Determination of Water in Methanethiol and Ethanethiol GEORGE MATSUYAMA Research Deparfmenf, Union Oil Co. o f California, Brea, Calif.
b The Karl Fischer titration method was studied for determining water in ethanethiol. The interference of mercaptan with this titration could not b e completely eliminated b y using isooctene as suggested in the literature. The reaction of ethanethiol with isooctene in the presence of borontrifluoride-ethyl ether complex proceeds to the extent of about 95%. A turbidity method for determining water in methanethiol and ethanethiol was developed based on the temperaturesolubility relationship of water in these mercaptans. Ethanethiol i s readily dried b y drying agents, such as Drierite and silica gel, or b y chilling in a d r y ice-acetone bath and filtering off the precipitated water.
W
in methanethiol and ethanethiol has a marked effect on the corrosion of mild steeI used to contain these materials. Very little work on the determination of water in mercaptans has been reported. Mitchell and Smith (1) determined water in ethanethiol and mercaptans of higher molecular weight using Karl Fischer reagent. The infrared spectrophotometric method has been used to determine water in organic solvents and should be applicable to mercaptans. The present study was undertaken to test three methods, in order to provide a simple, rapid plant control method for determining water in methanethiol and ethanethiol. The investigation gave indications that the Karl Fischer method proposed by hlitchell and Smith is not adequate for determining water in ethanethiol. The infrared spectrophotometric method is the best method for determining water but requires equipment not readily available on location. A method was developed utilizing the temperature a t 1%hich water or ice turbidity appears in the liquid mercaptans. This turbidity method, although not particularly accurate or specific, can be used n i t h very little equipment in the plant. ATER
APPARATUS
Infrarcd spectra n-ere recorded with a l’erkin-Elmer llodvl. 21 doublc-bcam spectrophotometer using a sodiuni chloride prism and samplc cclh hnl-ing 1.5mm. light paths and sodium chloride n indows. 196
ANALYTICAL CHEMISTRY
A conventional all-glass Karl Fischer titration assembly was used. Two platinum electrodes sealed through the titration flask were attached to the polarization connections of a Beckman Model H-2 or Leeds & Northrup Model 7664 pH meter for titration end point detection.
4 . 6 ~
REAGENTS
The thiols used were commercial materials. Mass spectrometric analysis showed the methanethiol to be 99.2+ mole 70pure (excluding water) with 2,3dithiabutane (dimethyl disulfide) as the principal impurity. Mass spectrometric analysis of the ethanethiol showed it to be 98+ mole % pure (excluding water). Principal impurities were 0.5 to 1.0 mole yo acetonc, 0.3 mole yo methanethiol, and 0.1 mole % 3,4-dithiahexane (diethyl disulfide). A standard sample of ethanethiol from the American Petroleum Institute containing 0.05 f. 0.04 mole % impurity was used as reference material. Phillips commercial grade iso-octene n-as used to inactivate ethanethiol in the Karl Fischer titration method. Eastman Organic Chemical P4272 boron fluoride in ethyl ether (45% BFa, practical) and hfatheson T6147 (Tech) boron trifluoride ether complex 47% mere used as catalyst for the reaction of olefin with mercaptan. No differences were observed between these preparations, except for slight differences in water content. All other chemicals were reagent grade.
-2
-1.5 Loglo of
-I
-0.5
(Weight % W a t e r )
Figure 1 . Turbidity temperature of ethanethiol
Mitchell and Smith, the acetic acid mas, therefore, omitted from succeeding titrations. Procedure. Into a dry titration flask, pipet 10 ml. of iso-octene, 5 ml. of boron trifluoride-ethyl ether complex, and 1 ml. of sample. Stopper the flask tightly, mix the contents, and allow to stand for a t least 30 minutes. Add 5 ml. of pyridine and titrate with Karl Fischer reagent. Carry out a blank titration to find the water content of the reagent.
A sample of presumably dry ethanethiol and samples to which known amounts of water vere added were titrated for water by the procedure (Table
I)
9
KARL PISCHER TITRATION METHOD
hlercaptans interfere in the Karl Fischer titration of water because of the reaction: 2RSH
+ 12
+
RSSR
+ 2 HI
(1)
Two moles of mercaptan are equivalent to 1 mole of water in reacting with Karl Fischer reagent. According to hlitchell and Smith ( I ) , mercaptans can be inactivated by addition to olefinic hydrocarbons with boron trifluoride as catalyst: RSH
+
\
/ BFI \
/
\
C=C
-+
C--C/
L ’
(2)
I!\
They recommend iso-octene as the olcfinic hydrocarbon. By their procedure they found 0.70yo water in a sample of ethanethiol. This procedure was tried on a sample of ethanethiol and a drifting end point was obtained in the titration. A 5 suggested by
Table 1.
