675
S E P T E M B E R 1947 carbon numbers. ii simple bromination of the three fractions thus gave the olefin and saturate distribution. Synthetic blends of the purest gases available were made in a special apparatus on a pressure basis. Partial pressures were corrected for gas law deviations. The purity of each gas used was determined by isothermal distillation at one or more of the following temperatures: O", -22.8", - 7 9 O , -114O, and -139" C. The purities were estimated to be: Component Ethylene Ethane Propylene Propane Isobutane n-Butane
Purity, Mole % 99 98 (2% propylene) 98.5 ( 1 . 5 % ethylene) 00 495 ( 5 % n-butane, trace of propane) 9 9 . 7 (0.3% isobutane)
+
I n order to correct for gas law deviation a t 760 mm. and 25' C. the volumes of components obtained in Fractions I to IV were multiplied by 1.003, 1.01, 1.02, and 1.03, respectively. These factors were derived from the well-known compressibility factors. The blends were first employed to test the accuracy of bromination as a method of olefin determination (Table IV). In order to minimize substitution, saturated bromine-water was not used, and the bromination was carried out with the pipet shielded with lightproof paper.
.
Table V shows a comparison between theory and experiment of distribution of propylene in F-I1 and F-IV and of ethane and butane in F-111. The analyses of the synthetic blends are compared with blend values in Table VI. ACKNOWLEDGMENT
Acknowledgment is made to K. E. Train of these laboratories for evaluating experimentally the limits of propane tolerance in isothermal distillation and contributing constructive criticism. A portion of the development of these methods was carried out while L. S. Echoh was engaged in graduate research a t Princeton University under the supervision of Robert N. Pease, to whom acknowledgment is made for his interest and sponsorship. LITERATURE CITED
Burrell, Seibert, and Robertson, U. S. Bur. Mines, Tech. P a p e r 104 (1915).
Lebean and Damiens, Compt. Tend., 156, 325 (191.3). Pease and Durgin, J . Am. Chem. Soc., 52, 1262 r1930). Rayleigh, Phil. Mag., 4 (sixth series), 521-37 (1902i. Shepherd, Bur. Standards J . Research, 2, 1145 i l H Z 9 ) . Ward, IND. ENG.CHEM.,ANAL.ED.,10,169 (10381. Zook, Oakwood, and Whitmore, Division of Perroleuin Chemistry, 108th Meeting, AMERICAN CHEMICAL SOCIETY, September 1944.
Titration of Acids by Electrolytically Generated Hydroxyl Ion JOSEPH EPSTEIN, H. A . SOBER', AND S. D. SILVER Gassing and Analytical Section, Medical Division, Edgercqod Arsenal, M d .
Microquantities of acid in aqueous solution are titrated by hydroxyl ion generated by electrolysis of sodium bromide solution. The end point is obtained potentiometrically by a Pinkhof system. A L-shaped electrolysis cell is employed, having a platinum electrolyzing electrode in each arm and filled with 0.15 M sodium bromide saturated with hydroquinone. The acid is ccllected in the cathode chamber which also contains a platinum titrating electrode and a quinhydrone reference electrode buffered at pH 4.3. A constant electrolyzir g cuirent
A
MOSG the gases or vapors usually found as air contami-
nants, a number are strongly acid and many others can be pyrolyzed to form strong acids, as is the case with halogenated hydrocarbons. A simple method of microanalysis, adaptable to recording, was developed in t,his laboratory in vihich acids in aqueous medium were titrated by hydroxyl ions generated electrolytically. When a salt solution is electrolyzed, the quantity of electrolysis product formed at, each elect,rode is governed by Faraday's la^. ;\lethods of ana1ys:s based on *his principle have been described. Szebelledy and Somogyi (4)reported a method of "coulometric analysis" in vivhich a constant current passed through a cell by means of platinum electrodes produced electrolysis products which reacted with the material t o be analyzed. Khen the end point was reached, as shown by the color change in an indicator, the cell was disconnected and the quantit,y of electricity was $etermined from the deposition of silver in a silver R-eight coulonieter which had been connected in series with the electrolysis cell. This method is both accurate and precise but has the disadvantages of being slow and not suitable for small quantities. 1
Present address, M t , Sinai Hospital, S e w York, N. Y.
is used, making the quantity of acid titrated proportional to the time of electrolysis necessary for neutralization. The end point at pH 4.3 prevents titration of carbon dioxide. When standardized empirically, the system gives results reproducible within * l O % for P = 0.05 with quantities of hydrochloric acid between 30 and 150 micrograms, using the standard deviation as a measure of precision. The method is adaptable to automatic and continuous analysis of gases in air which are acid or pyrolyzable to acids. Cell diagram and circuits are given.
