Titrimetric Determination of Unsaturation by Catalytic Hydrogenation WILLIAM SEAMAN Research Division, American Cyanamid Co., Bound Brook, N. 1.
b Gasometric determination of unsaturation b y catalytic hydrogenation lacks convenience. In the titrimetric method presented, hydrogen generated from standard lithium aluminum hydride solution i s used with syringe and hypodermic needle techniques. The excess hydrogen reacts with oxygen to form water, which is titrated with Karl Fischer reagent. The method offers simplicity of equipment and ease of manipulaiion and replication. The coefficient of variation ranged from 3 t o 13%.
T
use of hydrogenation in the presence of a platinum or palladium catalyst to determine unsaturation suffers from the inconveniences associated .rr ith volumetric or manometric gas measurements. This paper reports a nen approach. Instead of measuring out a volume of hydrogen gas, a measured volume of standard lithium aluminum hydride qolution is delivered into a methanolic solution of the sample to generate hydrogen. The platinum oxide catalyst is in suspension. After the sample has been hydrogenated by shaking for a short time, the excess hydrogen is reacted n i t h oxygen to form nater. The latter is determined by titration nith Karl Fischer reagent. Hypodermic needles and syringes are used for delivery. Titration is carried out through rubber serum-bottle stoppers which cover the mouths of sniall bottles containing sample and reagents ( 3 ) . The simplicity of apparatus and technique make it easy to run any desired number of replicate determinations. HE
EXPERIMENTAL
Reagents. Lithium aluminum hydride solution. T o a n 8-ounce nairon--mouthed bottle, containing about 100 ml. of dried di-n-butyl ether, over nhich a stream of d r y nitrogen is passing. add 5 t o 6 grams of lithium aluminum hydride. Shake cautiously a t the start until there is no further heat of solution. cap the bottle with a serum-bottle stopper. and allonthe mixture t o stand for several hour> with occasional shaking. Filter by vacuum through a sintered-glass filter in a stream of dry nitrogen into an
1840
ANALYTICAL CHEMISTRY
%ounce, narrow-mouthed bottle. This bottle is finally filled with nitrogen and capped with a rubber serum-bottle stopper. The solution should remain clear indefinitely. Too short a m-aiting period before filtering would cause the later separation of eolid matter and trouble in subseauent measurements and delivery. Karl Fischer reagent. 1 ml. of which is equivalent to aubout 3 to i mg. of n-ater, is placed in a narror-mouthed bottle which is sealed ivith a rubber serum-bottle stopper. Hydrogenation catalyst is catalytic grade platinum oxide. Determination of Unsaturation. All equipment must be oven-dried and stored in a desiccator before use. To 5 ml. of d i y methanol in a 60-ml. narrow-mouthed serum bottle (Fishei Scientific Co., S o . 3-220), a weight of sample is added t h a t T d l cause about 10 ml. of hydrogen t o remain unreacted after hydrogenation has been carritd out b y the hydrogen liberated from a standard 0.25-ml. volume of lithium aluminum hydride solution. T h t n 10 nig. of platinum oxide catalyst and a glass tube (about 6 em. long and 4 mm. in inner diameter), JThich has a thin closed bulb at the bottom but is open in the top, arc added. A rubber serum-bottle stopper (Fisher Scientific Co.. Xo. 14-126) is placed over the mouth of the bottle. Through a 25-gage hypodermic needle and a stopcock (Arthur H. Thomas Co., No. 9421), the bottle is evacuated. filled with nitrogen, and re-evacuated. A 25-gage hypodermic needle and a 1-nil. graduated syringe (1Iicrochemical Specialties Co.. KO. 288), provided with an adjustable stop for precise delivery of a desired volume (I), are flushed with nitrogen and used to withdraw 0.25 ml. of lithium aluminum hydride solution from the reagent bottle, The hydride solution is introduced through the serum-bottle stopper into the gas bulb in the reaction vessel. If the hydride is introduced directly into the methanol, erratic losses of hydrogen occur nhen the needle is IT-ithdrann. The bottle is first shaken vigorously to break the bulb and is then shaken under a n a r m water t a p to complete the hydrogenation: About 10 minutes should be sufficient. The brown color of the platinum oxide should change rapidly to the dark black of platinum metal povider. If this change fails to occur, as may happen for unknown reasons, the determination should be re-
