precision of the method on five samples from operating industrial systems and one standard solution. As is true of any titration with an indicator end point, a certain amount of subjectivity can be expected, especially on colored or turbid samples frequently encountered in industrial water systems. These data show that the precision of the method is quite adequate. Table IV shows a comparision of results between the dithizone extraction procedure (IO)and the DTP-4-dithizone titration. The samples selected were typical industrial water samples with varying concentrations of possible interferences. Even with iron concentrations as high as 8 p.p.m. and hardness levels in excess of 1000 p.p.m., the two methods show excellent agreement. This is even more significant when considering that
the dithizone extraction procedure is very sensitive, requiring very small sample aliquots for analysis. The favorable agreement over 8 p.p.m. of iron was not anticipated. However, the particle size of suspended iron is a factor in the extent of interference. Very finely divided iron will be more readily dissolved, whereas coarser material will be dissolved more slowly and thus interfere to a lesser extent. Iron interference must be considered with samples that require acidification. LITERATURE CITED
(1) Costa, A. C., Chemist-Analyst 47,
39 (1958). (2) Elo, A., Polky, J. R., ANAL. CHEM. 32, 204 (1960). (3) Medlin, W. L., Ibid., 32, 632 (1960). (4) Mehlig, J. P., Guill, A. P., Zbid., 23, 1876 (1 51). (5) Xegoiu, D., Vasilescu, C., Analele
Univ. “C. I . Parhon,” Ser. Stiint. S a t l . 9, 65 (1960).
(6) Reilley, C. N., Schmid, R. W., ANAL. CHEM.31, 887 (1959).
(7) Reilley, C. K., Sheldon, M. V., Chemist-Analyst 46, 59 (1957). (8) Reilley, C. N., Vavoulis, A., ANAL. CHEM.31, 243 (1959). (9) Sadek, F. S., Schmid, R. W., Reilley, C. N., Talanta 2, 38 (1959). (10) “Standard Methods for the Examination of Water and Waste Water,” 11th ed., p. 386, APHA, Inc., New York, 1960. (11) Wanninen, E., Ringbom, A., Anal. Chim. Acta 12, 308 (1955).
JAMES J. HICKEY CHARLES J. OVERBECK Analytical Laboratory Nalco Chemical Co. 6216 W. 66th P1. Chicago, 111. 60638 THISlaboratory has applied for a patent on this test procedure.
Quantitative Determination of impurities in Salicylic Acid SIR: h reYieFv of the literature indicates a lack of methods for the determination of impurities in salicylic acid in the 1i.p.m. concentration range. Various paper and column chromatographic methods ( 2 , S ,6 , 9 ) are available for the separation of the hydroxybenzoic acids; however, none was sufficiently sensitive or broad enough to cover all possible impurities. Phenol (E) has been separated by steam distillation followed by a bromate-bromide determination. Recently, Bailey ( 1 ) determined salicylsalicylic acid by a thin layer chromatographic method to a stated lo\\-er limit of o.05y0. The procedure can probably be modified for greater sensitivity. From an extension of earlier work on the separation of the isomers of hydroxybenzoic acid (8),neutral impurities and those of lesser acidity than salicylic acid were separated by nonaqueous ion exchange chromatography. The method developed did not require a special impurity-concentration step or the preparation of derivatives. Salicylsalicylic acid under specified conditions undergoes alcoholysis on the resin yielding equimolar amounts of salicylic acid and methyl salicylate, the latter moiety being used as a measure of the Concentration of this impurity. When w l i i j i-:ilicylic. wid was introduced to tl e r , 4 n in the sample environment only partial alcoholysis took place. However, neutralization of the sample before introduction to the repin produced a 9&lOO% conversion a t the 100- to 1000p.p.m. level. p-Hydroxyisophthalic acid has a pK value smaller than that of salicylic acid; therefore, it cannot be determined by the ion exchangeultraviolet method. 934
ANALYTICAL CHEMISTRY
Consequently a thin layer chromatography (TLC) method was devised for its determination. This compound exhibits very intense fluorescence when activated by ultraviolet radiation. After the acid was separated on the TLC plate, this property was used for concentration determination. EXPERIMENTAL
