method can be extende'i to quantities of an order of magnitude smaller by using the initial organic extracts directly and not diluting by a factcsr of five as was done in the present procedure. It would seem that this method using KPFs, standardized by the gravimetric method of Affsprung and Archer (4), as a secondary standard, might very well replace all other methods in the routine determination of hexafluorophosphate.
ACKNO\VL.EDGMENT
The KPFBused in this work was donated by the OzarkMahoning CO., Tulsa, Okla.
RECEIVEDfor review November December 27,1966.
14, 1966.
Accepted
Ultraviolet Dletermination of Tertiary Mercaptans as Thionitrites George W. Ashworth and Robert E. Keller Research Department, Organic Chemicals Dloision, Monsanto Co., St. Louis, Mo.
ULTRAVIOLET ABSORPTION METHODS are not applicable to mercaptans per se because of the absence of a resonating or absorbing species. This report covers work on the conversion of mercaptans to thionitrit :s with subsequent ultraviolet absorption measurement of the resonating thionitrite species. Kresze and Winkler (],I studied the conversion of tert-butyl mercaptan to tert-butyl 1hionitrite with nitrous acid in aqueous dioxane and followed tk e kinetics of this reaction spectrophotometrically. No other references were found in the literature covering ultraviolet absorption measurement of thionitrites. Likewise, only very lirriited work has been reported on the study of thionitrites by infrared spectroscopy. Philippe and Moore ( 2 , 3) prepared cthyl, n-propyl, and isopropyl thionitrites by reaction of appropriate mercaptans with nitrous anhydride and obtained infrared spectra of these compounds in their gaseous state. A sensitive and rapid ultraviolet absorption method for conversion and measurement of tertiary mercaptans as thionitrites is described. Primary r?ercaptans produce no significant absorption under the same experimental conditions. Secondary mercaptans show about 30% of the molar absorptivity of the tertiary. EXPERIMENTAL Apparatus. A Cary Model 11 spectrophotometer was used with 2-cm fused silica cells to make the ultraviolet absorption measurements. Standard laboratory glassware was used to carry out the experimental work. Reagents. Carbon tetrachloride, reagent grade; sodium nitrite, 50 solution; hydrochloric acid, 6N; and ammonium carbonate, 5 % solution. Recommended Procedure. Weigh (to the nearest 0.1 mg) the sample containing 1 0 mmole of tertiary mercaptan into a 100-ml volumetric flask. Dissolve in carbon tetrachloride and dilute to volume. Pipet a 5-ml aliquot into a 125-ml separatory funnel containing about 25 ml of water. Pipet 20 ml of carbon tetrachloride into the separatory funnel. Carry out the following manipulations in a dark room provided with incandescent illumination only. Pipet 1 ml of 50z sodium nitrite solution and 2 ml of 6 N hydrochloric acid into the separatory funrel and shake vigorously for 30 seconds, vent the funnel, and continue shaking for an additional
(1) G. Kresze and J. Winkler, Ber., 96, 1203 (1963). (2) R. J. Phillippe and H. Moore, Spectrochirn. Acta, 17,1004(1961). (3) R. J. Phillippe, J. Mol. Spectry., 6 , 492 (1961).
30 seconds. Draw off the carbon tetrachloride layer into a second separatory funnel, add about 25 ml of distilled water, and shake vigorously. Draw off the carbon tetrachloride into a third separatory funnel, add 15 ml of 5x ammonium carbonate solution, and shake vigorously. Filter the carbon tetrachloride through a small pledget of cotton packed in the stem of a funnel to remove suspended water. Record the absorbance of the carbon tetrachloride solution from 400 to 270 mp in a 2.0-cm cell using carbon tetrachloride in the reference cell. Measure the absorbance of the thionitrite at the absorption maximum (344 mp). Reagent and matrix blanks are normally zero. Calibration. Prepare a standard solution of the tertiary mercaptan to be measured in carbon tetrachloride. Process an aliquot of this solution containing about 0.05 mmole of mercaptan through the method and calculate the absorptivity at the absorption maximum (344 mp). Calculate unknowns using this absorptivity. RESULTS AND DISCUSSION Spectral Curves. The absorption spectra for butyl thionitrites shown in Figure l are typical of those for all thionitrites in carbon tetrachloride. The absorption peak of the tert-butyl thionitrite occurs at 344 mp. The sec-butyl thionitrite has a peak at 340 mp and an absorbance about 30% of that for the tertiary thionitrite. No characteristic absorption is obtained for n-butyl thionitrite and isobutyl thionitrite. tert-Butyl thionitrite gives two absorption peaks occurring at 229 and 343 mp in n-hexane. n-Hexane was not used as a solvent until completion of the work with carbon tetrachloride. Wavelength 229 mp could be a poor choice for analytical purposes because of possible interference from ultraviolet-absorbing contaminants in samples. sec-Butyl thionitrite gives only one peak occurring at 340 mp in n-hexane. Appreciable cutoff-type absorption occurs in the low wavelength region. Originally, isooctane was a solvent of choice because a lighter than water phase was desired for the extractmn manipulations. However, the reproducibility has better with carbon tetrachloride than isooctane and for this reason, the use of isooctane was discontinued. Chloroform was not used as a solvent because of interfering alcohol present in commercial product. The alcohol can be removed with water washes, but the washed chloroform is not always stable. The additional sensitivity gained with chloroform did not justify the additional work to prepare freshly washed chloroform. VOL. 39, NO. 3, MARCH 1967
