STUDIES OK PHOSPHORIC ACID*
I. The Decomposition of Formic Acid by Means of Phosphoric Acid
BY JAMES H. WALTON AND HENRY Y. STARK
Phosphoric acid as a dehydrating agent for certain organic compounds
possesses the distinct advantages of not acting as an oxidizing agent during
the process of dehydration and of not charring the reactants. On the other
hand it requires a much higher temperature than sulfuric acid. I n spite of the
fact that phosphoric acid has been used for a long time no systematic investigation has been made on the kinetics of the reactions in which this acid acts
as a dehydrating agent. Further, no attempt has been made t o systematically
study the effects of the ortho, the pyro, and the meta acids. I n using phosphoric acid for dehydration the procedure followed has been distinctly
empirical as evidenced by the fact that the method usually followed is to heat
the 85 percent solution of ortho phosphoric acid until a concentrated solution
sufficiently active for the purpose in hand is obtained. S o midence of the
composition of this product is available.
This investigation was carried out with the object of studying the dehydrating action of the different phosphoric acids on formic acid, according to
the equation HCOOH = H 2 0 CO. Similar studies made on the dehydrating action of sulfuric acid on such compounds as formic,' malic2and triphenyl?
acetic acids have yielded valuable information on the mechanism of sulfuric
acid dehydrations and the conditions influencing such reactions.
+
Experimental
Materials. A11 the chemicals used in this work were the purest obtainable.
The phosphoric acid was prepared from a good grade of the 8s percent sirupy
acid, three samples prepared as follows being used in these experiments.
Sample A. The acid was evaporated slowly in a large evaporating dish
the temperature being gradually increased until after a period of three or four
hours it reached 2 5 0 ' . It was held at this temperature for thirty minutes
after which it was transferred to a glass-stoppered bottle and used as a stock
solution. This sample contained 90.3% ortho acid, and 5.1% of the meta
plus the pyro acids.
Sample B. The 85 percent acid was concentrated under a pressure of
30-40mm. a t a temperature of I O O ~ - I O ~ ~The
.
resulting acid contained 94.6y0
ortho-phosphoric acid and gave no test for the pyro or meta forms.
Sample C. This was pure crystalline ortho phosphoric acid prepared by
Dr. E. 0. W i g of this laboratory.
* Contribution from the Laboratory of
General Chemistry, University of Wisconsin
Schierz: J. Am. Chem. SOC.,45,447 (1923).
* Whitford: J. Am. Chem. SOC.,47,953 (192j).
a Dittmar: J. Phys. Chem., 23, 533 (1929).
JAMES H. WALTON AND H E N R Y hl. STARK
360
Apparatus and Method. The reaction was carried out in a two-necked
Pyrex flask of IOO cc. capacity. One neck of the flask carried a mercury seal
which permitted the contents of the flask to be stirred while the whole system
was closed. The other neck of the flask was provided with a stopper carrying
a capillary tube that was connected with a water-jacketed gas burette. In
the experiments to be described the procedure was as follows: 4 0 cc. of phosphoric acid was placed in the flask together with a few glass beads or short
glass rods the object of which was to act as baffle plates during the stirring,
thereby helping to relieve the supersaturation of the solution with respect
to the gas formed. The stirrer was put in place and the flask then placed
in a thermostat which was operated a t 120' unless otherwise specified. After
the contents of the flask had reached the temperature of the bath a small
capsule containing the substance to be dehydrated was dropped into the
flask through the side neck that led to the burette, the stopper was then
quickly inserted, and readings on the gas burette were taken a t definite time
intervals giving the speed of dehydration of the formic acid.
Inasmuch as sodium formate is easy to purify and weigh out and also
yields formic acid with excess of phosphoric acid, it was used in these experiments instead of pure formic acid.
Completeness of the Reactzon. That sodium formate in the presence of
excess phosphoric acid yields formic acid which is quantitatively decomposed
into water and carbon monoxide is evidenced by the following data: Four
portions of sodium formate were weighed out, added to phosphoric acid and
the resulting volumes of carbon monoxide gas were measured. The calculated
volumes of gas in cc. were 37.8, 37.4, 37.8, and 50 respectively. The observed
volumes were 37.7, 36.8, 38.0, and 49.5 cc.
I n measuring the speed of the reaction it was found that an induction
period existed and that satisfactory constants were obtained only when the
zero point waa taken after 15-25 percent of the sample had decomposed.
These constants, which are for a unimolecular reaction, are sufficiently concordant as shown by the following data:
Time in Min.
