200
H. LOTZKAR, T. II. SCHULTZ, H. S. OWENS, AND W. D. MACLAY
EFFECT OF SALTS ON THE VISCOSITY OF PECTINIC ACID SOLUTIONS H. LOTZKAR, T. H. SCHULTZ, H. S. OWENS, AND W. D. MACLAY Western Regional Research Laboratory, Albany, California Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, U . S . Department of Agriculture Received December
'
4 , 1946
The effect of salts on the viscosity behavior of pectinic acid solutions has not been extensively studied, although the importance of salt effects t o the isolation (3,5, 6, 8, 10, 11, 16) and t o the gelling properties (1, 12) of pectinic acid is well recognized. Probably the most complete investigation, that of Baker and Goodwin (l), was confined t o single concentrations of many salts, and certain similarities which appear to exist among several cations were not observed. Very few viscosity data have been published concerning the industrially promising low-methoxyl pectinic acids. The purpose of this paper is to present data dn the viscosity of pectinic acids in solution of salts which are encountered in various phases of pectin technology. Explanations are offered, wherever possible, for the behaviors observed. EXPERIMENTAL
The pectinic acids used in this work were prepared by methods already published (4, 14). The analytical data for these acids are given in table 1. The technique of viscosity measurement has been described previously (13). The so-called natural pH is the pH of the pectinic acid solution without added hydroxide or acid and is near pH 3 a t the concentrations used. The salt effects studied a t pH 6 represent an effect on the pectinjc acid almost completely neutralized with sodium hydroxide. The pH of the solutions was adjusted to the , desired value after the addition of the salt t o be studied, because the addition of salt t o pectinic acid solutions causes a change in pH. The temperature was 25°C. f 0.03". The intrinsic viscosities were determined from the viscosity of solutions containing 0.155 M sodium chloride and adjusted t o pH 6. The significance of these values will be discussed in another publication. RESULTS
Alkali metal salts Figure 1 illustrates the effect of pH on a dilute solution of a typical highmethoxyl pectinic acid at different sodium chloride concentrations. The viscosity is reduced to a low and nearly constant value in the pH range from 3.5 t o 6 with 1 per cent sodium chloride. As the pH value approaches 1.5,where the pectinic acid is little ionized, the relative viscosity is not affected significantly by the sodium chloride. The results with low-methoxyl pectinic acid in the absence of salt, which are
201
VISCOSITY OF PECTINIC ACID SOLUTIONS
not included in the figure, are similar t o those with high-methoxyl pectinic acids at pH values above 4. Below that value the results are complicated by aggregaTABLE 1 Analytical data for pectinic acids used in this investigation SAMPLE IDENTIFICATION*
SOURCE
EQUIVALENT WEIGHT
[slt
10.7, No. 1 10.9 2.9A 7.5B 6.6B 5.6B 3.5B 10.7, No. 2 5.8E 3.6E
Citrus Apple Citrus Citrus Citrus Citrus Citrus Citrus Citrus Citrus
720 830 245 420 370 320 265 760 360 280
4.2 5.7 2.8 3.5 3.5 3.0 3.1 8.7 4.8 3.7
* The number refers to the methoxyl content in per cent and the letter following the number indicates method of deesterification. The letter A indicates acidic deesterification; B, basic deesterification; E, enzymatic deesterification. t [VI denotes intrinsic viscosity of a pectinic acid solution in which the concentration of the pectinic acid is expressed in grams per 100 ml. of solution.
't
a
PH
FIG.1. Effect of p H on viscosity of a 0.3 per cent solution of pectinic acid 10.7,No. 1 at various concentrations of sodium chloride.
tion (14),which causes a rise in viscosity. Addition of sodium chloride caused a further increase in viscosity, such as that observed by Olsen et d. (12) for low-methoxyl pectinic acids.
202
H. LOTZKAR, T. H. SCHULTZ, H. S. OWENS, AND W. D. MACLAY
The effect of sodium or potassium chloride on the relative viscosity of solutions of two pectinic acids is shown in figure 2, in which the abscissa is the square root of the salt concentration. The square root of the concentration was selected t o illustrate more clearly the effects of low salt concentrations. In all cases the initial effect is the depression of the relative viscosity. This is followed in most
I !
