Direct Method for the Determination of Methoxy Group in the Presence of Borohydrides ARTHUR P. ALEXANDER, PHILIP G. BOURNE, and DOUGLAS S. LITTLEHALE Analytical Laboratories, M e t a l Hydrides, Inc., Beverly, Mars.
Osmium tetroxide solution, 0 . O l A l . Ordered as perosmic acid crystals, Merck; Howe & French suppliers. Sitric acid, SAr, a 1 to 1 solution. Sulfuric acid, 6-V, 5 volumes of water to 1 volume of concentrated sulfuric acid. Standard methanol solution, Fisher Scientific Co.
A quantitative method for the determination of methoxy group in borohydrides is based on oxidation of methanol to formic acid w-ith a standard solution of ceric nitrate. A precision within 0.570 of the methoxy group present in borohydrides is obtained.
PROCEDURE
S
ODIUhI methylate and methoxy borohydrides are formed during the preparation of sodium borohydride; thus, it is of value to know t,he trace amounts of methoxy-group contam'nants present in the final product. Such a determination is also applicable in analyzing the by-product, sodium methylate. .\n:tlysis of methoxy borohydrides for methosy content is useful, for the present assays are based on boron analysis. Further, :I methoxy determination gives a more accurate value for t!ie per cent of oxygen present in borohydrides. The several methods tried prcvious to the direct method of methoxy determination proved unsatisfactory. When a watermethanol mixture is distilled from a solution of sodium methylate, varying amounts of boron are carried over. This makes it impossible to compare the specific gravity and/or the index of refraction of the distillate against standard solutions. A mirromethod, using a modified Zeisel procedure, also is unsatisfactory. The method has several disadvantages, inasmuch as sodium compounds present tend to interfere, and more specialized micro equipment is necessary ( 2 ) . A direct method of determining methoxy by sodium content, and a subsequent correction for sodium hydroxide, cannot be applied because of the presence of sodium hydride and other sodium compounds ( 4 ) . The principles of the current, method were first developed and applied during an investigation of the reactions of quadrivalent cerium with aliphatic alcohols in nitric acid solutions ( 3 ) . I n the present application, t,he assumption is made that methoxy group is present as methoxy borohydrides, sodium methylate, methyl borate, methanol, or similar compounds formed during the production of borohydrides. The proposed method gives an accuracy within &0.5% in the presence of such contaminants as sodium borohydride, sodium hydride, sodium borate, and carbon. Sodium methylate of analytical purity was unavailable a t the time, so the accuracy is best shom-n by a comparison with carbon analyses. This report shows that the methoxy group is determined quantitatively by oxidation to formic acid with qundrivalent cerium.
-4sample containing approximately 50 mg. of methoxy group is cautiausly dissolved in 10 ml. of 8-V nitric acid (the solution being kept cool v i t h an ice bath to prevent loss of methanol), and the solution is filtered ( S o . 42 Whatman) into a 500-ml. Florencetype reflux flask. ( A larger sample may be taken and appropriate aliyuots used.) The sample is refluxed in a bath of boiling water for 10 minutes and then cooled to room temperature; 100.00 ml. of 0.1N ceric ammonium nitrate are added, and the mist,ure is refluxed for an additional 10 minutes. After cooling, 20 ml. of 6iV sulfuric acid, 3 drops of osmium tetroxide (catalyst), and 1 drop of o-phenanthroline ferrous sulfate (ferroin) indicator are added. The excess quadrivalent cerium is then backtitrated with standard arsenite solution to the first trace of light pink. CALCULATIONS
The weight of methoxy group is calculated by means of the following formula:
( A X N - B X R ) X 0.007758
Mg. of CHaOwhere
A
total milliliters of quadrivalent cerium added in excess normality of quadrivalent cerium E = milliliters of arsenious oxide solution used for backtitration R = equivalence of quadrivalent cerium t,o arsenious oxide: ml. of C e f 4 ml. of As203
iY
= =
EXPERIMENTAL
A quantit,ative recovery of methanol, in the absence of other constituents, was established. Measured amounts of methanol were refluxed with standard ceric ammonium nitrate solution. Table I s h o w the recovery of methanol and verifies that methanol can be quantitatively analyzed by this procedure ( 3 ) .
