T H E RELATIOS B E T W E E S ACIDS A S D PECTIN
I S JELLY FORhlATION B Y GENE SPENCER
An “optimum acidity’’ range for fruit jelly-making by the hot method has been reported by different workers. Tarrl has attached pectin-acid compound significance to this and supports his hypothesis by a series of experiments in which the number of cc. of different acids required to give a definite pH value to a 1 7pectin ~ sol is taken as criterion of stoichiometric relation between pectin and the acids used. We have already seen that the “optimum acidity” for jelly-making with sugar denotes a characteristicof the hot evaporation method and not of pectin. Tarr’s conclusion derived from sugar-free pectin sols treated with different acids should be considered carefully since it involves an important colloid principle. The data to be studied were obtained by Tarr (page 20) as follows: “The acids were added in small accurately measured quantities to IOO cc. of distilled water, hydrogen ion concentrations being determined after each addition of acid. The acids were then added in a similar manner to IOO cc. of distilled water in which I g. of pectin was dissolved, hydrogen ion concentrations being determined as before. It should be mentioned here that all hydrogen ion concentrations for this work were calculated from potentials that were measured with a Leeds and Xorthrup Type K potentiometer.” For the purpose of more careful study Table VI11 of Tarr’s paper is transferred to Table I below: The data of this table were interpreted by Tarr (page 2 3 ) as follows: “It will be observed that the effect of each acid is different. For example, to produce a hydrogen ion concentration of pH 3.50 with the pectin dissolved in the water requires approximately 2 . 6 cc. of 0.1 N sulphuric acid, as compared with j.5 cc. of 0.1N tartaric acid, 7 . 5 cc. of 0.1 X phosphoric acid, or 8.5 cc of 0.1 N citric acid. The total amount of acid that is required varies with the particular acid employed. “Furthermore, there is a purely stoichiometrical relation existing between the combining power of the acids and the effect that they produce on the hydrogen ion concentration with pectin present in the solution. It is a relation that is based on the actual combining ability of the various acids. Loeb has already shown that this same relationship exists between proteins and acids, and, in our presentation, we make frequent recourse to his interesting discussions. “Weak dibasic or tribasic acids give off one hydrogen more readily than both or all three, depending entirely on the hydrogen ion concentration of the Univ. of Del. Expt. Sta. Bull., 134, (1923).
ACIDS AND PECTIN IN JELLY FORMATION
41 I
TABLE I (Table VI11 from Tarr) The Effect of Various 0.1 Kormal Acids on Distilled Water Distilled Water (100 cc) I g. of Pectin
+
3 . 4 . Water Water +Sui. Water Acid +Sul- phuric +PhosAdd- phuric Acid+ phoric Acid Pectin Acid ed cc. PH PH PH 0 4.2 I 5.50 5.50 I 3.06 4.06 3.55 2.81 2 3.28 3.64 2.64 3.41 3 3.15 3.21 3 .oo 4 2.53 5 2.47 3.04 2.92 2.89 6 2.41 2.81 7 2.33 2.77 - 2.65 8 2.58 2.25 9 2.73 I.
IO
2
.
-
2.18
2.50
14
-
2.44 2.39 2.34 2.31
I5
2.08
2.27
I1
I2
I3
-
2.13
-
-
18
-
-
20
1.97
2.14
16
22
-
24
-
25
1.92
26 28
-
30 35
1.88
40
45 50
1.81 1.78 1.75
-
-
-
2.68
2.59
-
2.63
-
5 . Water +Phosphoric Acid+ Pectin PH 4.17 4.05 3.95 3.84 3.77 3.68 3.60 3.54 3.46 3.40 3.34 3.29 3.24 3.19 3.14 3.09
-
6
5.50
7 . Water +Tartaric Acid+ Pectin PH 4.22
-
-
3.13
3.88
3.35
4.04
2.93
-
3.63
-
3.15
-
3.76
2.84
_
3.45
_
3.04
3.64
2.80
3.33
-
-
2.97
3.54
2.73
3.26
2.91
3.46
Water +Tartar$ Acid PH
-
-
_ -
2.67
2.61
2.91
-
-
-
2.58
2.44
2.79
2.52
-
-
-
2.39 2.36 2.31 2.29
-
-
.
-
_
-
-
-
3.17
_
3.12
3.07 3.01 2.98 2.96 2.92
_
8
.
9
.
-
-
2.83
-
-
3.34
-
2.76
3.25
-
2.72
3.16
-
3.11
-
2.88 2.85
2.68
-
-
2.71 2.62
2.54
2.83
2.67
3.06
2.55
2.49
-
2.74
2.60
2.97
2.43
2.67
2.54
-
_
-
+
Water Acetic Acid+ Pectin
-
-
11.