3.5 .
a
-I
Legend
W
a 2.5
:Natural p H 9 l p H 6 b . e n
m
0
4
.
0
. 2.0
E (0.2%)
( M O L A L SALT CONCENTRATION)"^
FIG.2. Change of viscosity of pectinic acid solutions with increase in the square root of the concentration of sodium or potassium chloride. The number in parentheses refers to the weight per cent concentration.
cases by a gradual increase in relative viscosity as the concentration of the salt is further increased. The depression is somewhat greater for the low-methoxyl than the high-methoxyl pectinic acid, and the study has indicated that this is a general phenomenon. This lowering of viscosity may have an application in decreasing filtration time with solutions of pectinic acids.
203
VISCOSITY OF PECTINIC ACID SOLUTIONS
As shown in figure 2, the change in cation from sodium t o potassium does not influence the salt effect materially. The differences observed are mainly the result of degradation' of the pectinic acid during storage. Kortschak (7) also observed that sodium and potassium salts had equal effects on the viscosity behavior of pectinic acids, but Baker and Goodwin (1) found that these salts TABLE 2 Effect of anions* o n the relative viscosity of 0.1 per cent solutions of pectinic acid 3.6Bat pH 6 RELATIVE VISCOSITY IONIC STRENGTE
0
0.001 0.0012 0.002 0.003 0.005 0.006 0.01 0.015 0.02 0.03 0.05 0.06 0.1 0.12 0.15 0.2 0.3 0.6
NaCl
NHL1
NaNOa
NazSO4
K*Fe(CN)s
1.885t 1.722
1.925 1.750
1.925 1:758
1.925
1.917 1.758
1.628 1.619 1.506
1.520
1.524 1.547
1.436 1.394
1:395
1.437 1.'403
1.404 1.363 1.370 1.350
1.347
1.354
1.342
1.349
1.360 1.341 1.344 1.345
* The effect of sodium hexametaphosphat,e (Calgon) was also measured. When the ionic strength of a 1 per cent solution of sodium hexametaphosphate was assumed t o be 0.06, there was good agreement between the viscosity of the pectinic acid solutions containing sodium hexametaphosphate and those containing the salts listed in this tabl-e. It may be used, therefore, instead of sodium chloride in determining the intrinsic viscosity of pectinic acid. I n view of the fact that sodium hexametaphosphate removes polyvalent cations, its value is apparent in determining the intrinsic viscosity of pectinic acids when they contain these cations as impurities. t The reason that this viscosity value is lower than the other values reported for 0.1 per cent solutions of pectinic acid 3.5B without salt is not apparent. did not yield results which were the same quantitatively, although they decreased the viscosity. Data on the influence of anions, presented in table 2, confirm preliminary results (9) which indicated that anions have little influence on the magnitude of the effects observed. The effect of ammonium chloride is also included to supOver a period of eight months the intrinsic viscosity of sample 10.9 changed from 6.4 to 5.7.
204
H. LOTZECAR, T. H. SCHULTZ, H. S. OWENS, AND W. D. MACLAY
port further the observation that sodium, potassium, and ammonium ions exert the same effect on viscosity under the same conditions.
*
5
Q)
0 0
E > w
3
t 4 .J
w
a
(MOLAL SALT CONCENTRATION)"'
F1a.3.. The effect of calcium chloride on the viscosity of pect,inic acid 10.7, No. 2 a t various concent$rationsof pectinic acid.
Alkaline earth and other polyvalent metallic salts Addition of calcium chloride to high-methoxyl pectinic acid solutions caused the viscosity to vary in several ways, depending upon pH, concentration, and intrinsic viscosity of the pectinic acid. Figure 3 illustrates the effect of calcium chloride on theviscosity behavior for various concentrations of sample 10.7, No. 2. The curves can be classified into three main types. At 0.1 per cent concentra-
205
VISCOSITY OF PECTINIC ACID SOLUTIONS
tion of the pectinio acid, the calcium chloride caused a decrease in the relative viscosity. An increa.se in concentration to 0.11 or to 0.15 per cent yields, with calcium chloride, a viscosity curve which goes through a more or less sharp maximum, depending upon the pH. At 0.2 per cent, the viscosity increases until gelation begins. The broken lines in figure 3 and in subscquent figures
$I
Baal
C U ( N O ~0 )~
NiCln
CaClt
@
CaClg,pH6
Sr(NO&
0
Sr(NOs)z, p H 6 0
AICI,
o
0.1
0.2
0.3
0.4
0.5
0.6
0.7
@
om
(MOLAL S A L T C O N c E NTRATI ON ) I t 2
FIG.4 . The influence of various salts on the viscosity behavior of 0.3 per cent solut,ionsof pectinic acid 10.7, No. 1. Unless otherwise noted, the results were obtained at the natural PH.
