Salt Enhancement of Acidity in Nonaqueous Solvents and Its Application to Determining Weak Acids in the Presence of Anhydrides H. WHITNEY WHARTON Research & Development Department, Miami Valley Laboratories, Procfer & Gamble Co., Cincinnati, Ohio
b The addition of low concentrations of certain inorganic salts to nonaqueous solvents enhances the apparent acidity of weak acids dissolved in these solvents. Thus LiCl present at not less than 10-2M in acetonitrile permits monoacidic acids with pK, values up to 5.5 to b e titrated with tri-n-propylamine. Since this titrant does not react with acid anhydrides, this method is ideally suited for the direct determination of fatty acids in the presence of their anhydrides. Investigations into the mechanism of the acidity enhancement suggest that the acidic proton is displaced by the Li ion to form HCI and the corresponding weakly dissociated Li salt through simple metathesis, The resulting HCI is then titrated by the tertiary amine. Titration of a homologous series of monocarboxylic acids (C, through Cz.) with tri-n-propylamine shows that as the carbon number of the acid increases, the potential break a t the end point (AEe D,) also increases, suggesting lower dissociation constants for the Li salts of the higher members of the series.
N
o
COMPLETELY satisfactory methods have been reported for the determination of carboxylic acids and their anhydrides in mixtures. Carboxylic acid anhydrides can be determined without interference from weak acids by a convenient titrimetric method using morpholine ( 6 ) , in which the anhydrides react with excess morpholine to form the amide and the carboxylic acid. The excess unreacted morpholine is then determined by titration with HCI. Free carboxylic acids having a pK, of less than 3 can be titrated with a tertiary amine without interference from the acid anhydride (fd). This pK, limitation, however, means that none of the aliphatic monocarboxylic acids can be determined by this procedure. Stepwise procedures for both acids and anhydrides use a morpholine-CS2 procedure (1) or a duplicate titration before and after hydrolysis of the anhydride ( 9 ) but lack accuracy a t high ratios of one qpecies to the other. rl
730
ANALYTICAL CHEMISTRY
spectrophotometric method has been applied to the determination of acetic acid in acetic anhydride (10). The present new method allows the direct independent determination of both carboxylic acids and their anhydrides in the presence of each other. The anhydrides are determined using the morpholine method of Johnson and Funk (6). Their potentiometric method avoids any interference from free weak acids regardless of pK,. The free carboxylic acid content is determined by potentiometric titration with tri-n-propylamine (TPA) with the sample solvent (CHICS) containing LiC1. This modification of the method of Siggia and Floramo (12) permits the determination of free monocarboxylic acids with pK, values as high as 5.5 and polybasic acids with pK,’s as high as 11.9 [only one polybasic acid (H3P04) having a pK, of >7.07 was titrated] as long as one acid group in the molecule has a pK, of less than 5.5. There are reports in the literature on the use of salts to enhance acidity in aqueous systems. Critchfield and Johnson ( 2 , s ) were the first to report the use of strong salt solutions to permit titrations of weak bases in aqueous solution. They titrated aniline in water containing 8M NaI with aqueous HC1 and demonstrated that it was the apparent acidity of the HCI titrant that was increased. Leithe ( 7 ) has reported the successful titration of weak acids a i t h XaOH in a supersaturated aqueous NaC1 solution. Rosenthal and Dwyer (11) have reported titrations in aqueous systems containing LiCl, following the lead of Critchfield and Johnson. Little work has been reported on the use of salts in nonaqueous titrations. Marple and Fritz (8) reported using tetrabutylanimonium bromide to enhance those weak acids whose conjugate bases are largely undissociated. This added salt had the same cation as their titrant. Their published titration curves do not, however, show much enhancement of the already excellent potential breaks obtained in their titrations with the strong base tetrabutylammonium hydroxide. LiCI, a t unspecified concentrations, has been reported to improve the stability of pH meter read-
ings (5)and to reduce solution resistance (4)during potentiometric titrations. The present work reports the use of inorganic salts in nonaqueous solvents to enhance the apparent acidity of weak acids. This is particularly vaIuable when the base titrant must be weak to avoid interferences from other sample constituents. Some observations on the proposed mechanism of this enhancement are reported. EXPERIMENTAL