Apparatus and Reagents. Gradient elution equipment, the ultraviolet monitor, resin preparation, and the spectrophotometer are described in an earlier paper ( 7 ) . However, 1-mm. capillary tubing was substituted for regular tubing in the gradient elution equipment. Dow U.S.P. sublimed salicylic acid containing 20 p.p.m. of p-hydroxybenzoic acid was recrystallized four times from glacial acetic acid and dried in a vacuum oven at 60” C. for 2 hours. KO impurities were detected within the limits of the method although a slight odor of acetic acid was present. This material was used for recovery studies. The following standards used had a minimum purity of 98y0: phenol, phenyl salicylate, methyl salicylate, salicylsalicylic acid, benzoic acid, mhydroxybenzoic acid, p-hydroxybenzoic acid, xanthone, and p-hydroxyisophthalic acid. ACS grade methanol and reagent grade glacial acetic acid were used without further purification. Ion Exchange Procedure. h 1.000gram sample of salicylic acid is dissolved in approximately 10 ml. of methanol and adsorbed on a 13- X 180-mm. bed of Dowex 2-X8 ion exchange resin (acetate form, 200-400 mesh). Column effluent together with washings are retained in a 100-m1. volumetric flask for phenol, xanthone, phenyl salicylate, and methyl salicylate (from alcoholysis
of salicylsalicylic acid) analysis. T o the mixing flask of the gradient elution equipment is added 200 ml. of methanol while the reservoir is filled with 15% acetic acid-methanol (v./v.), The ion exchange column is attached and, with the aid of approximately 2-p.s.i. nitrogen pressure, the gradient elution is carried out at 3 ml. per minute. Concentration determinations are made in 10-em. absorption cells using methanol as the reference solvent. p-Hydroxybenzoic acid is measured at 270 mp since acetic acid shows general absorption at 254 mp, the wavelength of maximum absorption for this particular compound. All other components are measured a t their respective ultraviolet maxima. The salicylic acid can be removed from the resin with approximately 400 ml. of glacial acetic acid. Following removal of the acetic acid with methanol, the resin can be reused indefinitely. TLC Procedure. T L C plates of 0.4-mm. thickness are prepared using silica gel G (E. Merck) and a Desaga spreader; they wei’e dried a t 105’ C. for 1 hour before use. One-gram samples of specially purified salicylic acid are weighed into 10-ml. volumetric flasks. Sufficient p-hydroxyisophthalic acid is added to produce concentrations of 0.0, 0.01, 0.02, 0.04, 0.06, 0.08, and O.lyo relative to salicylic acid. The contents are diluted to volume with methanol and 10-pl. aliquots are spotted, together with an equivalent amount of the samples being examined, using a Hamilton 10-J. syringe. The plate is developed with benzene/dioxane/ac&tic acid (90 : 25: 4) (4). (p-Hydroxyisophthalic acid has an Rt value of 0.31 using this system.) Following airdrying of the plate, concentration determinations are made by a visual comparison of the fluorescence of the unknowns and standards under short wavelength ultraviolet radiation (254 md.
FRACTION NUMBER
Figure 1. Separation of small amounts of the various acids in the pesence of a large amount of salicylic acid by ion exchange chromatography ( 1 0-ml. fractions) and monitered by an ultraviolet photometer RESULTS AND DISCUSSION
The chromatogram for the separation r acidic impurities in salicylic acid is illustrated in Figure 1. Seutral and very weakly acidic impurities such as phenol, phenyl Falicylate, and xanthone are found in the methanol eluate from sample introduction. Sensitivities for the determination of the various impurities are calculated to be as follows: phenol: 20 p.p.m., xanthone: 10 p.p.m., phenyl salicylate: 20 p.p.ni., salicylsalicylic acid: 40 p.p.ni.,p-hydroxybenzoic acid: 5 p.p.m., vi-hydrosybenzoic acid : 20 p.p.ni., benzoic acid: 50 p.p.m., p-hydroxyisophthalic acid by TLC: 200 p.p.m, The limits of detection were calculated on the bazis of obtaining an absorbance of 0.04 for a 100-nil. solution using a 10-cm. absorption cell and a 1,-gram sample of salicylic acid. Table I illustrates representative recoveries of the various possible impurities added to 1-gram samples of specially purified salicylic acid and taken through the procedure. Recoveries were quantitative within a n experimental error of about + 5% relative at the 100-p.p.ni. level. An exception to this was the determination of salicylsalicylic acid. When this compound is put through the resin in the sample environment, a moderately acidic medium, alcoholysis occurs to the extent of 30 to 40%. However, if the sample is first neutralized to form sodium salicylate, alcoholysis increases to greater than 90%. Because the sample is introduced to the resin in a nearly neutral or slightly basic medium, a portion of the methyl salicylate will be adsorbed on the resin. The remainder will be found in the methanol effluent.