indicate that the viscosity of the solutions changed with time, usually increasing about 5 per cent in 24 hr. The viscosity trends, however, are valid.
206
H. LOTZKAR, T. €1. SCHULTZ, H. S. OWENS, AND ITr. D. JIICL.4P
Similar curves were obtained with sample 10.7,No. 1 in figure 4. In this case a higher concentration of pectinic acid (0.3 per cent) was used because its intrinsic viscosity was only about half as great as that of sample 10.7,No. 2. For natural pH, howver, the curve for viscosit'y us. calcium chloride concentration goes 5.2 5. I 5.0
4.9 4.8
4.7 4.6 4.5 4.4 F
t 3.5 (0
0 0
2! !
>
3.4
3.3
w 3.2
>
a
3.1 3.0
a
PECTlNlC ACID
2.9 2.8 2.7
1.9
1.8
CONC".:
0.1% PECTlNlC ACID
1.7 1.6
1.5 1.4
0
002
0.04
0.06
6.08
0.1
( M O L A L SALT C O N c E NT RATION)
F I G .5 . Effect of cnlniu~nchloride on t,he relntivc, visc0sit.y of severnl pectinic acids or approximately llic sanle intrinsic viscosity but, tliffcrent methosyl coiltents i l l viirious cwncrnt rations.
through a minimum. Figure 4 also s h o w that strontium nitrate acts like calcium chloride, while barium chloride eshibits a greater tendency t)o increase the viscosity. Cupric nitrate and nickel chloride can be classed with barium chloride a t the natural pH. In its effect,aluminum \vas similar to calcium and strontium. Measurements were not made at higher pH values with certain of these salts
vIscosIw
OF PECTINIC ACID
SoLcwoxs
207
because t'hey hydrolyze to form basic salts which undergo mutual coagulation with negatively charged pectinic acids. Decreasing the methoxyl content of pectinic acids causes them tm be more sensitive to precipitation by alkaline earth salt's. The low-methoxyl pcctjinic acids exhibit) certain viscosity patterns that are similar to t,hosc of the highmethoxyl acids in figures 3 and 4. J f the pectinic acid is sufficiently dilute, n depression in the relat,ive viscosity is obt'ainetl, as shown in figure 5. At concent,ration 0.1 per cent', pectinic acids 5.GB at the natural pH and 7.5B both at the nat'ural p1-E m d at pH G give curves of the first type. With still more dilute solutions it is probable that similar curves would be obtained for t,he lowestmethoxyl pectinic acid. For pectinic acid concentrations of 0.4 per cent and greater, viscosity curves of the third type, represented in figure 5 by sample 7.5B at pH 6 , are obtained with all the samples. At intermediate concentrations the viscosity behaviors diverge in a manner dependent) upon the methosyl content of the pectinic acid. Sample 7.5B at pH G and 0.3 per cent concentration acts like sample 10.7,No. 1 in figure 4, although it yields a maximum that is less pronounced. At the same concentration and pH, sample 5.GB yields a minimum irhich is characterist,ic of lon--methoxyl pectinic acids under these conditions. Once again, there is evidence of a region of demarcation between pectinic acids with low-methoxyl and those wit'h high-methoxyl contents. The transitional samples appear t o have methosyl contents between G and 7 per ccnt, in agreement \vit8hthe value noted when viscosity behavior as a function of pR was studied (14). Increase from the natural pH to pH 6 seems to accentuate the minima observed with curves for the low-methoxyl pectinic acids. Liminal values (15) were determined with various salts and pectinic acids. They showed tha,t the enzymatically deesterified pectinic acids were more sensitive to precipitation by salts t8hanwere those of about the same methoxyl content deesterified by alkali. For example, 0.0225 molal strontium or calcium salt solution was required to precipitate sample 6.GB from a 0.1 per cent solution, while 0.0015 molal \vas sufficient to precipitate sample 5.8E. Salts of aluminum, ferric iron, lend, copper, and nickel were effective at concentrations as low as 0.0003 molal. The difference in quantity of these salts required by pectinic acids deesterified by enzyme and by alkali was only slightly greater than experimental error. The greater sensitivity of enzymatically prepa,red materials to precipitation by calcium and strontium is similar t o the findings of Hills, White, and Baker (-1) with pectinic acids deesterified with tomato pectinesterase, as compared wibh those deesterified with acid. DISCUSSION
The viscosity behavior of pectinic acids in solution with salts indicates the interplay of various factors which reveal themselves individually under certain conditions. tTnfortunately, so many factors are involved that it is impossible to be precise \\.hen only one factor is discussed. Generally, when the pectinic acid solution is sufficiently dilute, salts merely depress the electroviscous effect of the