373
Table I. Molar Absorptivities ( E ) of Thionitrites Peak wavelength Mercaptan E Solvent 17-B~tyl 10 344 Carbon tetrachloride n-Hexane 229 (slope) 130 344 Isobutyl 15 Carbon tetrachloride ii-Amy1 Carbon tetrachloride 10 344 Isoamyl Carbon tetrachloride 11 344 344 n-Decyl Carbon tetrachloride 9 Carbon tetrachloride n-Octyl 9 344 200 344 sec-Butyl Carbon tetrachloride n-Hexane 229 (slope) 1660 n-Hexane 165 340 Cyclohexanethiol 344 238 Carbon tetrachloride ti-Hexane 340 220 n-Hexane 229 (slope) 2700 229 n-Hexane reif-Butyl 9360 343 n-Hexane 707 978 339 Chloroform 687 343 Isooctane 771 344 Carbon tetrachloride 344 780a Carbon tetrachloride tert-Amyl 344 Carbon tetrachloride ter.r-Octy1 897 344 918" Carbon tetrachloride {err-Dodecyl 344 9363 Carbon tetrachloride terr-Tetradecyl 344 950a Carbon tetrachloride tevt-Hexadecy1
2.c
I.5
"
P)
B 1.0 4
0.5
Corrected for typical assay of mercaptan reported by manufacturer, Phillips Petroleum Co. All mercaptans reported in this table are commercial grade materials.
Molar Absorptivities. The molar absorptivities of a random selection of 14 thionitrites for normal, iso, secondary, and tertiary mercaptans are given in Table I. Results show that normal and is0 mercaptans have molar absorptivities no greater than about 2 % of the values reported for tertiary mercaptans. It is apparent that the tertiary mercaptan can be determined with little interference from these isomers. Secondary mercaptans have about 30% the absorbance of the tertiary and thus interfere. The data show that the molar absorptivity increases with increasing molecular weight. No work was done to explain this observation. As shown for tert-butyl thionitrite, the molar absorptivities vary with the solvent used. A higher absorptivity is obtained using chloroform than with the other solvents. A slight shift in the wavelength of the absorption maxima also occurs. The general character of the curve is the same in all the solvents. Beer's law is obeyed for terf-butyl thionitrite. It is assumed that it is obeyed for all the thionitrites, but this was not checked. Stability. Thionitrites decompose a t the rate of 0.5 to 0 . 8 x of the amount present per minute in daylight or in fluorescent light. The use of low actinic separatory funnels cuts this down by a factor of 10. The thionitrite is stable in incandescent light for a t least 16 hours. Reproducibility. Results for six replicates show that the procedure is reproducible to within about 2 % of the amount measured for the tertiary mercaptans. Trace acids remaining in the carbon tetrachloride will cause poor reproducibility. For this reason, the described procedures involving water and bicarbonate washes should be followed closely. Poorer reproducibility was obtained for tertiary mercaptans using isooctane as a solvent in place of carbon tetrachloride. This may have been caused by free acidity remaining after the water washes. The reproducibility of the method for secondary mercaptans is still poorer (+15 % average deviation) 374
e
ANALYTICAL CHEMISTRY
Figure 1. Absorption spectra for butyl thionitrites 0.125 gram of mercaptan per liter of carbon tetrachloride 2-cm path length cell 1. Normal 2. Is0 3. Tertiary 4. Secondary
using either n-hexane or carbon tetrachloride. Additional work should be carried out to optimize the method if it is applied to quantitative measurement of secondary mercaptans. Interferences. Any ultraviolet-absorbing species and any resonating species which can be generated from other components will interfere. The authors and others ( 4 ) have studied nitrites prepared from alcohols by this technique extensively. Results show that primary, secondary, and tertiary alcohols all give characteristic nitrite ultraviolet-absorption spectra having at least five well resolved peaks in the range 330 to 400 mp with molar absorptivities ranging from 55 to 100. Reaction Study. This ultraviolet technique was used by the authors to provide data for Zienty et al. on the addition reaction of rert-dodecyl mercaptan with maleic anhydride in dioxane at 25" C (5). Results showed the rate and extent of reaction under the experimental conditions used.
RECEIVED for review September 21, 1966. Accepted December 5, 1966.
(4) V. D. Kapkin, M. A. Ratonskaya, V. B. Belyanin, and A. N. Baskirov, Zh. Analit. Khim., 20, 364 (1965). (5) F. B. Zienty, B. D. Vineyard, and A. A. Schleppnik, J . Org. Chem., 27, 3140 (1962).