Table I. Methanol Present, Gram 0.02382 0.02382 0.02382 0,03970 0.03970 0 03970
REAGENTS
Standard ceric nitrate. The approximate 0.lK solution is prepared by dissolving 540 grams of ceric ammonium nitrate in 5 liters of water containing 810 ml. of nitric acid, and then diluting to 9 liters. (The nitric acid is boiled until colorless to remove oxides of nitrogen.) A 9-liter carboy is used to store the solution. This solution is standardized against standard arsenite solution as follows. About 40 ml. of ceric ammonium nitrate solution are measured into a flask. Ten milliliters of 6 N sulfuric acid, 4 drops of 0.01hl osmium tetroxide, and 1 drop of 0.025M ferroin are added, and the reagent is titrated with standard arsenite until the first faint pink color. Standard 0 . l N arsenious oxide solution (1). Pure arsenious olide, 2.4725 grams, is weighed and dissolved in 20 ml. of I N sodium hydroxide. Sulfuric acid, liV, is added to the solution until reaction is slightly acid to litmus. The solution is transferred to a 500-ml. volumetric flask and made up to the mark. F o r larger volumes of arsenious oxide, the solution may be standardized against a normal iodine solution.
Recovery of hIethanol Methanol Recovered, Gram 0.02383 0 02383 0.02384 0.03979 0 03976 0 03976
Recovery,
%
Average Std. dev.
100.04 100.04 100.08 100.23 100.14 100.14 100.13 fO.07
Sodium methylate hydrolyzes into stoichiometric amounts of sodium hydroxide and methanol; thus, per cent recovery of methanol in the presence of sodium hydroxide is established. SaOCH3
+ H20 +NaOH + HOCHI
Table I1 shows the per cent methanol recovered under these conditions. 105
106
A N A L Y T I C A L CHEMISTRY Table 11. Recovery of Methanol in Presence of Sodium Hydroxide Methanol Present, Gram 0,03970 0.03970 0,03970
Ceric .4mmomum Nitrate, M1. 100.00 100.00 100.00
Methanol Recovered, Recovery, Gram % 0.03976 100.14 0.03993 100.58 0.03976 100.14 Average 100.29 Std. dev. * 0 . 2 2
Table 111. Comparison between Analyses of Sodium Methylate by Carbon Combustion and Direct Oxidation with Quadrivalent Cerium Sample NO.
I-a
11-a 11-b 11-0 111-a 111-b
Carbon,
Carbon as Methoxy,
Methoxy with Quadrivalent Cerium Oxidation,
21.39 21.59 21.16 21.17 21.77 20.52
55.25 55.79 54.78 54.71 56.25 53.02
55.42 54.84 55.16 55.25 55.13 55,50
%
%
%
The contaminating effect of suspended carbon was eliminated by filtering it off from the sample. Experimentation showed that colloidal carbon was oxidized completely by refluxing with nitric acid. After refluxing a mixture of colloidal carbon, methanol, and nitric acid, a measured amount of quadrivalent cerium was added. The per cent recovery of methanol on three determinations was 99.55 f 0.17. Table V shows the methoxy determination of sodium trimethoxyborohydride and compares the results with those that were obtained by boron analyses. Table V shows that filtering off carbon and refluxing with nitric acid previous to refluxing with quadrivalent cerium resulted in more accurate and precise work. The results and precision obtained when commercial material was analyzed for methoxy are shown in Table VI. The precision increased several fold when the samples were first refluxed with nitric acid. DISCUSSION O F \IETHODS A N D RESULTS
The basic chemistry of the method involved the oxidation of methanol to formic acid with excess standard ceric nitrate and the subsequent quantitative Average 2 1 . 2 7 i 0 27 54 97 3= 1 27 55.22 i 0 . 2 9 _._.__ b a c k - t i t r a t i o n of the latter with standard arsenite soluTable IV. Determination of Methoxy Group in Simulated Commercial By-product Material tion.
Sample NO.