208
H. LOTZKAR, T. H. SCHULTZ, H. S. OWENS, AND 'CV. D. MACLhY
ionized pectinic acid molecule. The depression appears to depend upon the ionic strength at low salt concentrations, as shown in figure G . The relative viscosity follows the expression log q r = log qro
- KP
where p is the ionic strength of added salt, qro is the relative viscosity of ash-free pectinic acid solution, qr is relative viscosity with salt, and K is the slope, ivhich varies with pH and kind of pectinic acid. It may also depend upon the concentration of the pectinic acid. The magnitude of the depression of electroviscom
I NoCl
0
'
0001
4CoCl0
0.002
IONIC STRENGTH
FIG.6. Log of relative viscosity of solutions of several pect,inic acids plotted against thr ionic strength of the added salt.
effects increases with decrease in methoxyl content. Increased charge per unit weight of the low-methoxyl pectinic acids could account for this increase in depression. A more comprehensive study of the electroviscosity of pectinic acid may show that a change in electrostatic attractions influences osmotic effects (2), so that an explanation based on either would be satisfactory. At the other extreme, when the concentration of pectinic acid is appreciably higher, metallic cations, except those of the alkali metal's, cause an increase in the relative viscosity, followed by gelation. Submicroscopic aggregation of a type which increases viscosity, either through an increase in asymmetry of the
VISCOSITY OF PECTINIC ACID SOLUTIONS
208
dispersed material or through increased solvation, probably precedes microscopic gelation, accounting for the increased viscosity. At intermediate concentrations of pectinic acids, it is possible to obtain maxima or minima in the curves for relative viscosity vs. salt concentration. The factors that are probably involved are reduction in electroviscosity and the formation of aggregates of various shapes, sizes, and degrees of hydration. These factors may be competing if more asymmetric aggregates are formed, OF complementary if aggregation in parallel bundles occurs. So little is yet known about the physical state of pectinic acid in solution that a definite explanation is impossible at the present time. From the measurements of viscosity and liminal value certain deductions may be made. Salts of cations of the alkali metal series decrease the electroviscous effect of the pectinate ion and apparently cause precipitation only by “salting out” the acid pectinate. Inasmuch as this “salting-out” phenomenon ’was noted only with low-methoxyl pectinic acids, it offered a means of fractionating pectinic acids according to methoxyl content. Experiments of this type have been carried out and show some promise. Inasmuch as pectinic acids are polybasic, cross linking through polyvalent cations is possible. This phenomenon is evident in the viscosity curves given in figure 4 for copper, nickel, and barium. Generally, even dilute solutions of polyvalent cations that are good complex formers increase the viscosity and precipitate pectinic acid solutions. Intermediate results are attained with polyvalent cations which form complexes less readily. Thus calcium, strontium, and aluminum do not increase the viscosity nor precipitate the pectinic acids at as low a concentration as copper or nickel. Barium is jntermediate in its action, increasing the viscosity but not precipitating high-methoxyl pectinic acids except when its concentration is above 0.5 molal. If the probability of cross linking is increased by increasing the number of free carboxyl groups by deesterification, polyvalent cations are effective precipitants in very dilute solution, and may afford an industrjal method for isolating low-methoxyl pectinic acids. SUMMARY
The effects of various salts on the viscosity at 25%. of solutions of pectinic acids have been investigated. The salt effect depends upon the nature and concentration of the pectinic acid, the pH, and the concentration and kind of salt. Reduction of electroviscosity occurred with pectinic acids and many salts, provided the concentration of both salt and pectinic acid was low. The viscosity of pectinic acids was increased by many polyvalent cations, especially those that are good complex formers. Intermediate effects were noted in the curves for viscosity vs. salt concentration, in which maxima and minima were obtained. The authors are grateful t o A. D. Shepherd for the preparation of some of the pectinic acid samples and to H. A. Swenson for some of the analyses.