jected. A4fter completion of the hydrogenation, the bottle is cooled to room temperature, and hydrogen is introduced in tlyo or three portions by a 10-ml. syringe and a 20-gage hypodermic needle. The bottle is shaken under warm tap water after each addition. Sufficient oxygen is added to create a definite back pressure after the last portion has been added, and the bottle is cooled. Vsually 30 ml. of oxygen will suffice. The contents, of the reaction bottle are cooled to room temperature and titrated with Karl Fischer reagent, delivered with a IO-ml. graduated syringe through a 25-gage needle. The syringe should have been filled with the proper volume of air, previously dried by having been pulled through a column of a drying agent. With a little practice, it is possible to titrate the mixture in a good light to a satisfactory visual end point in spite of the suspended platinum. Titration to an electrometric a d point, with n-ire electrodes inserted through the serumbottle stopper, was not convenient ( 2 )* B l a n k Values. A blank value for 5 nil. of methanol plus 10 nig. of platinum oxide amounted to about 0.6 t o 0.7 ml. of the usual Fischer reagent. T h e contribution of oxygen and nitrogen t o the blank was negligible. With the indirect method of standardizing the lithium aluminum hydride solution, blanks need not be determined. They cancel out if they do not change significantly between the time of standardization and determination. Standardization. Karl Fischer reagent is standardized b y titrating known weights of water in methanol in t h e described apparatus. Standardization of the lithium aluminum hydride solution was a t tempted by carrying out a determination as described for a n unsaturated sample, but n-ithout using such a sample. The pressure of t h e relatively large volume of free hydrogen caused erratic losses upon the insertion and withdrawal of hypodermic needles, so that it \\as impossible to get a precise value. An indirect method of standardization v, as adopted accordingly. A substance capable of rapid and complete hydrogenation, such as cinnamic acid or sorbic acid, and of known purity was hydrogenated in the usual manner with lithium aluminum hydride. From this determination, a standardization value was
calculated in terms of milligrams of n a t e r equivalent t o unit volume of hydride solution. I n a typical standardization, 1 meq. of cinnamic acid (148.2 mg.) n-as hydrogenated with 0.26 ml. of a hydride solution. A typical run required 6.2 ml. of Karl Fischer reagent which had a standardization value of 1.87 mg. of water per ml. For calculations, let m = mg. of sample, a = mg. of water equivalent to the volume of hydride used, b = nig. of n a t e r equivalent to 1 ml. of Fischer reagent, and c ml. of Fischer reagent required for titration of n ater formed from the r e d u a l hydrogen. Then 18.016 m ' ( a - be) = equivalent \T right per double bond. For the purpose of stnndardizing the lithium aluminum hydride solution, the equivalent n eight per double bond iq knonn, and the equation is solved for a. For a sample containing only one unsaturated constituent, if m is chosen to equal 1 mg.-equivalmt per double bond for that constituent. then 100 (a - bc)/18.016 = '7, unsaturated condituent in the sample.
Table 1.
Substance Cinnamic acid
Sorbic acid
Date 6/8/56 7/19/56
5/18/56 5/22/56
Standardization Values Mg. H?O Equiv. to 0.25 111. LiAlH4 Soln. 2 7 . 2 , 2 7 . 7 , 25 8 , 2 7 . 7 , h v . 2 7 . 1 2 8 . 9 , 2 5 . 9 , 2 6 . 6 , 2 7 . 7 , 2 8 . 5 , 2 7 . 0 , 2 6 . 6 , .iv. 27.3 Grand av. 2 7 . 2 Std. dev. 1 1 . 0 or ~ k 3 . 7 5 ; Std. error of mean =kO..?O or i l . 15; 2 6 . 8 , 27.4, 2 7 . 0 , 2 7 . 9 , Av. 2 7 . 5 2 7 . 8 , 2 8 . 3 , 2 0 . 3 , Av. 2 8 . 5 Grand av. 27. $1 Std. dev. 1 0 .77 or f 2 . 8 5 . i Std. error of mean 1 0 . 2 9 or ~ k 1 . 0 ~ ~
Table 11.