Water in Ethanethiol
(Karl Fischer titration) Sample Added EtSH, HaO, ml. rng. 1 1 55:0 1 61.2
H20 Found, Mg.
H?O Calcd., 1Ig.
6.9 61.9 67.4
si:9 68.1
As the titration indicated about 1% water in the mercaptan, several methods were tried to obtain anhydrous samples. Ethanethiol was shaken with Drierite, silica gel, and phosphorus pentoxide. Another sample was chilled in a dry ice-acetone bath to freeze out water. All four samples and a standard sample from the American Petroleum Institute showed 7.3 i 0.5 mg. of water per ml. t q Karl Fischer titration. The sample
The reaction time was varied from 15 to 90 minutes to see if this influences the extent of reaction. T i t h 15 minutes shaking the reaction went only to 92%, but with 30 to 90 minutes shaking the reaction went to 94%. Doubling the amount of mercaptan or iso-octene caused the reaction t o proceed to the extent of 95.5%. As the boiling point of methanethiol is 6.5' C., the determination of water in this mercaptan by the Karl Fischer titration or infrared method WT'&R not tried. W e i g h t % Water
TURBIDITY METHOD c
Figure 2.
Turbidity temperature of ethanethiol
dried over phosphorus pentoside turned cloudy on standing several days. and the water titer increased.
results obtained by using the ethanethiol dried over silica gel as the reference point are given in Table 111.
Because of the high water content obtained by this method for the BPI standard sample and other dried s m ples, i t was decided to investigate another method.
Table 111.
INFRARED ABSORPTION METHOD
The large infrared absorption of the O H stretching frequency a t 2.8 microns was selected for the measurement of water content of the ethanethiol samples. IJsing the infrared apparatus, samples of silica gel-dried ethanethiol to which known amounts of water had heen added were used to establish a n instrument calibration. The dry material was used as a reference in all measurements. The absorbance values shown in Table I1 indicate a constant e.ctinction coefficient for water in this medium.
W a t e r in Ethanethiol
(Infrared absorption) Ethanethiol H,O, Sample \Tt. L ; Dried over SiOz Used as reference Dried over Drierite 0 00 Dried by dry ice freezing 0 04 API standard 0.04
Y 0
!k
--
c'
K a t e r or ice precipitates froni ethanethiol when a sample is chilled. If the equilibrium involved in this precipitation is H20 (1 or s)
H20 (solution)
(3)
the equilibrium constant for the system when the solution just begins to become turbid is equal to the activity (or approximately the concentration) of the water in solution. According to the Gibbs-Helmholtz relation,
Therefore, the logarithm of the concentration of water in the mercaptan should be directly proportional to the reciprocal of the temperature (absolute units) at which the solution begins to become turbid. This approach was investigated as a possible simple, eni-
.9
c
z \
0 0
2
I
Table 11.
Instrument Calibration for Water in Ethanethiol
(Silica gel-dried ethsriethiol reference) Water Added, Absorbance, A/Wt. Go \vt. t C A HD 0 05 0 148 3.0 0 06 0 266 44 0 075 0 263 3 5 0.093 0.296 3.2 0.11 0.390 3.5 0.14 0.428 3.1
Using the previous calibration, measurements were made of t h e dried ethanethiol with the API standard sample (0.05 0.04 mole % impurity) in the reference beam of the spectrophotometer. The spectral data showed that the dried mercaptan samples contained less water than the API standard. The
*
-2
I
-1.5 Logloof
I
I
-I
I
I
I
-0.5
(Weight % Water)
Figure 3. Turbidity temperature of methanethiol
Later experiments shoved that Linde molecular sieve Type 4A possibly gave slightly better drying than silica gel. A sample of ethanethiol dried over molecular sieve showed about 0.1% less water than a sample dried over silica gel. These data indicate that anhydrous ethanethiol can be obtained and that the reaction of iso-octene with ethanethiol does not go to completion. If it is assumed that ethanethiol dried over silica gel is indeed dry, a Karl Fischer titer of 7.3 rt 0.5 mg. of water per ml. shows that the iso-octene-ethanethiol reaction goes to the extent of 947,.