210
S, E. SHEPPARD, R. H. LAMBERT, AND R. D. WALKER
REFERENCES (1) BAKER,G . L., A N D GOODWIK, &I.: Univ. Del. Agr. Exp. Bull. No. 216 (1939). (2) BRICCS,D. R., AND HANIG, M.: J. Phys. Chem. 48, 1 (1944). T. W., A N D MAES,L. A. F.: U. S. patent 1,385,525 (July 26,1921). (3) DOELL, (4) HILLS,C. H., WHITE,J. D., JR.,AND BAKER, 6. L.: Proc. Inst. Food Tech. 1942,47. (5) HUNT,C. H . : Science 48, 201 (1912). (6) JAMESON, E., TAYLOR, F. N., A N D WILSON,C . P.: U. S. patent 1,497,884 (June 17,1924). (7) KORTSCHAK, H. P.: J. Am. Chem. Soc. 61,681,2312 (1939). (8) MAGOON, C. A., A N D CALDWELL, J. S.: Science 47,592 (1918). (9) MERRILL, R. C., AND PETERSON, M.: Unpublished data. (10) MYERS,P. B.: U. S. patent 2,163,621 (June 27, 1939). (11) MYERS,P. B.: U. S.patent 2,165,902 (July 11, 1939). (12) OLSEN,A . G . , STUEWER, R., FEHLBERG, E. R., A N D BEACH,W. >I.: Ind. Eng. Chem. 31, 1015 (1939). (13) OWENS,H . s.,LOTZKAR, H., ERRIL ILL, R. C., AND PETERSON, M.: J . Am. Chem. S O C . 66,1178 (1944). (14) SCHULTZ, T. H., LOTZKAR, H., OWENS,H. S., AND MACLAY, W. D . : J. Phys. Chem. 49, 554 (1945). (15) THOMAS, A. W.: Colloid Chemistry, p. 180. McGraw-Hill Book Company, Inc., New York (1934). (16) WALLERSTEIN, L.: U. S. patent 2,008,999 (July 23, 1935).
DESENSITIZING BY DYES I N RELATION TO OPTICAL SENSITIZING OF THE SILVER HALIDES’ S. E. SHEPPARD, R. H. LAMBERT,
AND
R. D. WALKERa
Eastman Kodak Company, Rochester, New York Received December 91, 1946
Historical and practical aspects of the problem of desensitizing photographic materials by dyes may be found in articles by Luppo-Cramer (18), Hamer (15), and Brooker (9). These articles also discuss in greater or less degree the theoretical question of the physicochemical mechanism involved. Luppo-Cramer himself, who introduced as the first practical desensitizer in the developing bath the dye phenosafranin (9)) has proposed two different explanations: firstly, an oxidation hypothesis; secondly, what he terms one of “insulation of the latent image,” by which is intended a sort of “poisoning” of the latent image as a catalyst for development or, in another terminology, as an impenetrable potential barrier. Modificatjons of the oxidation theory have been proposed, as that involving the participation of atmospheric oxygen (7), or the dualistic electrochemical hypothesis of Baur (4),but not much seems to have been attempted in the direction of systematic experimentation on the redox potentials of the systems involved; nor as to whether, as in the related process of photographic Communication No. 1059 from the Kodak Research Laboratories. Most of the experimental work was done by Mr. Walker, who has now joined the University of Florida. 1
2