Hydrogenation Values
Equivalent TVeight per Double Bond Substance Theoretical Found Aconitic acid 1 7 4 , l l 171.2) 1 7 7 . 7 , 1 7 7 . 7 , 1 8 4 . 8 , Sorbic acid
56.06
Oleic acid
282.45
Oleic acid
282.45
Fumaric acid
116.07
h v . 177.9 50 0 , 54.3, 57.2, 5 5 . 1 , 5 1 . 2 , Av. 53.6 311.8, 371.6, 340.3, 272.9, h v . 324.2 264.0, 255,1, 256.3 290.3, Av. 266.4 109.9, 140.2, 128.3, 1 2 3 . 2 , Av. 125,4 ~
Std Error of Std Dev Mean hbso- Rela- Abso- Relalute tive, yo lute tive, % 5 6
3 1
2 8
1 6
2 9
5 4
1 3
2 4
409
126
205
6 3
164
6 2
8 2
3 1
126
100
6 3
5 0
RESULTS AND DISCUSSION
Standardization of Hydride. Table
I gives standardization values obtained with cinnamic acid on t n o separate days for one lithiuni alumin u m hydride solution and with sorbic acid for another solution, also on two separate days. T h e cinnamic and sorbic acids had assay values of 99.4 and 99.7%, respectively, by acidimetric titration. T h e agreement of the values with a given standard on separate days indicates the stability of the hydride solution. A single standard deviation was calculated for both groups. Determination of Unsaturation by Hydrogenation. Table I1 gives hydrogenation values for several unsaturated acids, which were of technical or reagent grade and not specially purified. T h e relative standa r d deviation for aconitic acid compares favorably n i t h those for cinnamic a n d sorbic acids given in Table I. T h e values for t h e other samples were less precise. For such samples, the precision may be improved b y making replicate determinations and averaging the results. For determinations which are carried out to obtain structural information, the precisions given should be adequate. For samples n i t h a low unsaturation value, the terni (a - bc) in the expression for calculating pcwentage unsaturation ivould be small, and the calculated percentage would tend to have a higher relative error than for highly unsaturated samples. Increased precision might be achieved b y additional replication and by improving the method.
Except for the cinnamic and sorbic acid samples, no data are available for purity, but the average values for the samples reported in Table I1 differ from the theoretical equivalent weights by no more than about twice the standard error of the respective means. I n other n ords, the 957, confidence interval includes the theoretical value. For sorbic acid, of known 99.7% purity, there is no systematic error, or bias, greater than the limits fixed by twice the standard error of the mean, when unsaturation is determined by using the standardization based on cinnamic acid. The same would apply to cinnamic acid, if sorbic acid were used for standardization. Before the new niethod \vas developed tu-0 facts were established: Elemental hydrogen is completely oxidized by air in the presence of a catalyst under reduced pressure such as exists in the new method. This was determined by oxidizing a known volume of hydrogen in a suitable apparatus under those conditions and titrating the water formed. The influence of errors in determining the excess hydrogen upon the analysis is minimized : The amount in excess is subtracted from the larger value for total hydrogen liberated, and the method may be adjusted to make the amount in excess as 1017 as is desired. The second fact was that an unsaturated acid, oleic acid, consumed under the same conditions as much as 96% of the theoretical volume of hydrogen needed to saturate the double bond. I n working out the actual method, the difficulty of getting a
precise direct standardization of lithium aluminum hydride made it impossible to determine the accuracy of the method evcept through intervention of a secondary standard. Interferences and Suggested Improvements. More woik should be clone on t h e present niethod including t h e study of more types of compounds. There should be even lcss difficulty t h a n in the classical hydrogenation method in determining unsaturation in compounds nhich have other reducible groups. Such groups as carboxyl, ester, acid anhydride, and others are reducible by lithium aluminum hydride under suitable conditions, generally different from those of the method described. However, the reaction of the hydride with methanol to form free hydrogen is so niuch more rapid under the conditions of this method that the interference of other reducible groups probably would be negligible. If such a n interference should be significant, two bulbs of differing thickness could be used instead of the one bulb noiv used for the hydride. The sample might be placed in the less fragile bulb, nhich would not be broken until the bulb containing the hydride had been broken. Thus, all of the hydride Jvould have reacted t o form hydrogen before the sample n ould be released. Some compounds, such as cinnamic acid, may also undergo direct reduction b y the hydride a t the double bond. Vnlikely as this may be under the conditions of the method, its occurrence would cause no error, because the stoichiometry would be VOL. 30, NO. 1 1 , NOVEMBER 1958
1841
idcntical with that based upon the liberation of hydrogen and reduction by the released hydrogen. The precision of the method niight be improved by using a more dilute hydride solution and a smaller excess, as well as more refined methods of measurcnient, drlivery, and titration, including more precise hyringes or niicroburets. Titration by weight instead of by volume might be desirable but would complicate the technique. However, for determining the number
of double bonds in a compound, the reported precision is satisfactory, particularly because of the ease with which replicate values may be obtained. ACKNOWLEDGMENT
The author nishes to thank H. C. Craig for carrying out the experimental work and H. C. Craig and 2. F. Smith for carrying out some of the preliminary experiments on which the method is based.