Gradient elution of the resin will remove that portion adsorbed with the first 40 to 50 ml. of effluent collected. In general it is best to examine the first fractions of the gradient elution for the presence of weakly acidic components. If salicylsalicylic acid and phenyl salicylate are both present in a salicylic acid sample, it is generally difficult to distinguish between the two. However the presence of one or the other can be distinguished by the slight difference between their neutral and basic ultraviolet spectra. A possible method of determining whether a mixture of the two is present would be to pass the sample through separate ion eschange columns in neutralized and unneutralized form. The presence of salicylsalicylic acid causes the absorbance value to be lower for the unneutralized sample. The alcoholysis of salicylsalicylic acid is definitely attributed t o its contact with the resin. A stability study of salicylsalicylic acid in 0.1% sodium hydroxide-methanol solution, as measured by ultraviolet spectrometry, does not indicate the formation of
Table 1.
either sodium or methyl salicylate. Further evidence to support the alcoholysis theory is an analogous reaction observed for salicylsalicylanilide. This compound alone or in the presence of salicylanilide undergoes alcoholysis with the formation of equimolar amounts of methyl salicylate and salicylanilide. Quantitative alcoholysis takes place without neutralizing the sample before it is introduced to the resin. If the analysis of sodium salicylate is attempted, weakly acidic species such as phenol and phenyl salicylate are adsorbed by the resin. I n the unneutralized sample analyses, these components are found in the methanol eluate together with any neutral components. This is explained by the fact that the acidity of salicylic acid is sufficient to prevent the adqorption of these components by acting either as an acidic eluent or as a contributor of a common ion suppressing dissociation of the weakly acidic compoucdi. Because these materials are weakly adsorbed, they appear in the first 50 nil. of effluent following the start of the gradient elution. The remainder of the weak acids are removed in the usual manner as illustrated in Figure 1. The stability of salicylic acid in methanol solution mas determined because of possible formation of methyl salicylate. One-gram samplrs of salicylic acid were weighed into respective 10-ml. volumetric flasks and diluted to volume with methanol. These were examined according to the method at 0, 1, 2, and 4 d a y intervals and showed the presence of < l o , 107, 350, and 650 1i.p.m. of methyl salicylate, respectively. Therefore the sample .solution should be put through the resin immediately. However, it is not essential that the gradient elution be started at this same time. A 1-gram sample of specially purified salicylic acid fortified with 10 fig. of p hydroxybenzoic acid (10 1i.p.m.) taken through the method gave an 80% recovery (8 p.p.m,). There appears to be no marked drop in recovery a t the lower limit. The only impurity consistently observed to be present in Dow U.S.P.
Analysis of Known Salicylic Acid-Impurity Mixtures
Mixture 1 Component p.p.m.: Added Found Phenol Xanthone 104 101 Salicylsalicylic acid 130 46 Phenyl salicylate Benzoic acid 102 101 p-Hydroxybenzoic acid 91 91 m-Hydroxybenzoic acid
Mixture 2 Added Found 100 104 106 105
103 102
102
99
Mixture 3 a Added Found 104 101
103
102
109 96
91
97
Sample was neutralized before the ion exchange separation.
VOL. 38, NO. 7, JUNE 1966
935
sublimed salicylic acid within the sensitivity of the method was, on the 100 P . P . ~of. hydroxyb benzoic acid. On rare occasions p-hydroxyisophthalic acid was observed a t the limit of detection. The possibility does exist that an impurity may have been overlooked in this analysis. However, from examination of salicylic acid mother liquors, it concluded that no other significant ?Oncentraimpurities Of tion were present.