Veloc. const. k X io3
4
258
5
260
6
256
7
8
258
257
IO
253
Additional evidence that the reaction over the greater part of its period conforms to the unimolecular type is given by the fact that a variation in the
amount of phosphoric acid used did not alter the value of the velocity constant. Reaction velocity measurements made when 50, 40 and 2 5 cc. of
TABLE
I
Temperature Coefficients-Acid
Temp.
k X
103
IO0
65.2
I IO
I 2 1 .o
Temp.
Coeff.
A with HCOONa
Temp.
I20
I
.86
130
k X
103
228
409
Temp.
Coeff.
I .88
I .78
STUDIES ON PHOSPHORIC ACID
36 1
phosphoric acid was used with the same quantity of sodium formate gave
identical velocity constants.
Temperature Coeficzent. The effect of temperature on the reaction is
shown in Table I.
Effect of Dzfferent Preparations of Phosphoric Acid. I n the beginning of
this investigation an attempt was made to use ordinary syrupy phosphoric
acid for the dehydration, but five or six hours were required for the reaction
to take place, and complete decomposition of the formic acid was never obtained. At the temperature of I Z O O , moreover, water vapor was given off,
which condensed and interfered with the measurement of the gas evolved.
For this reason and also because of its more rapid rate of reaction, the concentrated mixture of acids (acid A) was used in the major portion of this
work. The effect of the kind and concentration of acid used is shown in
Table 11.
TABLE
I1
Effect of Different Acids on Reaction Rate
k
Acid used
Ordinary syrupy ( 8 j y b ortho)
Acid B (94.6c/c ortho)
iooYc ortho acid
x
103
20
Acid used
k X io3
274
Acid A (90.3% ortho)
Pure pyro acid
2-4
6
I21
72.1
Eflect of Water on the Reaction Rate. Since water is formed as a result of
the action of phosphoric upon formic acid, the effect of its addition becomes
of considerable importance from the viewpoint of its effect on this reaction
and also in the comparison of the relative dehydrating properties of sulfuric
and phosphoric acids of different concentrations. Table I11 gives the results
obtained by adding water in varying amounts to a fixed concentration of the
concentratcd acid (acid A).
TABLE
I11
Effect of Addition of Water to Reaction Mixture
Amt. of water added
%H20
Molalitv
k x 103
(cc. per 250 cc.)
none
0.5
I
2
4
6
IO
present
4,61
4.61
4.86
5.22
j .62
6.13
7 .34
of water
2.68
2.76
2.81
3.16
3.31
3.63
4.39
k X
103
(Y2S04)
274
274
235
99
99
89
225
79
196
159
68
94
24
51
Table I11 shows that water is very effective in retarding the dehydration
reaction. The effect of water in decreasing the velocity constant of the
reaction of H3P04on HCOOH is evidently not very different from the effect
on the corresponding reaction with H2S04 at z j o , as the values given in the
last columns indicate. This is also evident from the slopes of the curves in
362
JAMES H. FT’ALTON AND HENRY M. STARK
Fig. I , which show further that the effect of water on the rate of reaction
becomes less marked as the amount of water is increased. The form of the
curve also suggests that the relation is exponential in character. When the
logarithm of the velocity constant is plotted against the amount of water
added a straight line is obtained (Fig. I , curve 3). When this logarithmic
curve is extrapolated to the ordinate axis to obtain the value for the velocity
constant a t zero molality of water a value of 2340 for k X 103is obtained.
This value can hardly be considered as a reliable oonstant for zero molality
of water because of equilibrium changes that take place in the complete
FKG.I
Relation between Amount of Water Present and Rate of Dehydration of Formic
Acid by Sulfuric Acid.
2 . Same as above for Phosphoric Acid (using Sodium Formate).
3. Logarithmic Representation of (2). (Referred to right hand axis).
I.
dehydration of the phosphoric acid solutions. The value simply represents
the maximum velocity constant which would be reached if the rate of reaction
increased by the same proportion with decreasing water content, over the
range of composition o to 4.6% water as it does between 7.34 and 4.6%
water.
E f e c t of Additlon of Various Phosphoric Acids to the Reaction Mixture.
To determine which of the various modifications of phosphoric acid was
effective in dehydrating formic acid, the pyro and meta forms from different
sources were added to the reaction mixture and the effect on the velocity
constant determined. If the dehydrating action of the mixture was due
entirely to the action of the pyro and meta forms present, the addition of
these forms in the pure state should increase the reaction rate. The results
obtained by the addition of various acids to acids A and B are shown in
Tables IV and V.