Weight
*/e
Water
Figure 4. Turbidity temperature of methanethiol
pirical method for determining water in mercaptans. Standard solutions of water in methanethiol and in ethanethiol were prepared by weighing water and dry mercaptan into borosilicate glass ampoules and sealing, The mercaptans were dried iyith silica gel and by chilling in a VOL. 29, NO. 2, FEBRUARY 1957
197
dry ice-acetone bath and filtering. The ampoules were m r m e d to room temperature, mixed thoroughly, then cooled s l o ~ l y and , the temperature a t which a turbidity was first observed was recorded. A Dewar flask, a thermometer, some acetone, and dry ice were used t o cool the sample and observe the turbidity temperature. The observations are shown in Figures 1 to 4. As anticipated from Equation 4, straight-line plots are obtained in Figures 1 and 3. Of interest, however, is the indication of two straight lines in each plot. This probably arises from a change in the turbid phase from ice to water. The heats of solution for these two phases would be different, causing a change in s!ope of the straight line. The temperature a t which the change occurs indicates that
the turbid phase is not pure water but mercaptan in water. To make the curves more readily usable for determining tvater in the mercaptans, they are plotted in more convenient units in Figures 2 and 4. The turbidity method for determining ivater in methanethiol and ethanethiol is not intended to be extremely accurate. The turbidity temperature was determined with an accuracy within about 3’ C., which is equivalent to 0.027, water a t a concentration of 0.2 weight 70. An ever-present source of error is the presence of foreign substances in the mercaptan sample, which can give rise to turbidity. On the other hand, the method is very rapid and simple and has been used in these laboratories for several years.
ACKNOWLEDGMENT
The author wishes to express his appreciation to D. 0. Alford and A. B. Menefee for assistance in obtaining infrared spectrophotometric data, and to H. E. Howard for mass spectrometric data. Permission of the Union Oil Co. of California to present and publish this paper is gratefully acknowledged. LITERATURE CITED
(1) Mitchell, J., Jr., and Smith, D. bI.,
“Aquametry. Application of the Karl Fischer Reagent to Quantita-
tive Analyses Involving Water,”
pp. 134 ff, Interecience, New York, 1948.
RECEIVED for review June 11, 1956. Accepted December 4, 1956.
Other papers presented in the group session on analytical research will be published in the March issue of ANALYTICAL CHEMISTRY
Thermogravimetric Analysis of Complex Mixtures of Hydrates EDWARD J. GRIFFITH Research Department, Inorganic Chemicals Division, Monsanfo Chemical
b
The Chevenard thermobalance has been used to determine the quantity of water present in each phase of complex mixtures of anhydrous salts and hydrates, containing as many as six phases. The method is based on the fact that when the proper rate of heating is employed, a selective decomposition of the phase with the highest dissociation pressure occurs. When the phase of the highest dissociation pressure is completely decomposed, the substance with the second highest dissociation pressure begins to decompose, etc. The water present in each phase may b e determined with an average error of about 1 %.
M
ANY OF THE SOLID CHEMICALS
used in household and industrial applications are complex mixtures of hydrates. Common examples are fertilizers, detergents, pharmaceuticals, and industrial poisons. The physical and chemical properties of these solids often depend upon the quantity and location of water of hydration in the products. 198
ANALYTICAL CHEMISTRY
Co., Dayton,
S o satisfactory method has been available for quantitatively determining the water in the various phases of partially hydrated substances, as the water moves toward an equilibrium condition. Simple crystalline mixtures of anhydrous salts and their hydrates may be quantitatively analyzed by x-ray analysis. K h e n the mixture becomes complex, the x-ray method is inadequate and phases are often missed. It is sometimes desirable to know not only how much water is present in a mixture of hydrates and in which phase the water resides, but, even more important, how much water is present in each phase. At any fixed temperature, a system of anhydrous salts and their hydrates exhibits a definite pressure of water vapor, sometimes referred to as the dissociation pressure. If the vapor is slowly removed, the hydrate maintains the same dissociation pressure until that phase is completely dehydrated. If a lower hydrate exists, the pressure drops to the equilibrium pressure of this phase when the higher hydrate disappears. The equilibrium between hy-
Ohio
drates and the rapor phase a t constant temperatures is well known in phase studies. K i t h the development of the modern thermobalance ( I ) , a tool i s available in which temperature may be varied while the atmospheric partial pressure of water vapor over the sample is nearly constant. The dehydration is then followed by measuring the loss of weight as a function of time. The rate a t which a hydrate loses water in an open system at a fixed temperature is dependent upon its dissociation pressure. Thus, the water in two different phases of a mixture of hydrates may be observed by the rate of loss of water from the mixture ( 2 ) . EXPERIMENTAL
The thermobalance employed in this work was manufactured by the A.D.A.M.E.L. Co., Paris, France, and was equipped with a strip recorder obtained from the same company. The temperature of the furnace could be automatically marked upon the pyrolysis curves by an especially designed shorting circuit across two terminals of the
