STUDIES ON PHOSPHORIC ACID*
11. Compound Formation between Ortho Phosphoric Acid and Certain Acidic Organic Compounds BY J A M E S H. WALTON AND RAYMOND J. KEPFER
The tendency of sulfuric acid to form molecular complexes with many other compounds has been shown by the senior author of this paper to be of great importance in explaining the decomposition of certain compounds by treatment with concentrated sulfuric acid. The compounds investigated were formic,' malic,* and triphenyl acetic3 acids. I n an investigation of the decomposition of formic acid by phosphoric acid it has been shown that the tendency of formic acid to form addition compounds would explain in a similar manner the dehydration of the formic acid. The present investigation was undertaken to show the tendency of compound formation between phosphoric acid and certain organic compounds. The method of investigation used was the one described by Knox and R i ~ h a r d s . This ~ method is based on the fact that when certain strong acids are added to solutions of a weak acid the solubilities of the latter at first decrease bccause of the common ion effect of the hydrogen ions. In certain cases, depending on the relative strengths of the two acids, the decrease in solubility takes place with increase in concentration up to a certain point only, after which an actual increase in solubility occurs. This increase in solubility is due to the formation of oxonium compounds, consequently solubility curves may be used to detect compound formation. The method is limited by the solubilities and the relative strengths of the acids used,
Experimental The ortho acid used was the ordinary syrupy phosphoric acid (8jyGSp. Gr. 1.710). Oxalic, succinic, and citric acids were purified by recrystallization. Malic acid was a d-1 mixture prepared by Eastman. This acid however was not entirely pure, for it had a slight burnt odor and a melting point of 125'-I 28' after recrystallization. The iso-valeric acid was distilled between 17 j oand 178". The phenol distilled between 182.joand 183'. Method of Experzmentation. About 2 0 cc of the solution of the solvent acid was placed in a large test tube together with an excess of the solute acid. The mixture was then heated to about joo and agitated fairly vigorously for *,Contribution from the Laboratory of General Chemistry of the University of Xiscomm. Schierr: J. Am. Chem. SOC.,45, 447 (1923). 'Whitford: J. Am. Chem. SOC., 47, 953 (1925). a Dittmar: J. Phys. Chem., 33, 533 (1929). Knox and Richards: J. Chem. SOC.,115, 508 (1919).
5 44
JAMES H. WALTON AND RAYMOND J. KEPFER
a few minutes in order to nearly saturate the solution a t that temperature. The tube was then placed in a thermostat at zs0, the contents stirred for several hours, and the saturated solvents then allowed to separate from the excess solute. I n the case of citric, malic, and the more concentrated solutions of succinic acid, a complete separation did not take place within a reasonable length of time so the solutions were filtered a t the temperature of the thermostat. z or 5 cc. portions were drawn into a pipette and analyzed. Method of Analysis. Three different methods of analysis were employed. For the run with phenol the amount of phosphoric acid present was determined by titration with standard sodium hydroxide solutions. The phenol was then determined by a method outlined by Redman, Worth, and Brock.' For all other combinations the total acidity was determined with standard sodium hydroxide using phenolphthalein as an indicator and comparing the final end-point color with that produced when phenolphthalein was added to a solution of disodium phosphate. With oxalic acid the amount of solute present was determined by an ordinary potassium permanganate titration and the phosphoric acid by the difference between the total acidity and acidity due to the oxalic acid. For the other solutes, citric, succinic, malic, and isovaleric acids, the amount of phosphoric acid was determined gravimetrically. The amount of solute was then determined by difference between the total amount of acidity and the acidity due to this amount of phosphoric acid.