'
Furthermore, indicators had to be selected rvhich T V ~ T I =not dcstroyed by electrolysis. I n another method of coulometric analysis, dwrrihcd h y Lingane (Z),the potential was held const'ant, the current varying, and a gas (hydrogen-oxygen) coulometer was employed. -4lthough it is extremely useful for mixtures of reducible sul,stxnces, it cannot , be used for the analysis of gases in air since oxygen must be excluded from the cell. The system described herein is similar to that reported by Szebelledy and Somogyi, in that a constant current is employed. The end point is determined potentiomet,rically b y the use of a Pinkhof syst'em ( I ) , thus eliminating the need of indicators. The quantity of electricity is determined by measuring the electrolysis time by means of a stopwatch or a drum rotating a t constant speed and the current by a milliammeter. Although the method described herein does not give so accurate result3 ns those obtained by Szebelledy and Somogyi, yet it makes it possible to estimate the quant,ity of acidity in t h e titration chamber rapidly and continuously. A U-shaped cell is used, so that the products of electrolysis a t either electrode may be used. I n this work a solution of sodium bromide ~ s electrolyzed, s
676
V O L U M E 19, NO. 9
hydrogen and hydroxyl ions being generated at the cathode and bromine at the anode. This is particularly well suited to the type k at' this laboratory, since the gases to he anaof ~ o r conducted lyzed fall into two broad categories, first, those which can be osidized by bromine, and scrond, those n-hich are acid or can be converted to acids by pyrol is confined to the EXPER131ENT.4L
Electrolysis Cell. The cell is shown in Figure 1. I t had an anode and cathode chamber separated by a glass tube containing an asbestos-covered perforated glass plate. Each chamber contained a platinum electrolyzing electrode, a gas inlet, and an outlet tube, TThich was connected to a vacuum line. These electrodes were made of platinum gauze about 2.5 X 2.5 cm. ( 1 X 1 inch), rolled up and welded to platinum a-ires leading from the cell. -1ir was drawn through the anode bubbler to remove the bromine formed by electrol The acid gas was to be drawn through the cathode bubble absorbed by the medium in the cathode chamber. The passage of air through this chamber also served to remove the hydrogen formed during the electrolysis and to agitate the contents. Titration Electrodes. The electrodes for potentiometric titration were inserted into the cathode chamber as shown in Figure 1. The titrating electrode was made of platinum gauze similar to the electrolyzing electrodes. The reference electrode was a glass tube, 10 mm. in diameter, drawn to a tip and filled with citrate buffer (LIcIlvaine) a t p H 4.3 saturated with quinhydrone, into n-hich was inserted a platinum wire. The tio of the reference cell *was filled with a 1 0 7 agar Dolution containing5$, G C C, sodium bromide to prevent leakage of the cell contents. Circuits. The circuits which were used are shown in Figure 2. Thc electrolyzing circuit consisted of a battery, a m i l l i a m meter. a 10,000-ohm variable resistance, and an off-on switch all connected in series with the platinumelectrodes of the electrolvtic cell. In the circuit for the potentiometric t i t r a E D tion a gzlvanometer (sensitivity 0.025 X microampere per mm., resistance 1000 ohmj) \vas connected through a 15,000-ohm variable series resistance to the titration e l e c t r o d e s . A2000ohm wriable resista n c e vas s h u n t e d Figure 1. T i t r a t i o n Cell across the galvaycmA.41. Air outlets eter. BBi. Air inlets D e t e r m i n a t i o n of C G . Electrolyzing electrodes D . Anode chamber Precision. The reproE . Cathode chamber ducibility of the stopF. Reference cell natch-m i 11i a m m e t e r G. Titrating electrode c o m b i n a t i o n as a coulometer \yas first measured. I n these experiments the titrating electrodes xere removed. The cell was filled with 0.15 M sodium bromide solution. .4ir was drawn through both chambers at a rate of about 1 liter per minute. A 6-volt battery was used as the source of current. The current was turned on and simultaneously a stopwatch was started. The reading on the milliammeter was noted and the current maintained at that level by adjusting the esternsl resistance. After varying intervals, the current n-as shut oP and the liquid in the cathode chamber was washed into a 150ml. Erlenmeyer flask. One drop of methvl orange was added and the solution was titrated with 0.1 S hydrochloric acid. The results are shown in Table I.
I
Thus this technique shows a precision of better than *47,
(2 S.D.1 for the production of quantities of sodium hydroxide equivalent to a range of about 4 to 18 mg.