363
STUDIES ON PHOSPHORIC ACID
From these tables it is plain that addition of pyro and meta varieties of
phosphoric acid has little accelerating effect on the reaction. Metaphosphoric acid seems to have an inhibitory effect on the reaction of HCOOH with
acid A. Pyro acid does not affect the rate of reaction with acid A, but
slightly accelerates the reaction of the ortho acid solution (acid B). The
addition of pyro and meta acids in quantities more than sufficient to bring
the percentage of these acids in the solution to the same value as is present
in acid A, in no case gives a rate of reaction approaching that of acid A.
TABLE
IV
Effect of Addition of Various Phosphoric Acids to Acid A
Acid added to Acid A
None (pure acid A)
15% meta acid (prepared by heating (NH4)*HP04at temperature of 280')
I 5% meta (same as above) but solution was held in thermostat
I hour before starting reaction
25% meta (same as above) but solution was held in thermostat I hour before starting reaction
I z % pyro acid (prepared by heating H 3 P 0 4a t 2 I 5')
127' pyro (prepared by heating equimolecular quantities of
H3P04 and H P 0 3 at 100')
P20s(varying quantities, 10--25%)
k
x
103
2 43
238
I94
I54
232
230
294-413
TABLE
V
Effect of Addition of Various Acids and Salts to Acid B
Substance added to Acid B
None (pure acid B)
11% meta acid (commercial C.P. Grade)
12% meta acid (prepared from (NH,) 2HP04)
12% pyro (prepared by heating H3P04 a t 215')
I 2% pyro (prepared by heating equimolecular quantities of
HPO3 and H 3 P 0 4at 100')
5% pyro (commercial C.P. grade)
12%
50%
"
acid A
I,
9,
k X 108
16.1
17 .o
17.6
20.7
18.3
15
.s
20.2
45.4
The effect of pure pyrophosphoric acid on the rate of reaction of 1007~
orthophosphoric acid with formic acid was also determined. Pure ortho
acid gave a velocity constant of 74. This is less than one third the value
obtained for acid A. When pure pyro acid was added to this pure ortho
acid in amounts sufficient to make the composition 10% by volume of pyro
acid, the value k X 103 was 72, a difference well within the experimental
error.
Effect o j Heating Acid B to 260' i n a Sealed Bomb. I n order to determine
whether the increased action of the concentrated mixture of acids present
364
JAMES H. WALTON AND HENRY M. STARK
in acid A was simply due to a loss of water or to a molecular change in the
phosphoric acid, a quantity of acid B (94.6% ortho acid) was heated to z5o0
in a sealed bomb and held a t that temperature for I / Z hour. The acid was
thus given approximately the same heat treatment as had been given to acid
A. When this acid was used as a reaction mixture in the reaction with
formic acid the value for k X 103 was 20.1, an increase of about 2 5 percent,
but a value still far below that for acid A.
Efect of Various Salts on Reaction Rate of Acid A . When 5 g. of K2SOa
were added to IOO cc. of phosphoric acid mixture (acid A) the reaction rate
was increased from 240 to 328. In order to determine whether this might
be due to the formation of sulfuric acid an experiment) was carried out in
which one cc. of 95% HzSO, was added to 40 cc. of reaction mixture. The
reaction rate was increased from 240 to 694. Since even two drops of H,SO,
to forty cc. of acid A increased the reaction rate considerably, it seems probable that the sulfuric acid exercises a catalytic influence on the dehydrating
action of the phosphoric acid mixture. This small amount would be insufficient to produce the increased reaction rate by dehydration alone.
The effect of the addition of Na2HP04in varying quantities to acid h
is shown in Table VI.
TABLE
VI
Effect of the Addition of N a 2 H P 0 4to Acid A
Gms. Sa2HP04
in 40 cc.
k X io3
Gms, NalHPO,
in 40 cc.
k X
103
2
93.6
3
5.5
85.9
28.1
From the above it will be seen that rVIa2HP04exerts a distinct inhibitory
effect on the reaction. This is true also for the tri-basic potassium phosphate.
When one gram of K 3 P 0 4was dissolved in 40 cc. of acid mixture and the
solution used as a reaction mixture, a value of k X 103 of 149 was obtained.
This value is slightly higher than the corresponding one for the same amount
of Na2HP04, indicating that the inhibitory effect of Na2HP04on the dehydration reaction of acid A with HCOOH is greater than the effect of
KBPO~.