Titrant. Tri-n-propylamine (Distillation Products Industries) was prepared as 0.05.1.1 in reagent grade acetone and standardized against reagent grade succinic acid (Matheson Coleman Br Bell) or primary standard benzoic acid. Standardizations were made by potentiometric titration in acetonitrile made 0.035M (saturated) in LiCI. Solvent. Acetonitrile (Matheson Coleman Br Bell, reagent grade), was thoroughly dried by storing overnight over anhydrous C a S 0 4 followed by passage through a column of fresh anhydrous C a S 0 4 (1 liter of CH&K per 100 grams of C a S 0 4 ) , saturated with dried reagent grade LiCl (Mallinckrodt Chemical Works) and assayed for final chloride content, if desired, by a potentiometric titration with hgSO3. Final LiCl concentration a t saturation mas 0.034 to 0.036.M. Samples. All samples (0.1 to 0.3 meq.) were dissolved in 30 ml. of LiC1-CH3CN solvent or prepared as approximately 0.1Y in CH&N or a 60/40 chloroform-acetone solvent for pipetting into 30 ml. of LiCI-CH3CN solvent. Electrodes. The conventional glass electrode (Beckman 40498) was used for all potentiometric measurements of the hydrogen ion concentration. The high p H , low sodium error electrode (Ueckman 40495) behaved generally the same and offered no advantages. Attempts to follow changes in the lithium ion concentration by using the I3eckman Cationic electrode (Beckman 39173) were unsuccessful. The response of this electrode towards LiCl in C H I C S varied considerably with concentration in a non-Nernstian manner and was severely influenced by changes in the hydrogen ion concentration. Since this electrode also responded in a
400k0
300
2 00
-1004 0
I.o
2 0
3.0
4 0
ml. 0 05 M Tr~-n-PropyI Amine
Figure 1 . Titration of typical monocarboxylic acids in acetonitrile with and without LiCl Trl-n-propylamine as titrant
highly variable fashion to changes in the hydrogen ion concentration (introduced as either HC1 or HClOJ, reliable correction of the Cationic electrode response for the hydrogen ion influence was not possible. The response of the Cationic electrode towards the hydrogen ion varied from 15 to 150 mv. for a tenfold change in HC1 (or HC104) concentration. depending on the absolute concentration of the strong acid. As reference electrode, the standard sleeve-type saturated calomel electrode (Beckman 41240) was used. The aqueous KC1 internal electrolyte was replaced by methanol bolutions of NaCl (saturated) for all titrations using T P A and CaC1, (0.231) for all titrations involving measurement of the Li+ concentration with the Cationic electrode. All manual potentiometric titrations mere made with either a Beckman Model G pH meter or the Beckman Model 1019 Research p H meter. RESULTS AND DISCUSSION
T h e titration curves for three typical monocarboxylic acids titrated with T P A in t h e presence and absence of LiCl are shown in Figure 1. T h e curve for the corresponding acid anhydrides, when titrated u i t h T P A , may be represented by the carboxylic acid titration curve obtained in t h e absence of LiC1. I n the presence of acid anhydrides, weak acids should be titrated in acetonitrile. Although both methanol and acetone dissolve sufficient LiCl and enhance acidity somemhat, neither solvent shows as large a AE, p . as the LiCl-CH3CN combination. Furthermore, methanol reacts in an erratic fashion with acid anhydrides, leading to high free acid values, and acetone tends to makc. the titrations to each end point of a dibasic acid-e.g., maleic acid-unequal. Typical recoveries of acids alone and in intentional Analytical Aspects.
mixtures with acid anhydrides are shown in Table I along with the appropriate anhydride analyses as determined independently by the method of Johnson and Funk (6). T h e analytical results reported in this table were obtained by automatic recorded potentiometric titrations using a n integrated system consisting of the Sargent constant rate buret (S-11120-1), p H adapter (S72172), and 1-mv. recorder (S-72180). The standard deviations are f 0.30% acid in the 0 to 100% acid range and i 0,41% anhydride in the 60 to 1 0 0 ~ o anhydride range. The precision of the free acid analysis tends to increase with increasing chain length of the monocarboxylic acid, since the AEe also increases. Applicability of LiCl Acidity-Enhancing Properties. Unless otherwise indicated, all remarks regarding t h e use of LiCl t o enhance the acidity of weak acids pertain to titrations in acetonitrile with tri-n-propylamine.
Table 1.