Titration of Hydrochloric Acid by Electrolytic Hydroxyl. It was assumed that a n acid introduced into the cathode chamber could be titrated by electrolytically generated hydroxyl ion, even though its decomposition potential were lower than that of the salt t o be electrolyzed. To test this assumption small amounts of hydrochloric acid were pipetted into the cathode chamber of the cell which already contained sodium bromide solution. The titrating electrodes were not used. The current and stopwatch were started simultaneously. After an excess of hydroxyl ion was produced, the current and stopwatch were 1 ELECTROLYSING stopped and the cathode chamber contents washed into a flask. The excess of the hydroxyl ion was titrated with standard h y d r o c h l o r i c a c i d solution, using methyl orange indicator. In a series of trials, the current was varied from 2 to 10 milliamperes, the voltage from 3 to 6 volts, and the concentration of the sodium bromide solution from 0.15 t o 0.60 M . CIRCUIT The electrolysis time in each case was 30 minutes. The quantity of acid initially introduced into the cathode chamber was added t o the amount necessary to neutralize R3 the excess base formed. The Figure 2. Electrical Circuit total acid used, calculated as milliequivalents, was compared B . Battery, 3 t o 6 volts with the theoretical RI. Variable resistor, 10,000 ohms valents of base proEk:$q?i R ? . Variable resistor. 15.000 electrolysis. ohms
BhE, 'i I
k
*
R3. Variable resistor, 2,000 ohms E , . Electrolyzing electrode (anode) Ez. Electrolyzing electrode (cathode) Ea. Titrating electrode E l . Reference cell .VI. Milliammeter, 0 t o 20 ma. M z . Galvanometer, 0.025 X 1 0 - 0 microampere per m m . S . Single-pole, single-throw switch
The results shown in Table I1 indicate that hydrochloric acid can be titrated by the electrolysis products of sodium bromide. Variation of current, voltage, or molarity of the sodium bromide did not aDureciablv affect the results. Potentiometric Titration of Hydrochloric Acid by Electrolytic Hydroxyl. I n this series of experiments the end point of the titration of known quantities of hydrochloric acid was determined potentiometrically by means of a Pinkhof system. The cell medium was 0 . 3 iV sodium bromide saturated with quinhydrone. I n the cathode chamber mere inserted the platinum titrating electrode and the reference electrode which were connected to the galvanometer. I n this system the end point (galvanometer reading zero) was reached when the pH of the cathode chamber contents reached the pH of the reference cell-namely, 4.3. Although this did not represent the true value for strong acids, especially in dilute solution, it had the advantage of not, being affected by the presence of carbon dioxide, which mill not be indicated by this system. In the analysis of pyrolyzed gases, therefore, t'he presence of rttt ,ndant carbon dioxide should not be a complicating factor. By the same token, however, the determined values should be some-
Table I. Precision Stopwatch-Milliammeter C o u l o m e t e r C irrent Jin
.
5 00 6 60 7 40 7 75
8 00 8 00 8 25
Time
Coulombs
See 1800 1800 1800 3600 1800 5520 1600
9.00 11.86 13.32 27.90 14.40 44.16 14.65
Expected
hlilliequivalents Found Difference
% 0 0 0 0 0 0 0
093 123 138 289 150 457 154
0 093 0 119 0.135 0 277 0 150 0 441 0 154
Mean Average deviation from mean Standard deviation
0.0 -3 2
*
-2.2 -4.2 0.0 -3.5 0.0 -1.9% 1.6% 1.9%
677
S E P T E M B E R 1947 Table 11. Titration of Hydrochloric Acid by Electrolytic Hydroxyl Concn. of NaBr M
E.M.F.
Current
Volts
Ma.