Discussion of Results
Before any attempt is made to explain the action of phosphoric acid on
formic acid, evidence must first be obtained to show which of the phosphoric
acids is responsible for this reaction. Unfortunately such evidence is not
available. That ortho phosphoric acid is not the active substance is evidenced by the fact that 100 percent ortho acid is less active than solutions
containing smaller concentrations of this acid (see Table 11). That the meta
and pyro acids are not primarily responsible for the decomposition of the
formic acid is evident from the data in Tables I V and V where the addition of
these acids to a solution of orthophosphoric acid is seen to be practically
without effect on the activity of the orthophosphoric acid alone. There re-
STUDIES ON PHOSPHORIC ACID
365
mainsthe possibility of the existence of other phosphoric acids formed either by
the association of molecules of the known acids or by the interaction of t.hese
acids with each other or with water. Such acids would undoubtedly break
down upon dilution and consequently would not be detected by the ordinary
analytical procedures. They would exist in solution in equilibrium with water
and the other acids and it is quite possible that insolutions of such high concentration such equilibria would be reachedslowly. This would explain the increase
in the activity of acid B after heating in a sealed tube, and also the marked
effect of the addition of traces of sulfuric acid to the reaction mixture. The
existence of other phosphoric acids is mentioned in the literature. Berthelot
and AndrB1 state that forms of phosphoric acid intermediate in composition
between the pyro and meta forms are formed in the hydration of the meta
acid. These are said to have the formulas HSPSO1Oand HBP&, although no
proof of the existence of these acids is given. Holt and Meyers2state that the
meta and pyro acids also give solutions containing complex molecules.
I n comparing the dehydration of formic acid by phosphoric acid with
its dehydration by sulfuric acid, certain similarities in the behavior of the
two acids lead to the conclusion t,hat the mechanism of dehydration is the
same and depends in each case upon intermediate cornpound formation. The
dehydrating action of sulfuric acid upon formic acid is believed to be due t’o
the formation of an unstable oxonium compound which breaks down into
water and carbon monoxide. The tendency of sulfuric acid to form such
compounds has been demonst’ratcd by James Kendall and his students, who
state that the tendency of compound formation of sulfuric acid with other
acids depends upon the difference in acidit,y between the two acids. The
decomposition of formic, malic and triphenyl acetic acids by sulfuric acid
has been studied by the author of this paper and his students, and the decomposition of oxalic acid has been investigated by Rredig and L i ~ h t y . ~
Dr. H. R. Dittmar4 says “Considering the tendency of compound formation
of sulphuric acid with another acid as being dependent upon the difference
of acidity, and assuming that this tendency to form a molecular complex is
proportional to the velocity of decomposition, then in the case of oxalic,
malic, formic and triphenylacetic acids, the weakest, acid would most readily
form an intermediate compound with sulphuric acid, so the speed of decomposition would be greatest. The order of decreasing acidity of these four acids
is: oxalic, malic, formic, and triphenylacetic. The velocity of decomposition
increases in the same order from oxalic, to malic, formic, and lastly triphenylacetic which decomposes most. rapidly.”
The tendency of phosphoric acid t.o form oxonium compounds has long
been known and has recently been demonstrated in the case of several organic
acids by Mr. R. S. Kepfer of this laborat,ory. Phosphoric acid is a much
weaker acid than sulfuric acid, consequently its tendency to compound
Berthelot and AndrB: Compt. rend., 123, 773 (1896).
J. Chem. SOC.,99, 384 (1911).
Bredig and Lichty: 2. Elektrochemie, 12, 450 (I-).
Dittmar: J. Phys. Chem., 23, 533 (1929).
* Holt and Meyers:
a
JAMES H. WALTON AND HENRY M. STARK
366
formation would be less, which is borne out by the fact that its action upon
formic and oxalic acids is much weaker than the action of sulfuric acid.
I n the study of the action of sulfuric acid upon the acids listed above it
was found that many substances, among which water is an outstanding example, acted as negative catalysts towards this reaction. These substances
all form molecular addition compounds n<th sulfuric acid. Similarly, water
and certain phosphates, substances which are known to combine with phosphoric acid, have a decided inhibitory action upon the reaction between
phosphoric acid and formic acid.
This investigation is being continued from the standpoint of the composition of concentrated solutions of phosphoric acid.
summary
Formic acid when heated with concentrated phosphoric acid a t a
temperature of 120' breaks up quantitatively into carbon monoxide and water.
The speed of this reaction has been followed by measuring the rate of
2.
evolution of gas. The reaction is unimolecular.
3. The temperature coefficient of the reaction is approximately 1.82.
4. The addition of small amounts of water and of certain soluble phosphates inhibits the reaction.
5 . The relationship between the amount of water present and the velocity
constant has been shown to be exponential.
6. The velocity of the reaction is greatly accelerated by the presence of
small amounts of sulfuric acid.
7 . The active dehydrating agent is neither the ortho, meta or pyro form
of phosphoric acid. It is believed that in the high concentrations of phosphoric acid used in these experiments another form of phosphoric acid is
largely responsible for the dehydration of the formic acid.
I.
Madison, vis.