Discussion Results with Phosphoric Acid. The results of the above experiments are given in Table I and are also represented graphically in Figs. 1-3. Of the various possible methods for expressing concentrations the one used by Knox and Richards, namely, normalities of the two acids in the resulting solution, has been chosen. The normalities given are an average of two determinations in each case. Since in all cases the solvent differed in strength from the solute two types of curves might be expected. ( I ) If, as has been shown by James Kendall, the solute acid is strong there is little tendency for compound formation to take place between the two acids. However since water can be regarded as a weak acid it will tend to form an addition compound with a stronger acid and thus hydration of the solute will result. Two factors, common ion effect and hydration of the solute will then tend to reduce the solubility of the solute acid. On the other hand there will be only a feeble tendency to compound formation and hence only slight cause for increased solubility. Consequently for acids 0.f thzs type one would expect to obtain curves which show only a slight rise as the solvent acid becomes very concentrated. This was found to be the case with citric and malic acids. The citric acid curve makes no turn whatsoever so far as determined, but no doubt there would be a slight indication of compound formation if a more concentrated solution of phosphoric acid had been used. This will be more clearly ~
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STUDIES O X PHOSPHORIC ACID
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shown later when a comparison with the solubility of citric acid in sulphuric acid is made. (2) As the solute acid becomes weaker the tendency towards compound formation becomes stronger while the amount of hydration usually decreases, hence for these weaker solute acids curves which make more of a turn a t higher concentrations of the solvent acid would be expected. Again as can be seen from Figs. I and 3 the results are in general in accord with what one would predict. Succinic, oxalic, and iso-valeric acids all show compound formation. There does however seem to be some inconsistency in the case of
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phenol. Since this is a very weak acid one would expect it to show extensive compound formation but the curve makes only a slight turn. However this can be at least in part attributed to the fact that the run wm not made in as high a concentration of phosphoric acid as in the other cases. This was due t o the fact that at concentrations of the phosphoric acid greater than 37 or 38 normal crystallization took place, evidently due to the compound formation found by Hoogewerff and van Dorp,’ so that runs could not be made. However t.he curve was turning up at 36 or 3 7 normal so there seems to be no doubt but that distinct compound formation would have been indicated had it been possible to work on up t o 41 or 4 2 normal solutions of phosphoric acid. It should also be possible to obtain further information on the subject by making the complete run a t some higher temperature a t which crystallization would not take place so readily. It is evident that the results agree with the general rule formulated by Kendall2 in his stiidy of the addition compounds of acids by the method of Rec. Trav. chim., 21, 349 (1902). J. Am. Chem. SOC.,43) 1545 (1921).
547
STUDIES ON PHOSPHORIC ACID
freezing point curves. If there is a great difference in the acidic strength of the components used there are strong indications of compound formation, while for components with less difference h acidic strength the indications of compound formation are weaker. I n nll cases the turn does not come until after the solvent acid is highly concentrated. This would seem to indicate that the addition compound is highly dissociated in water solutions, and true addition does not take place until the relative concentration of the water has been greatly reduced. From I M#hc u c h f h Phhosphni uoh' rime nr/*rh Soehuric ncLf fNar
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FIQ.2 what has been said concerning the weak acidic property of water and thus the tendency for hydration of the solute acid to take place, this would be expectcd. Comparison of Phosphoric and Sulphuric Acids. To compare the solubilities obtained with citric, succinic, and oxalic acids in phosphoric acid with the corresponding values obtained by Knox and Richards in sulphuric acid, the values for solubility obtained by Knox and Richards have been plotted in Figs. I and 2 with the corresponding phosphoric acid curves. For the sake of convenience in plotting, the normalities of the sulphuric acid have been mnltiplied by two. The similarity of phosphoric and sulphuric acids as to tendency to form addition products is very clearly illustrated for in all cases the curves are very nearly duplicates. This is evidence for assuming that the solubility of citric acid in phosphoric would have increased had a higher concentration of phosphoric acid been used. It will also be noticed that, in all three cases, the minimum solubilit,y of the acids in question is slightly less in phosphoric acid than it is in sulphuric acid and is always reached at a lower concentration of sulphuric acid
JAMES H. WALTON AND RAYMOSD J. KEPFER
5 48
than of phosphoric acid. This is no doubt due to the fact that the tendency to form addition compounds is slightly less with phosphoric acid than with sulphuric acid, which is what one would expect, since there is not so much difference between the acidic strengths of the organic acids used and the phosphoric acid as there is between these same acids and sulphuric acid. This is likewise in accordance with what Knox and Richards found concerning nitric, hydrochloric, and sulphuric acids. Finally, the first part of the curve with sulphuric acid always falls below that with phosphoric acid. This
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can be attributed to the fact that for the same concentration the sulphuric acid is much more highly ionized and therefore its common ion effect is much more pronounced than that for phosphoric acid.
Summary The solubilities of phenol and of citric, malic, oxalic, succinic and isovaleric acids in ortho phosphoric acid solutions of various concentrations have been determined. 2. With the above compounds there are indications of the formation of addition compounds with phosphoric acid which can be explained by the formation of oxonium compounds. 3. Phosphoric acid is very similar to sulphuric acid in its tendency to form addition compounds with the compounds investigated, the chief difference being that the former acid does not form these compounds quite so readily as does the latter acid, this difference probably being due to phosphoric acid being the weaker of the two. I.
Madison, Wis.