4-1 4-1
4- 1
&-1 4-3
Sodium Sodium Methylate, Borohydride, Wt. % % 21.9s 52.79 66.19 21.95 2.4 92,36 9.03 74.94 5,36 89.93
Sodium Hydride,
Sodium Borate,
%
%
22.81 10.55 2.23 15.13 0.70
2.46 2.02 2.96 10.90 1.01
Methoxy, Gram TheoRecovRecovery. retical ered % 0.06395 0 06406 100.11 0.08523 0.08485 99,55 0.05403 0.05414 100.20 0.07078 0.07126 99,34 0.07215 0.07330 98.83 Average 99.61 Std. dev. z t O . 3 8
++
++
CHIOH 4Ce+++' HzO = 4Cec++ HCOOH 4H+
With the removal of intertering carbon from commercial material, the precision of rcsults was improved from about f0.8 to f O . l % . The per cent r e c o v e r y of m e t h o x y group which was obtained from simulated by-product mixtures is &0.45%. It is felt that methoxy group can be accurately determined by a fairly
T h e total carbon content of sodium methylate was assumed to be present as the methoxy group. A sample of material \sa8 obtained (Rlatheson, Coleman, and Bell), and further investigations were made, using this material. Analyses by carbon combustion in comparison with the present method were made on Table V. Rlethoxy Group in Sodium Trimethoxyborothree separately sampled portions. hydride The results in Table I11 show an agreement of better than Methoxy 0.4% of total methoxy present when the material was analyzed Group Average by two different methods (direct and indirect). Of the two (Theoreti- Methoxy % cal from Group Recovery Standmethods, better precision was obtained by the direct-ouidation Boron Recovof ard Sainple .4nalysis), ered, Methoxy Devimethod. No. % % Group ation Remarks Synthetic mixtures containing sodium methylate, sodium 5-1 6 8 . 8 6 6 9 . 8 3 1 0 0 . 4 8 + I 09 Carbon not filtered borohydride, sodium hydride, and sodium borate were made, off (three determinations) these compounds being the chief constituents in the commercial 5-2 69.24 69 29 100 07 1 0 34 Carbon filtered off: product. Table IV shows the recovery of methoxy when it refluxed with HNOa (two dewas present with varyterminations) ing amounts of other eompounds. Table VI. Methoxy Group in Cominerrial By-products Poor precision during early Ceric Sample Ammonium Methoxy hlethoxy analyses of methoxy in comWt., Nitrate, Group, Group, Sample mercial material was probably NO. Gram M1. Gram % .4verage Remarks due to the presence of "sus0.0319 6-1 0.0918 39.69 o,05813 34.04 i0.72 No reflux 0,1743 70.18 with HNOa pended" and/or colloidal car0.04864 36.49' No reflux 6-2 0.1333 54.18 bon (some organic decomposi37'20 with HSOa 0.05535 37.91) 0.1460 66.99 t i o n o c c u r r i n g during the 6-3 No reflux 0.2331 0'05599 23'98) 23.45 + 0 . 5 3 production of sodium borohy0.1252 36 66 .. 7840 0.02945 22.91, with HNOa dride). When colloidal and 0.03388 22.231 Reflux with 6-4 0.1524 32.00 0.04600 22.483 22'36 "08 amorphous carbon were re0.2046 56.11 fluxed w i t h q u a d r i v a l e n t 0.1669 58.04 0,04857 Reflux 6-5 05062 29,39 + 0.11 0.1699 61.49 cerium, there was only a partial 6-6 0,04383 0.1263 8 2 . 5 7 recovery of the ceric solution. 34.65 + 0 . 0 6 Reflux 0.1987 58.58 Thus, this consumption of 31.77 i0 . 0 4 Reflux 0.1284 49.98 0.1284 6-7 q u a d r i v a l e n t cerium gave 0.1265 49.30 0.1265 erroneously high results.
::i$j
*
*
i!: E:}
:;so)
:::;:}
V O L U M E 2 7 , N O . 1, J A N U A R Y 1 9 5 5
107
simple method in the presence of expected interfering compounds and carbon.
LITERATURE CITED
(1) Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative
Inorganic Analysis,” 3rd ed., New York, Macmillan Co., 1952.
“Scott’s Standard Methods of Chemical Analysis,” 5th ed., Vol. 11. D. 2527. New York. D. Van Nostrand Co.. 1925. (3) Skogg, D.-A., and Sister Monica, ANAL.CHE‘U.,25, 822 (1953). (4) s. Industrial Chemicals c o , New York, 8sLaboratory Procedure. Purity of Sodium LIethylate,” 1954.