-
-
2.49
IO.
-
2.55
_
cc) and on
Water Water +Citric Water +Citric Acid+ +Acetic Acid Pectin .Acid PH PH 4.16 5.50
-
-
(100
-
2.89
solution. At the hydrogen ion concentration that we employ, there would be only one hydrogen liberated from such acids. With a strong dibasic acid like sulphuric acid, however, both hydrogens are held with so small an electrostatic force that even a t a hydrogen ion concentration of pH 3, or considerably below, the acid liberates both hydrogens. The action of sulphuric acid would consequently be similar to that of a strong monobasic acid like
412
GENE SPESCER
hydrochloric. It would follow, therefore, that a t the hydrogen ion concentrations employed in these investigations, three times as many cc. of 0.1N phosphoric acid should be required to produce a given pH as are required of 0.1 N sulphuric acid. And in the same manner, twice as many cc. of 0.1N tartaric acid and three times as many cc. of 0.1?u’ citric acid should be required. “From the data presented in Table VI11 (Table I above), it will be observed that the relations just described are quite closely maintained. For convenience, let us establish the given p H at 3.50. Approximately 2.6 cc. of 0.I N sulphuric acid are required to produce pH 3.50, while approximately 7 . 5 cc. of 0.1N phosphoric acid are required to produce the same value. This maintains rather closely the 1 : 3 ratio that should exist between these two acids. About j.5 cc. of 0.1 Pi tartaric acid are required, as compared with 2.6 cc. of 0.1 N sulphuric acid, a proportion that approximates the I :2 ratio that should exist. This is not an exact relation, however, and when citric acid is compared, it will be observed that there is an even wider variation between the ratio that exists and the I :3 ratio that we said should exist; I O cc. of 0.1 K citric acid are required as compared with tthe 2.6 cc. of 0.1 r\’ sulphuric acid. About 45 cc. of 0.1 N acetic acid are required t o produce a hydrogen ion concentration of pH 3.50.’’ The number of cc. of different acids compared in the above discussion were obtained by mathematical interpolation on the assumption that the curve between any two points is a straight line, as perhaps it is within the limit of experimental error. T o avoid the necessity of this interpolation, however, and t o be able to see the data more completely in this respect, Table I1 has been arranged from the odd columns of Table I. These odd columns represent the p H value resulting from increasing the size of the added increments of sulphuric, citric and tartaric acids. Acetic acid is not considered since it offers no basis for comparison, and Tarr omits i t from discussion. Since the pH measurements on the same pectin sol to which no acid was added do not agree within 0.03 points from the average, this variation was taken as the recognized experimental limitation of the series. Accordingly pH values varying within this limit were taken as comparable and their average recorded in the first column of Table 11. The number of cc of the different acids which, added to the pectin sol, gave it the designated pH values, have been copied from Table I to the “Used” column of Table 11. The calculated number of cc of each acid which bears the claimed stoichiometric relation t o the number of cc of sulphuric acid used, is recorded in the “Calculated” column. There is no “Calculated” column for sulphuric acid since the number of cc “used” of this acid was chosen as the basis for the stoichiometric comparison. The data for each acid of Table I1 are conspicuously separable into the two groups divided by the horizontal divisions. Above this dividing line the theoretical ratio holds within a fraction of a cubic centimeter in 7 out of I O cases. Below the line of division, the ratio fails completely.
ACIDS AND P E C T I N I N J E L L Y FORMATION
413
TABLEI1 Hapor
PH
Used
4 .os 3.86 3.77 3.64 3.54 3.46 3.42 3.34 3.24 3.18 3.I2
I
3 4
7 8 9 IO I2
I3 I4
Calc.
Tartaric Acid IJsed Calc.
2
4 5.3 6.6 8 8 9.3
3.07 2.98 2.92 2.89
15 -
-
20
I3
-
I2
2.77 2.65
25
2
.so
2.14
35 4s -
Citric Acid Used Calc.
IO
I4 16 -
A comparison of the number of cc of different acids experimentally required to give the same pH value to a pectin sol, compared with the number theoretically required by the stoichiometric ratios as claimed for pectin and acids by Tarr.
The failure of the data to hold a t higher acidities has been accounted for thus, page 24: “It will be observed that as the hydrogen ion concentration is increased, these relations become even more distant. This is exactly what is t o be expected if compounds are formed with pectin and the acids which yield ions that are common to the acids. That is, if it is assumed that the addition of tartaric acid to pectin in solution forms a pectin-tartaric acid compound, it is entirely possible to conceive of such a compound as yielding an ion that is common t o the tartaric acid. The effect of an ion that is common to an acid is to decrease the dissociation of the acid to the extent to which the dissociation is affected, depending upon the particular acid involved. It is only intended to present this point here, because it offers a possible explanation of the variations that occur between the theoretical stoichiometric relations and actual relations as determined from these data.” This argument is untenable in view of the facts of Table 111. This table is an arrangement of the data of the even columns of Table I and represents changes in p H of water free from pectin, resulting from the addition of the same acid increments.