Typical Results for Analyses of Carboxylic Acids, Anhydrides, and Mixtures of Acids and Anhydrides
Acid samples Acetic acid Propionic acid Stearic acid Maleic acid Succinic acid Acid anhydride samples Acetic anhydride Propionic anhydride Stearic anhydride A
B Maleic anhydride Succinic anhydride
F
Table I1 summarizes the potential breaks a t the end point for the titration of a variety of acids. I n t h e absence of LiCI, only acidic groups having pK, (HzO) values less than 3.13 (citric acid) could be titrated, supporting the observations of Siggia and Floramo ( 1 2 ) . With LiCl present, monocarboxylic acids with pK, values up to 5.5 (picolinic acid) could be titrated. I n addition, all other acidic groups in a molecule having one acidic group with a pK, of les5 than 5.5 were also titratable up to a pK, of 11.9 (HP04-2). Differentiation between acidic groups in the same molecule was not generally possible; normally a single potentionietric inflection equivalent to the sum of all titratable groups mas obtained. Thus all three equivalents of &Po4 (pK1 = 2.1, IjK, = 7.1, 11K3 = 11.9) titrated to one end point, as did both acidic groups of phenylphosphonic acid (pK1 = 1.83, pKz = 7.07). =In exception was maleic acid (pK1 = 1.92,
31.5 120.0 37.3 148.2 157 5 569 0 57.7 220,0 59.3 336.2 Acid content,
70
3.5 32.3 0.37 1.10 0.61 7.75 0 9 62 86 91 97
0 6 6 5 8 9
Found, mg. 32.4 124.8 37.7 149,l 158 4 563 0 57.9 217.0 60.0 339.2
Acid content Recovery,
7c
103.8 104.0 100.9 100.8 100 5 99 0 100.3 98.6 101.2 100.8
Anhydride content, % 97.6 67.3
Total, yc 101.1 99.6
102.3 99.6 98.5 91.7
102.7 100.7 99.1 99.5
99 79 38 13 7 2
3 2 3 6 4 4
99 88 100 100 99 100
3 8a
9 1 2
3 Known mixtures of acids and Acid content Anhydride content anhydrides, acid + Taken,b Found, Recovery, Taken.c Found. Recoverv. " , e7 ( I anhydride meq.' meq. /O meq. meq. ./c Acetic 0.338 0.340 100.6 0.297 0 294 99.0 98 1 0.229 Propionic 0.409 0.408 0.227 99.2 Maleic 0.302 0.306 101.3 0 294 0 291 99.0 Succinic 0.323 0 346 105.0 0.287 0.285 55.4 Thin-layer chromatography indicates 10-15% nonacidic impurity. b Corrected for anhydride assay value as reported above. 6 Corrected for acid assay value and acid derived from anhydride as reported above. " I
(1
VOt. 37, NO. 6, MAY 1965
731
Table II.
Typical End Point Potential Breaks (AEe,p.) for Titration with Tri-npropylamine in Acetonitrile Containing LiCl KO. of
Acid Oxalic Slalonic Adipic Sebacic Citric Lactic Phenylphosphonic Xicotinic Picolinic Benzoic
P K (H,O) ~ 1.27, 4.27 2.86, 5 7 0 4.41, 5 28 4.55, 5 52 3.13, 4.76, 6.40 3.86 1.83, 7.07 4.87
equivalents titrated
AEe,p,,mv.
2b
437a 255a
2
2
280a
415= 310a 2705 475a 6OC 185c
5.50 4.17 2 . 1 , 7.2, 11.9
2
3b 1 2b
1
1 1 3b
9.55
&PO4 38P i5a (53) Acetic 4.76 Hexanoic (caproic) 4 85 1206 (110c) 230a (19@) Dodecanoic (lauric) Octadecanoic (stearic) 290= ( 2 5 O C ) a Glass-calomel (saturated NaCl in CHsOH) electrode pair. b 1 equivalent t'itrated in absence of LiCI. Glass-calomel ( 0 . l M LiCl in CHIOH) electrode pair.