(2)
ACKNOWLEDGMENT
The authors nish to acknowledge their indebtedness to 33 B. O’Connor for performing some of the analyses required for this work.
Purity of n-Bu tane, lsobutane, n-octane fro,m Freezing Points
RECEIVED for review M a y 7, 19.54. l c r e p t e d August 16, 1954.
Is0butene,
and
N E D C. KROUSKOP, GEOFFREY PILCHER, and ANTON 1. STREIFF Carnegie lnstitute o f Technology, Pittsburgh 73, Pa.
Measurements were made of the lowering of the freezing point of n-butane, isobutane, isobutene, and n-octane on the addition of known amounts of probable impurities. The data, covering the range 100 to 92 mole % of the major component, show that these systems follow the ideal solution laws in this respect.
z w 0.5(L
3
0
S E of the most important general methods for evaluating
the purity of chemical substances is to compare the freezing point of the actual sample with the value for zero impurity. The theoretical principles have been fully described (4, 5 ) . The method depends, however, upon the physical assumptions that the impurity fornis an ideal solution with the major component, and that it is solid-insoluble. It is of considerable importance to test these assumptions for a wide variety of systems to see for n h a t mixtures thr, idpal solution law is obeyed, and to extend the r:tnge of substances for which this criterion of purity can be used with certainty. This has been done for n-butane, isobutane (2-niethylpropane), isobutene (2-methylpropene), and n-octane by measuring the i‘i,eezingpoints of these suhstances with known amounts of known impurities added. The impurities were chosen to be of type similar to those rspected to be prescnt in the highly purified samples. The mixtures of the volatile substances, n-butane, isobutane, and isobutene were made by the method previously described
t-
x
5 LL
O a‘ Li
1.0-
Ln
m W
1
z
1.5-
2 3
9 C 2.0I)
a LL
0
100
98
96
94
92
MOLE PERCENTAGE OF n-BUTANE Table I. -~
Lowering of the Freezing Point of n-Butane, Isobutane, Isobutene, and n-Octane System
Uajor coinponent n-Butane
Isobutane
Isobutane
n-Butane
Isohiitene
Isohutane
.____
SOlLltL.
Vole % ’ of Solute 3.717 7.195 3.605 6.692
Figure 1. Lowering of Freezing Point of n-Butane on Addition of Known Amounts of Isobutane
Lowering Freezingof Point,a ’ C. 1 . 2 3 6 h 0.003 2.412 + 0.004 0.901f0.005 1.666 & 0 . 0 1 6
1-But.ene
(8, 6). For n-octane, the mixtures were made up by weight using a stoppered bottle. The apparatus and experimental procedure for measuring the freezing points have been described (4, 6). All Ca hydrocarbons used in this investigation were research grade hydrocarbons from the Phillips Petroleum Co. All other compounds were highly purified samples from the -American Petroleum Institute Reseaich Pioject 6.
cis-%Butene trans-%Butene ti-Octane
2,2,4-Trimethylpentane 2,4-Dimethylhexane 1-Methyl-3-ethylcyclopentane (cis trans) I-cis-2-Dimethylcyclohexane 1-trans-2-Dimethylcyclohexane E thylbensene 1,4-Dirnethylbensene (p-xylene) Average of 2 experiments.
+
7 461 7 746
1 486 =t0 004
8.531 8.506 8.642 8.100 5.283
1.678 i0.001 1.687+0.005 1 . 7 1 4 f 0.001 1 . 5 4 O i 0.005 1.014 i 0 . 0 0 1
1 572
zt
0 003
The data obtained for the four substances are collected in Table I and are illustrated in Figures 1, 2, 3, and 4, where the experimental points are shown, together with the ideal line for the lowering of the freezing point. For the mixture of n-octane and p-xylene, only 5 mole % of p-xylene was used, in order to keep below the eutectic composition near 7 mole yo of p-xylene. The ideal lines were calculated ( 4 , 5 ) from the cryoscopic constants for each compound, as taken from the tables of the API Research Project 44 ( I ) , as follows, tT being the freezing point for zero