GENE SPENCER
414
TABLE I11 PH
H~POI Used
3.35 3.28 3 .I4 3.03 2.97 2.92 2.84 2.81
2.76 2.73 z .68 2.62
2.59 2.54 2.53 2.43 2 ,32 2.27
-
Tartaric Acid
Citric Acid Calc.
2
3 4
5
6
9 I1
‘3 15
20 25
40 45
A comparison of the number of cc of different acids experimentally required to give the mime pH value to water, compared with the number of cc which correspond to the stoichiometric ratios claimed for pectin and these acids.
The significant thing about this table in comparison with Table I1 is that below the i ~rizontallines, there is the same falling away from an approach to a definite ratio. The “suppression of ionization” by a common ion argument is, therefore, less convincing since, according to these two tables, the same conclusions could be drawn for acid and water as for acid and pectin. I n the case of citric and tartaric acids the stoichiometric ratio fails to hold after 6 cc. of acid have been incorporated in the sol. When pectin is absent (Table 111) the ratio fails to be approximated after 4 cc of these two acids have been added. There seems to be no substantiation of the hypothesis of a pectin-acid compound from these data. As to the “buffer action” of pectin to which one frequently finds reference in the literature, Tarr saysJ page 32: “It is evident from the data that the presence of pectin does materially depress the dissociations of the acids.” This impression is certainly to be derived from Table I for in the case of every acid considered, the p H value indicates that the mixture is less acid in the presence of pectin than when pectin is absent. A careful study of Table I, however, shows that, although the addition of I cc of sulphuric acid to water resulted in an increase in acidity represented by 2.44 (5.50-3.06) points as against an increase of only 0.15 (4.21-4.06)
ACIDS AND PECTIN IN JELLY FORMATION
415
points when pectin was present, the next added increments, 19 cc in all, increased the p H value of water only 1.09 (3.06-1.97)points as against 1.92 (4.06-2.14)points when pectin was present. That is to say, after the added acid had disposed of some non-pectin influence in the pectin sol, presumably salt impurities, the presence of pectin instead of “depressing” the dissociation of acid increased the hydrogen ion concetration. I n the case of phosphoric acid it took 7 cc to dispose of the salt influence. The first 7 cc of the added acid caused a pH change of 2.69 in the absence of pectin and 0.63 in the presence of pectin, but the next 38 cc caused an increase in pH of only 0.52 in the absence of pectin and 1.05 when pectin was present. I n the case of tartaric and citric acids the first 2 cc disposed of the salts and in the case of acetic acid it took 20 cc. Horizontal lines in Table I indicate the acidities for each acid a t which the salt impurity influence appears to be overcome. These lines divide each column of data into two groups: (a) The group in which the increase in pH value is greater in the absence of pectin than in its presence and (b) the group in which the increase in pH value is less in the absence of pectin than in its presence. Table IT’ summarizes the p H values for these two groups.
TABLE IV Added Acid Increment
(a)
0-1
(b)
1-20
(a) (b)
0-7 7-45
(a) (b) (a) (b)
HrS04
H20 Pect.
2.44 0.15 1.09 1.92
Changes in pH Values HsPOd Tartaric Acid H?O Pect. H20 Pect.
-
-
-
-
-
-
-
-
-
-
2.69 0.63
-
-
0.52
0-2
-
-
-
2-50
-
-
-
-
0-20
-
-
-
-
-
20-50
-
Citric Acid HrO Pect.
-
-
Acetic Acid H 8 Pect.
-
-
-
-
-
-
-
-
-
-
2.37
0.34
2.1 j
0.12
-
0.70
1.21
0.18
1.15
-
-
-
-
2.25
0.56
-
-
-
0.18
0.20
1.05
-
-
Data calculated from Table I to compare the change in pH value of HrO and a 1 7 ~ pectin sol, resulting from the addition of acids (a) u to, and (b) beyond the point where the presence of pectin ceases to cause an abnormal Ewering of the pH values of a pectin sol as compared with a pectin free solution.
I n this case the change in pH value caused by the addition of a given increment of acid was more significant than the actual pH. A consideration of the pH value alone gave the wrong impression as to the properties of pectin for it seems probable that a property was attributed to pect’in which belonged to the salt impurities. I n this connection it is interesting to note that phosphoric acid, a stronger acid than tartaric or citric, required more than three times as many cc to dispose of the effect of the salt impurities.