pK2 = 6.23) which showed stoichionietric t,itration of each acidic group separately, with the second equivalent exhibiting the larger potential break a t the end point in acetonitrile. Differentiations between acids are possible in the LiC1-CHsCS solvent, system. Acids differing by about 4 pK.(H20) units can be differentiated when present in nearly equivalent amounts, if the comparative stability of their lithium salts is nearly the same or if the lithium salt of the weaker acid is less dissociated. Of particular interest is the fact that mixtures of equivalent amounts of monocarboxylic acids can be differentiated if their chain lengt'hs differ carbon atoms. The ration curves, however, are not as well defined as for the individual acids alone. The magnitude of the p o t e ~ t i a break l a t t'he end point increased with the carbon number of inonocarboxylic acids from C z (acetic) through C2* (behenic). Since that portion of the titration curve beyond the equivalence point is unchanged for any of the acids titrated, it is evident that the apparent acidit,y of the sample is increased by the presence of LiCl. In the homologous series of monocarboxylic acids, the half-neutralization potential ( H S P ) of the acids also incrpases with increasing carbon number. Since the pK, values of these acids in water vary only from 4.76 for C z to 4.96 for C9, the autodissociation of the acid? is not considered to be a significant factor in this acidity enhancement'. Formic acid (pKa = 3.75) not unexpectedly had an HAT higher than acetic acid s n d about equal to that, of the Ci iiionoc.arbo~~-li(~ acid. 'The rate of incrmsc in HSI' is linear from C2 through Ci, u i t h a slol)c of 14.1 niv. per carbon number. S o such corrclations were found for the homologous series of di-
732
ANALYTICAL CHEMISTRY
1 1 1 1
carboxylic acids from oxalic through sebacic acid (Clodicarboxylic acid). Mechanism of Enhancement. hlthough the effect of the presence of a n inorganic salt in this nonaqueous system is the same as that observed by Critchfield and Johnson in their titrations of aniline with HCI in aqueous 851 SaI-an increase in the effective acidity of the acid involvedthe mechanism of this acidity enhancement is not considered to be the same for the two solvent systems. Critchfield and Johnson, in their work in aqueous solutions, proposed that the mechanism of the acidity increase in the presence of high concentrations of inorganic salts involved an increase in the activity of the hydrogen ion ( 3 ) . This incremed activity was brought about, by dehydration of the hydronium ion by the particular salts chosen, thus freeing the hydrogen ion. As the present syst'em is nonaqueous, such a dehydration mechanism is not a suitable explanation for the acidity enhancement observed. I proton "desolvation" iiiechanisni is also not entirely satisfactory for t'he present nonaqueous system, since the t'otal concentration of LiCl is too low (0.035.11) to prevent a significant number of solvent molecules from reacting with (solvating) the protons of the sample acid. Thus t'he mechanism operating in this nonaqueous system is probably not one involving solvent-salt interactions. The observations for the homologous series of monocarboxylic acids suggests that the mechanism of the LiCl enhancement of acidity is through the simple metathetical reaction : HA KzJr
H+
+
+ ,1-
LiCl K3Jr
Li+
+ C1-
-
LiA K4Jr
Li+
+ A-
HC1 KSJl H+ C1-
+
where K 1 is t'he equilibrium constant for the forward metathetical reaction, K , and K5 are the normal acid dissociation constants, K B is the autodissociation (ionization) constant for LiC1, and Kq is the dissociation constant of the lithium salt of the weak acid, HA. ( K 4is considered t'o be either a soluble coniplex dissociation constant or a solubility product constant, whichever is appropriate. Both are valid representations for a niet'athetical reaction.) Thus the acid really being titrated by TPA is HC1 and the increasing found as the chain length of the acid increases reflects the increased concentration of HC1 present a t the beginning of the titration. This increase in HC1 concent'ration indicates that the equilibrium reaction above is displaced further to the right (increasing K,) as the chain length of the acid increases and this shift in the equilibrium is brought, about by an increase in the degree of association (decrease in K4) of RCOOLi with increasing chain length, For any acid, then, the magnitude of the AEe.p,is directly relat,ed to the stability of the corresponding lithium salt,. To establish t'he