Quantitative Infrared Analysis of Some Water-Soluble Acids and Salts SIR: The acid-binding and solubilizing properties of long-chain aliphatic amines are well knobln ( 1 , 5 ) . One such amine, .imberlite Lh-2 liquid resin ( 9 , 4 ) , is an excellent reagent for exracting water-soluble acids into nonpolar solvents. Becauc,e of this ability and also because the resin has relatively weak infrared absorbaiice, it offers the interesting possibility of solubilizing insoluble acids in infrared transmitting solvents such as c a r b m disulfide, for quantitative analysis by infrared spectrophotometry. In previous work ( 2 ) this technique was usec for quantitative infrared analysis of sulfonic acids; its u4e has now been extended to other acids and salts both organic and inorganic.
corbic acid are not solubilized by either the extraction or solvent procedures. Weakly bound volatile acids such as acetic acid may be lost when the solutions are heated on the steam bath (HCI, which is tightly bound, does not volatilize) ; trichloroacetic acid can be solubilized by the extraction pro-
cedure but decomposes in the solvent procedure; nitric acid oxidizes the LA-2 resin if heated on the steam bath beyond dryness. Despite these and similar problems which may occur with other individual acids, most of the common water-soluble acids can be solubilized in CS,by one or both of the procedures
EXPERIMENTAL
Strong acids in general can be extracted directly from tqueous solution into a carbon disulfide solution of LA-2 resin. Weaker acids in general are not extracted quantitztively. By replacing the carbon disulfide with chloroform, the extraction i i improved, but weak acids are best :solubilized by a solvent procedure in which the acid is dissolved in a n alcohol or acetone solution of resin and then evaporated to dryness. Extraction Procedure (Strong Acids). Proceed as directed under “Extraction Procedure” for sulfonic acids ( 2 ) except that the addition of hydrochloric acid is omitted. Solvent Procedure (Weak Acids). Proceed as directed urlder “Ethanolic Procedure” for sulfonLcacids. Water Soluble Salts. Pass a suitable aliquot of s o l u i o n through a cation exchange ccdumn (Dowex 50W-X1, 50-100 nie:h; acid form; or equivalent resin:. Elute with water a n d proceed as directed under the extraction or solvent procedurc. Typical spectra of inorganic and organic acids obtained by this technique are shown in Figures 1 to 5 . Qualitative spectra were obtained on a Perkin-Elmer Infrat-ord. Quantitative measurements w r e made on a Perkin-Elmer infrared spectrophotometer Model 21. (’ell thickness is 0.5 mm. in all cases.
1-01
Figure
1.
HCI, 1.43 rng./rnl.
1.o 00
4
5
8 9 10 11 WAVELENGTH (MICRONS)
6
7
Figure
HgS04.(LA-2)2 - _ _ H2so4. LA-2
2.
12
13
14
12
13
14
Hz.504, 2.09 rng./rnl.
DISCUSSIC)N
Ysing the procedures outlined i t is possible to analyze quaititatively many mixtures of acids. However, several limitations should be pointed out. Nature of Acids. The method is riot applicable t o all acids. Some very weak acids such as glutamic acid, glycine, and other amino acids, nitrous acid, sulfurouq acid, and as-
4
5
6
7 8 9 10 WAVELENGTH (MICRONS)
Figure
’’
5
3
-HCIOB,3 . 21 - - - - HCIO,, 3 . 18
rng./ml. rng./ml.
VOL. 36, NO. 7, JUNE 1964
1383
described. In addition to the acids listed in Table I, the following acids were satisfactorily solubilized for infrared analyses: oxalic, malonic, succinic, malic, cyanoacet,ic, formic, maleic, fumaric, pht,halic, ni-hydroxybenzoic, p-hydroxybenzoic, glyceric, iodic, and hypophosphorous. Phosphoric acid is an exception in that it is not solubilized by the solvent procedure and is only partially solubilized in the pxtraction procedure, apparently in a hydrat’ed form (cloudy solution). On evaporating to dryness, the anhydrow acid-resin salt does not dissolve in CS,or CHCls. Nature of Spectra. Some acids yield spectra with few peaks; with broad, poorly defined peaks; or with peaks which are interfered with by CS2 or reiin absorbance. For example: HC1 (Figure 1) has only one strong peak, near 3.75 microns. HBr shows the same spectrum. Sulfuric and perchloric acids (Figures 2 and 3) have very similar spectra. For these and other mixtures, it may be difficult or impossible to find analytically useful peaks. Sulfuric acid can combine with the resin in a 1 : 1 or 2 : 1 ratio (equivalent,sof acid per equivalent, of resin) (4, and each salt has a characteristic infrared spect,rum (Figure 2). For analyt’ical purposes it may be expedient to keep the sulfuric acid concentration sufficiently low so that only the 1: 1 salt is formed.
D ’
3,
4
5
6
7
8 9 10 WAVELENGTH (MICRONS)
13
14
12
13
14
1 15
.10 u
g.20
2 8.30 g.40