416
GENE SPENCER
If the presence of salt impurities is the correct and the whole explanation of the p H changes just considered, one might expect this change to start a t the same p H value in t h e case of each acid. This is not the case as may be seen by reference to Table I. With sulphuric acid the change starts a t the pH value 4.06; with citric at 4.04; with tartaric a t 3.88; with acetic at 3.64, and with phosphoric a t 3.54. Furthermore, the pH value in the water solution resulting from the addition of the same increment of acid does not show the same order of variation as is shown by the pectin sols. This is brought out better in Table V.
TABLE V I
Acid
H8Oc Citric Tartaric Acetic Phosphoric
2
pH of Pectin Sol
4.45) 4 .04(4) 3 .88(3) 3.64(2) 3'54(1)
?
A
cc acid per 1 0 0 cc. Solu. I .o 2 .o 2 .o 2 0 .o 7.0
pH'of water Soh.
v
3.06(2) 3.35(5) 3.13(3) 3.25(4) 2.81(1)
Data transferred from Table I: The pH of pectin sol (column 2 ) at which the effect of the salt impurities of the 1% pectin sol are overcome by the acid designated in column 3. In the last column (4) the pH value of water acidified by the same number of cc of acid is indicated.
The difference in the order of the acids, placed according to their pH values, in the absence and in the presence of pectin must be explained. Since pectin increased the hydrogen ion concentration of the sol after the salt impurities were neutralized, it must have been functioning similarly during the neutralization of the salts. We know from cataphoresis experiments that pectin adsorbs anions preferentially and we believe that this is responsible for the characteristic acidity of pectin sols, since such a preferential adsorption would always leave a predominance of hydrogen ions in the dispersing medium. From this standpoint it is interesting to note that I cc of HzSO, gives a greater acidity increase to water than does 2 cc of either citric or tartaric, as would be expected even in the presence of salts; but, when pectin is present, the order is reversed, and the organic acids give a greater increase in pH to the pectin sol containing salt than does the inorganic acid. This observation can be explained by the assumption that pectin shows a greater preferential adsorption for organic anions than for the sulphate ion and so increases the hydrogen ion available for neutralizing the salt impurities, in the case of the organic acids. If the pectin had not thus functioned to increase the hydrogen ion concentration more of the organic acids would have been necessary to overcome the salt effect; so the order of the pH would have been the same as for water. For example, if 4 cc. of tartaric acid had been used in Table V, Column 3 , instead of 2 cc., this amount of acid added to water would have changed
ACIDS A S D PECTIPi I N JELLY FORMATION
517
the pH value in Column 4 to 2.93, making the order in that column the same as in Column 2 for tartaric and sulphuric acids. It seems then that anion adsorption by pectin has played a definite part in the establishing of the pH values of Table I. If the stoichiometric relation as it exists at the beginning of each series in Table I1 is attributable entirely to the salt impurities, this relation should cease to hold a t the same pH that the buffer effect of the salts is overcome. This is not exactly the case as is shown in Table VI.
TABLE TrI I
Acids
Sulphuric Phosphoric Tartaric Citric
2
pH values taken from Table I1
pH values taken from Table V
3.07 3.46 3.64
3.54
-
4.06
3 Differences (Col. 2-Col. I )
-
0.47
3.88
0.42
4.04
0.40
A comparison of the pH values at which the stoichiometric relations fail to hold, aa per the horizontal divisions of Table 11, and the pH values at which the buffer salt effect is overcome, aa per Table V, Column 2.
The difference between these two pH values for each acid may be considered as constant, since the experimental determination of p H values for the same pectin sol to which no acid had been added (Table I) varied between 4 . 2 2 and 4.16, that is, by 0.06 points. This may mean that at the p H values, as of Table V, at which the buffer salt action is overcome, the salts are all in such a form that they no longer act as buffers to the particular acid, but that they are not completely neutralized until the pH values, as shown by the values from Table 11, are realized.
Conclusions There is no substantiation for the hypothesis of a pectin-acid compound either on the basis of “optimum acidity” in jelly formation in the presence of sugar, or a stoichiometric relation between acids in their relation to pectin the absence of sugar. 2. Pectin does not reduce the hydrogen ion concentration of an acid solution. On the other hand it increases the hydrogen ion concentration of the mixtures as a result of preferential anion adsorption, 3. The ignored presence of salt impurities together with a consideration of the actual pH value, rather than the change in pH value, resulting from the addition of a given increment of acid, is responsible for the mistaken impression that pectin has a “buffer” action tending to suppress acid ionization. I.
Cornell University