mechanism of this enhancement' of apparent acidity, solutions of acetic, valeric, heptanoic, dodecanoic, and octadecanoic acid were titrated with TPA in acetonitrile containing various amounts of LiC1. Because of the similarities of the K , values for these acids and the constancy of K a and K5, the large differences in AEe,p, ( K , ) obtained for the extreme members of this series must be due to differences in KqIthe degree of dissociation of the lithium carboxylate. The shapes of those titration curves obtained when less than stoichiometric amounts of LiCl were present made quantitat,ive interpretation dificult. However, it was definitely established that for the short-chain monocarboxylic acids, a Considerable excess of LiCl was required before all the acid present could be titrated, indicative of a high Kq value. The long-chain (C18)acid, however, required only a slight excess of LiCl over that required for stoichiometry to permit quantitative titration, indicat,ing a lower Kq value than that for the shorter chain length acids. Since a t least a stoichiometric ainount of LiCl must be present before essentially all of the longchain acid can be titrated, the hypothesis t,hat metathesis is the principal mechanism whereby the inorganic salt increases t'he apparent acidity of these weak acids is reasonable. Furthermore, since a considerable amount of LiCl in excess of stoichiometry is required for the shorter chain acids, the equilibrium nature of the proposed mechanism, with its associated constant, K 1 ,is also reasonable. In t'he titration of octadecanoic (stearic) acid with TPA in the presence
10% chloroform was tolerable without of LiC1, a precipitate formed. This decreasing the AE,,, values signifiwas isolated and analyzed for lithium. cantly. Theoretical for lithium stearat,e: 2.39% Extensions to Other Solvents and Li; found: 2.367,. Inorganic Salts. Using succinic acid More readily interpretable titration as a reference acid, a variety of salts curves were obtained for t h e s e T P h titraa n d solvents was examined. T h e tions when CaC12 was substituted for LiCI-CH3CK combination showed t h e LiC1. Recause anhydrous CaC1, is ingreatest enhancement of acidity (AEe,,. soluble in CHICK, the CaCI, was added of 230 mv.) using anhydrous salts, and as a inethanol solution with the t'otal solvents that were inert towards acid alcohol content held constant. The anhydrides. The solvents tried were same conclusions were reached for this acetonitrile, nitromethane, acetone, tertem as for the LiCl case. The butyl alcohol, and methanol. The led titration curves led to the alcohols, however, are not suitable for quantitative findings reported in Table anhydride-containing samples. The 111 and further support the conclusions anhydrous salts investigated were LiCI, regarding the mechanism reported for LiRr, LiC104, X'aC1, NaRr, S a I j and LiC1. I n this system, the amount of CaCI,. Hydrated salts cannot be used acid titratable is clearly a function of in systems containing acid anhydrides, the amount of CaC1, present and the inbecause the water of hydration has been fluence of the chain length of the acid experimentally observed to hydrolyze on the amount of the acid titrated conithese anhydrides, leading to false high pared to the amount of CaClz present is free weak acid values. The next most clearly shown. I n this CaC1, system, the AEe,p, effective salts for succinic acid in solvents other than CHICK were S a 1 in values are larger for all the monocartert-butyl alcohol (AEe, 190 mv.) and lic acids than those obtained using CaC1, in methanol (AEe,p. 240 rnv.). suggesting a larger K , value for I n an attempt to increase the AE, , t,he metathetical equilibrium reaction for short-chain monocarboxylic acids, 15 proljosed. Since less CaC1, than LiCl salts w r e investigated in CHICY. is required for each acid, the Kh values These were the best' (largest AE, u , ) of a for the calcium carboxylates are smaller series of 47 salts (F-, C1-. Br-) I-, than the corresponding K 4 values for the C104-, K03-,SO4-, salts of the alkali lithium carboxylates. This argument and alkaline earth metals and silver) assumes that the KBvalues for LiCl and examined in methanol. The survey CaCI2are nearly equivalent and that the acid was acetic acid. ;Ilthough sodium small amount of methanol required in and potassium perchlorate were without the CaC12 cases was without significant effect, Ca(C104), gave the largest AEe p . effect. (260 mv.) for acetic acid and in addition In the CaC1, system, the precipitate provided a AE, p , of 380 