0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.30 0.30 0.30 0.30 0.30 0.60 0.60 0.60 0.60 0.60
6 6 6 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
7.5 7.8 8.1 3.9 4.0 4.3 4.8 6 9 6.4 6.6 6.9 7.0 7.0 2.0 4.0 6.0 8.0 10 0
Theoretical Base Produced
Total Acid Used
Meq. 0.139 0,145 0,151 0,0738 0,0746 0,0803 0,0895 0,124 0.119 0.123 0,128 0.130
0.130 0,0373 0.0746 0,112 0.149 0.193
'
Meq. 0.133 0.148 0.153 0.0713 0.0738 0.0786 0.0910 0.123 0.120 0.122 0.126 0 126 0.130 0 0353 0 0779 0.113 0.149 0.197
AIean Average deviation from mean Standard deriation
Difference
% -4.3 +2.l f1.3 -3.4 -1.1 -2.1 +1.7 -0.8 +O 8 -0 8 -1.6 -3.1 0 0 -5 4 +4 4 +0.9 0.0 T
X
-0.6% 2.0% 2.5%
Table 111.. Potentiometric Titration of Hydrochloric Acid by Electrolytic Hydroxyl "21 Added Micrograms 29.9 59.8 149.5
No. of Determinations 29 19 11
Average Recovery
DeTViation from Mean
%
%
91.7 86.3 90.5
3 5 2 5 3 1
StandRrd Deviation q 4 3 2 9 3 8
what lower than the theoretical. This is shown in Table 111, which gives the results of a series of titrations of known amounts of hydrochloric acid by this method. DISCUSSION
Accuracy. As may be expected, the potentiometric titration is not stoichiometric. This is of little consequence for the projected
use of this system, since calibration is easily accomplished empirically.~ bv maintaining a constant current and meawiing the time necessary to titrate known quantities of added acid. I n actual practice the electrolysis time has been recorded on a drum rotating a t constant speed. The quantity of acidity was then directly proportional to the length of the recorded line. Precision. Using the standard deviation as a measure of precision, the chances for a determination to be within * l o 5 of the mean calibration are better than 96 out of 100. For the purposes of this laboratory this precision is satisfactory. The precision is shown to be practically unaffected by the quantitl- of acid determined for the range of experimental amounts, ea. 30 to 150 micrograms. Sources of error are the measurement of current and time and the Pinkhof titrationitself. Since thissystem depends on constancy of current, precautions must be taken to prerent polarization. Usually, adequate stirring by the air stream is sufficient. The milliammeter and the stopwatch should be calibrated. The Pinkhof system is known to be not too precise but its advantages in speed and simplicity are in its favor when a *lo% precision is allowable. Applications. This method can have considerable application in the sampling of air contaminated by nosious gases, since it can be made continuous, and the cell is portable, comparatively nonbreakable, and above all remotely controlled. Because both the generation of ions and the observation of the end point are accomplished electrically, it can be readily adapted to automatic control by simple electronic devices. Examples of such instrumentation have becn developed by S o d h r o p (3:and have been i n use in this laborctory for several years. LITERATURE CITED
(1) Kolthoff. I. 11..and Furman. N. H., "Potentiometric Titrations." pp. 115-52. New York. John Wiley & Sons, 19%. (2) Lingane, J. J., J . Am. Chsm. SOC.,67,1916-22 (1945). ( 3 ) Northroo. J. H.. uersoiial communication. (4) SzebellcdL L., and Somogji. Z , 2 . anal. Chem 112, 313, 323, 332, 385 391, 395, 4'10 (1938).
Determination and Nature of Leaf Sterols RfOYROE E. W-ALL A ~ EDWiRD D G. KELLET Eastern Regional Research Laboratory, L . S . Department of Agriculture. Philadelphia 18, Pa. Methods for the determination of unsaturated and saturated sterols in leaf meals are described. Alternati\e procedures using micro- or macrosamples and colorimetric or graJimetric techniques are giten. The close relationship hetw een the time us. density curtes, absorption curies, and E of the color reaction product is demonstrated.
L
EAF sterols may have potential value a; sources for the
preparation of vitamin D-active compounds and sex hormones. I n vie!v of that fact it seemed desirable to determine the sterol content of a number of leaf meals and extracts. No method has been published for the quantitative determination of leaf sterols, although many procedures are available for the determination of cholesterol in blood and animal tissue snd sterols in vegetable oils. Sterols in such materials can be determined by macro gravimetric methods or micro colorimetric procedures. The former are all based on the work of Windaus ( l 7 ) , who shoved that on the addition of digitonin, cholesterol or phytosterols in alcohol solution form a characteristic insoluble digitonide. The sterol digitonide is then filtered and 'Lveighed. All the plant sterols isolated to date can be precipitated by digibDin.
The macroprocedures devised for cholesterol are accurate but have the disadvantages of being time-consuming and requiring
large samples and considerable quantities of expensive digitonin. Microprocedures devised for cholesterol eliminate these difficulties. Of these, the Liebermann-Burchard reaction, i n iyhich the sterol is t,reated with acetic anhydride and sulfuric acid, has been the most extensively studied. Bloor (2) adapted the reaction to determination of cholesterol in blood and many workers have introduced modifications. Schoenheimer and Sperry (1.2)precipitated cholesterol as the digitonide and, after dissolving the compound in acetic acid, carried out the colorimetric reaction in the presence of digitonin. Kelsey ( 7 ) also precipitated cholesterol as the digitonide, which he then decomposed with boiling benzene and, after removing the insoluble digit,onin, carried out the color reaction on the free sterol. Sperry (13) has shown that determination of cl!olesterol by the Schoenhcimer-Sperry microprocedure has an accuracy comparable with that of macromethods. By utilizing a number of the foregoing procedures and intro-