LITERATURE CITED
( 1 ) Bailey, R. w., AKAL.cHEM. 36, 2021 (1965). ( 2 ) Lederer, AI., Australian J . Sci. 11, (3)208 Marvel, ( 1949).C. S.,Rands, R. D. Jr., Chem. sot, 72, 2642 (1950). J. (4)Pastuska, G., Z. Anal. Chem. 179, 355 (1961). ( 5 ) Shcherbachev, K. D., Khim. Form. Prom. 1934 ( 5 ) , 37. ( 6 ) Sinton, F. C., J . Assoc. Oflc. Agr. Chemists 13, 344 (1930).
(7) Skelly, N. E., ANAL. CHEM.33, 271 (1961). (8) Skelly, N . E., Crummett, W. B., Ibid., 35, 1680 (1963). (9) Van Oame, H. C., J . Assoc. Oj'ic. Agr. Chemzsts 43, 593 (1960).
NORMAN E. SKELLY Special Services Laboratory The Dow Chemical Co. Midland, Mich. Division of Analytical Chemistry, Winter Meeting, ACS, Phoenix, January 1966.
Stability Constants of Some Dihydrazide Complexes of Cadmium SIR: A recent polarographic investigation of the complexation of cadmium by alkyl and aryl carboxylic acid hydrazides (6) led to the conclusion that the complesing moiety was the neutral hydrazide rather than the anion, as had been found for other metal ions such as copper (1, 3 ) . At least three soluble cadmium complexes were found, and distinct differences in stability between the alkyl and aryl comple.;es were noted. Therefore, it a1)peared that, in the case of cadmium, the coniplexing hydrazide ligand was behaving like an amine. I n a manner analogous to the situation with organic mono- and diamine complexes, it was felt that the complelation of metal ions by dicarboxylic dihydrazides might be significantly different from that found with monohydrazides. Therefore, a study of the behavior of cadmium in the presence of some dihydrazides was undertaken. The success obtained with the polarographic approach for the determination of complexity constants in the previous study (6) led to its use in the present case. The method of DeFord and Hume
( 2 ) was utilized to calculate stability constants from the polarographic data obtained. EXPERIMENTAL
Chemicals. Carbohydrazide, succinic dihydrazide, and adipic dihydrazide (Olin Mathieson Chemical Corp.) were recrystallized twice from water, dried a t 100" C., and stored under dry nitrogen. All other chemicals were reagent grade and were used without further purification. Apparatus and Procedure. Detailed descriptions of the apparatus and procedure have been given previously (6). RESULTS AND DISCUSSION
Previous studies on the complexation of cupric ion by isonicotinic acid hydrazide established that the over-all reaction involves release of protons (1, 3 ) . Titration of the hydrazide with copper solution lowered the pH about 0.6 unit below that for the corresponding copper alone. The reaction of monohydrazides with cadmium, however, did
not seem to be of the same type; no release of protons was found (6). I n the present instance, titration of the dihydrazides with C d f 2 produced the data shown in Figure 1. The p H of the solutions did not drop below that for Cd+2 itself. Therefore, any reaction of the dihydrazides with cadmium must occur without release of protons and, again, through a neutral hydrazide species. Polarographic studies of complexation are well established ( 5 ) . If single, strong complexes are involved, a simple plot of Elidus. log ligand concentration can give sufficient information for elucidation of the complex. With successive eomplexes, more elaborate calculations are required. DeFord and Hume ( 2 ) first derived the necessary equations for calculation of the stability constants of consecutive complexes. For a reversible reduction of a complexed metal ion to the amalgam, they have defined functions, F J ( X ) , such that
-+
EH,V.
Carbohydrazide t
Carbohydrazide
I
Cd++Blank
I
I
I
I
-
1.5
I -1.1
I
- 8.8
I 0
lag [Hydrarid4
Figure 2. Half-wave potential of cadmium as a function of log hydrazide concentration
936
ANALYTICAL CHEMISTRY
