13.75 N acetic acid in presence of freshly precipitated ferric hydroxide is greatly increased. But with more dilute acetic acid the reverse is the case. Thus with 2.38 N acetic acid the conductivity is decreased on the addition of ferric hydroxide. It is well known that a positively charged ferric hydroxide sol is obtained by shaking freshly precipitated ferric hydroxide with dilute acetic acid. The whole of the colloid can be readily coagulated by the addition of electrolytes like (NH4)2S04, NazSOI etc. In this case practically the whole of the iron remains as a colloid and not as ferric acetate. The decrease in the conductivity of dilute acetic acid on the addition of ferric hydroxide is certainly due to the adsorption of acetic acid by ferric hydroxide and hence the concentration of the acid is decreased. In a foregoing paper from this laboratory it has been shown that by shaking freshly precipitated ferric hydroxide with arsenious acid a negatively See Zsigmondy: “Chemistry of Colloids”, I66 (1917); Pauli and Matula: Kolloid-Z. 21,49 (1917). 2 Wintgren: loc. cit. Compare Dhar: Z. anorg. Chem. 1913, SO, 43 (1913).
I 28
N. R. DHAR
charged sol of ferric hydroxide is obtained. The decrease in the conductivity of arsenious acid on the addition of ferric hydroxide is also certainly due to the adsorption of arsenious acid by ferric hydroxide, which is a good adsorbent of arsenious acid. Exactly similar results are obtained with boric acid. With concentrated acetic acid the case is different. If ferric hydroxide is added to glacial or very concentrated acetic acid the conductivity is enormously increased due certainly to the formation of the electrolyte ferric acetate. That ferric acetate and not colloidal ferric hydroxide is formed is shown by the fact that no coagulation takes place on the addition of electrolytes to these solutions. In these cases the formation of the conducting electrolyte ferric acetate more than counteracts the adsorption of acetic acid by ferric hydroxide and hence the conductivity is greatly increased. Hence the formation of ferric acetate or colloidal ferric hydroxide on the treatment of acetic acid with freshly precipitated ferric hydroxide depends on the concentration of acetic acid and this is guided by the following mass action equilibrium Fe(OH)3+3CH3 COOH =Fe(C H3C00)3+3Hz0 Looking a t the whole problem it seems that the blue substance obtained by the adsorption of iodine by starch is something like an unstable chemical compound which can give off ionic micelle in water. It will be interesting to find out whether there is any change in the conduc tivity of the reactingsubstances(viz. sulphur dissolved in SZCLor CCLor CSZ and the raw caoutchouc). That there is some chemical action in the process of vulcanisation will be evident from the fact that its temperature coefficient for a 10' rise is 1.8. This value is greater than the average temperature coefficient of homogeneous reactions.' In our experiments on the adsorption of electrolytes by manganese dioxide no effect of temperature on the phenomenon of adsorption was observed. The blue colour of starch iodide is readily destroyed by chlorine, silver nitrate, mercuric chloride, iodic acid and other oxidising agents, like nitric acid, chromic acid, etc. This fact can be easily explained on the view that the blue substance is an unstable iodide. The following results were obtained in the decolourisation of the blue substance with mercuric chloride:IO.CC of the starch solution with different volumes of iodine solution titrated against M/5 HgC12 Solutions. Volume of N/IO iodine Solution. Vol. of M/5 HgCl2 Sol. 0.5 cc 2 . 2 cc I cc 8 . 4 cc I cc 8 . 4 cc 2 cc 1 6 . 5 cc 3 .cc 2 4 . 7 cc I n order to decolourise with nitric acid far greater quantity of the oxidising agent is necessary. 1
Compare Dhar: Proc. K. Akad. Weten. Amsterdam, 21, 1042 (1919).
T H E STARCH-IODINE REACTION
I
129
I O cc starch and 0.5 cc of alcoholic iodine require I I O cc of 3.75 N "03. There has been a great deal of controversy as t o whether iodide ions are necessary in the generation of the blue substance. In Mylius' formula of starch iodide, H I is regarded as a part of the blue iodide of starch. This view has been corroborated by Roberts1, Lonnes2,Hale3 and Sen4;while Rouvier5, MeinekeG, Berczeller', Euler and Myrbacks have shown that potassium iodide is not necessary for the production of the blue substance. Lottermoser (loc. cit.) appears to have taken an intermediate position and believes that iodides are required temporarily for the production of the blue colour. I have observed that on the addition of a freshly prepared alcoholic solution of iodine to starch paste or colloidal starch the blue colour immediately appears. In this case iodine did not get time to undergo hydrolysis according to the equation 312-l-3 HzO= gHI+HI03, hypoiodous acid being formed as an intermediate product. The hydrolysis of iodine being a non-ionic change is expected to be slow; hencc the immediate appearance of blue colour on the addition of alcoholic iodine to colloidal starch does not support the view that iodide ions or H I are necessary for the production of the blue colour, because at the beginning there is hardly any H I in the mixture. Of course, according to the conception advanced in this paper the blue substance itself is an unstable iodide, which gives off iodide ions in solution. The great amount of H N 0 3 necessary for the discharge of the blue colour seems to be against the view that H I is necessary for the formation of the blue colour. The effect of several electrolytes on blue starch iodide has also been investigated. In thie line of research there has been a considerable difference of opinion amongst previous investigators. Standard solutions of KCl, (NOa)2, Ba KzS04, KI, Alz(S04), KBr and BaClz were prepared and in clean test tubes gcc of each of the solutions added to I O cc of the starch iodide obtained by mixing IOO cc of starch ( I S grams per litre) with .2 or .3 or .4 or .5 cc N/IO alcoholic iodine. I n all cases g cc of water was added instead of the electrolyte to I O cc of starch iodide and this tube scrved as the blank one for comparison with other tubes. The effect of electrolyte on starch iodide seems to be complicated specially that of KI, which markedly intensifies the blue colour. With other electrolytes the general effect seems to be coagulating in its nature. On the addition of the electrolyte the particles of the colloidal starch iodide coalesce with one another and become larger and hence the blue colour
Am. J. Sei. (3))47,422. Z.anal. Chem. 35,409. 3Am. Chem. J. 28,438 (1902). Proc. K.Akad. Wet. Amsterdam (1923). Compt. rend. 114,749. Chem. Z.18, 157 (1894). LOC.cit. * LOC.cit. Compare Bray: J. Am. Chem. SOC.32,932 (1910). 1
'
N. R. DHAR
130
is intensified though to a varying extent depending on the nature rather than the valency of the ions in question. It will be interesting to investigate whether the coagulation of blue starch iodide follow the Schulxe-Hardy rule. Summary When alcoholic iodine is added to starch sol, the conductivity of thc (I) substance formed is much greater than the sum of the conductivities of the individual substances. The subEtance obtained by the adsorption of iodine by starch is (2) appreciably conducting and behaves something like an unstable iodide. It seems probable that micellar ion? are given out in sol. ( 3 ) It eeems probable that the presence of outside iodide ions is not necessary for the formation of the blue substance. The hydrolysis of iodine is a slow process. (4) Electrolytes have the general effect of intensifying the blue colour by the coalescence of the smaller particles into bigger ions caused by thier coagulating effects. Chemical Laboratories, Sllahabad University, India.