mv. for stearic formed during the titration of stearic acid, 230 mv. for benzoic acid, and 360 acid was isolated and the calcium conmv. for succinic acid in CH&X when tent determined. Theoretical for caltitrated with TPA. Although the hycium stearate: 6.597, Ca; found: 6.60%. drated forms of Ca(NO& and C a I h Eecause of the methanol required to gave A I T ~ , ~values , comparable to Cadissolve sufficient CaCI,, this system (C104), for each of these four acids, their cannot be applied to samples containing anhydrous forms, required for anhydride anhydrides. containing samples, were insoluble in The sleeve-type calomel reference CHsCN, as was anhydrous CaCL electrodes, containing CaC12 or LiCl as Hydrated CaI, was also as effective as the electrolyte, frequently allow enough Ca(C104)2,but upon dehydration, some leakage to introduce sufficient salt into free I2is formed that interferes with anthe titration vessel to permit complete hydrides, and is therefore also unsuitatitration of the sample acid without any ble. Strictly anhydrous Ca(C104)2,howintentionally added salt. Because of ever. is difficult to obtain because of its this, XaC1 was used as t,he reference powerful desiccating action, and the electrode electrolyte in these studies, small amount of residual water retained having been previously proven to be even after rigorous drying operaticns is inert as far as enhancing weak acid still apparent in TP.4 titrations of acid acidity. anhydrides. The pot,entialusefulness of It was found that the coniposition of Ca(C104),as a replacement for LiCl in the internal electrolyte of the calomel this method is obvious from the greatly reference electrode influenced in a reincreased b.Ee,D values, especially for producible fashion the potential breaks short-chain monocarboxylic acids (260 a t the end point in the TP-4 titration of mv. us. 50 mv. for LiCl using acetic the nionocarboxylic acids. These acid) and even significantly helljful for values, summarized in Table 117, may long-chain acids (380 mv. cs. 220 mv. for prove useful in selecting the proper elecLiCl with st,earic acid). However, trolyte for specific analytical applicaCa(C104), cannot yet be recommended tions. for use where samples contain waterKhere solubility properties of samples reactive, acid-producing components ww a problem, up t o 2OY0 acetone or
Table 111. Amount of Monocarboxylic Acid Titratable with TPA as a Function of Chain Length and Amount of CaClz Present
CaC12
present,
meq. 0 40 0 80 100 4 80
Acid titratable
(1.00 meq. present), meq. Cz Cs C? CI~ CI~ 0 29 0 30 0 34 0 37 0 37 0 61 0 66 0 73 0 76 0 76 0 7 8 0 8 8 0 9 3 100 1 0 0 1.00 1 00 1 00 1 00 1 00
Table IV. Influence of Reference Electrode Electrolyte on AE,,,
Internal electrolyte in methanol Saturated S a C l Saturated LiCl 0.1M LiCl 0.2M CaCls
AE,
,, mv.
C2-C3
acids 75 50 70
20
Cl4-Cl6 acids 280 240 240 150
such as acid anhydrides because of the residual water accompanying even carefully dehydrated Ca(ClO&. Referring to the above equation describing the proposed metathetical reaction, the differences in Al?e,p. ( K , ) found for different salts with the same acid simply are due to the combined influences of K3 and K4, since K 5 will be essentially the same for most mineral acids. That other lithium salt? were generally less effective than LiCl in enhancing the acidity of these acids can be attributed to lower Ka values for these salts compared to LiC1. The ability of a given salt to enhance the acidity of weak acids is considered, for a given solvent system, to depend upon the degree of dissociation of t,he salt' and the stability constant for the metal-acid anion compound formed. Such measurements were not made in this study. LITERATURE CITED
(1) Critchfield, F. E., Johnson, J. R.,
A N A L . CHEM. 28,434 (19Wj. ( 2 ) Zbid., 30, 1247 (19513). (3) Zbid., 31, 570 (1969). (4) Fritz, J. S., Lisicki, ?;. Jl., Zbid., 23, 589 (1951j. ( 5 ) Grove, E. L.,Talanta 4, 205 (1960). ( 6 ) Johnson, J. B., Fiink, G. L., ANAL. CHEM.27, 1464 (1955). (7) Leithe. IT., Z . Anal. Chem. 189. 396
(19621.
(, 8,) llarole. L. FY..Fritz. J. S . C ~ m i .3' 5 , 1431 (1963). ' (9) lIinczewski, J., Hojnacka, A , , Anal. ( W a r s a w ) 4 , 89 (1959). (10) llitra, B. C., Ghosh, P., Palit, AXAL. CHEM.36, 67.3 (1964). (11) Iiosenthal, I]., Dayer, J. S., 35, 161 (1963). (12) SLggia, S., Floramo, S . A , , 2 5 , ( 9 7 (1953).
RECEIVED for review September 28, 1964 Accepted 1)ecemher 11, 1064 1)tvtsmti of Analytical Chemistry, 149th Jleetlng ilCS, Iletroit, l I i c h , April 1965 VOL. 37, NO. 6 , M A Y 1965
a
733