LITERATURE CITED
(1) Chaney, A. L., IND.EKG. CHEK, AXAL.ED. 10, 326 (1938). ( 2 ) I.evy, G. B., Murtaugh, J. J., Rosenblatt, hl., Zbzd., 17, 193 (1945). (3) Smith, I). M.,Mitchell, J., Jr., Billmeier, A. XI., ANAL.CHEY. 24, 1847 (1952). RECEIVED for review February 26, 1958. Accepted June 11, 1958. Division of Analytical Chemistry, 133rd Meeting, ACS; San Francisco, Calif., April 1958. Meeting-in-Xiniature, Analytical Group, S o r t h Jersey Section, ACS, South Orange, T.J., January 27, 1958.
Determination of Halogen in Organic Compounds by Automatic CouIo metric Titration 0. E. SUNDBERG, H. C. CRAIG, and J. S. PARSONS Research Division, American Cyanamid Co., Bound Brook,
b An automatic titrator, utilizing the coulometric principle, i s used for the determination of halides, following sodium peroxide Parr bomb fusion. The method involves precipitation o f the halide b y generation of silver ions from a silver anode. The electricity required i s measured b y an integrating motor incorporated in the instrument, and the titration is automatically terminated at the end point b y the Beckman titrator control unit. The aulomatic coulometric titration technique affords several advantages over the generally used Volhard titration method for halogen analysis; it i s direct, obiedive, and timesaving. It minimizes the human error and yields greater precision.
N.J.
nitrate reagcnt and facilitates automatic operation of the rquipment. The Lingane nicthod m s applied to the automatic titration of halides in solut'ions obtainrd from Parr bomb fusions. The method n-as d s o adapted with some success to thc titration of mixtures of bromidr and chloride. During thr coursc of this work, an automatic coulometric titrator was built. T h r instrumentation was simplified over previous models by incorpomting a low-inertia integrating motor into the circuitry. This motor eliminates the need for a precise constant current supply and a precisr timing device. Details of t,hc instrumentation havr been described (,4). EXPERIMENTAL
D
halogen in organic compounds is iniportant in a number of laboiatoriee. Dwomposition of the organic sample by a sodium peroxide Parr bomb fusion, follon-ed by :t T'olhard titration, has been the most satisfactory technique for routine analys w However, colored iiiipuritirs mag ohscwe the end point. or a fading end point niay result if tht, precipitated si1vc.r halide is not filtered off. A method involving c1irrc.t electrometric tit,ration n.ith silver ion is advantageous. c,liminating thc need for filtrat'ion of the silver halide. The automatizing of the titration also saves tinir. Recently. Lingane ( I ) shon-rd that autoniatic coulometric titration of small amounts of halide with electrolytically generated silver ion was feasible. Coulometric generation of silver ion eliminates the need for standard silver ETERVISlTIOS Of
1842
ANALYTICAL CHEMISTRY
Apparatus. il photograph of the simplified automatic coulometric titrator used is shown (Figure 1). From left to right. ai(' the electrolysis eel1 assembly, the integrating motorcounter unit. t h e Beckman Automatic Titrator Rlodel K. and the ~ o \ v e r supply unit. -4 block diagram of tlie autoniatic coulometric titrator is presented in Figure 2 . A simple unst,abilized current of 250 ma. is provided from a direct current sonrce (selenium rectifier bridge). The quant,ity of electricity is measured in terms of motor counts by the integrating motor 11-hich integrates current and time. Silver ion is generated in the electrolysis cell from a silver wire anodc. The potential of the solution is controlled by a Beckman titrator unit equipped with a silver-calomel electrode system. An agar-agar bridge with 0.1-Ysodium nitrate is used brtween thc calomel electrod(, and the halide solution. The antiripstor con-
trol circuit of the Beckman titrator, automatically regulates the addition of proportionally smaller amounts of silver ion a s the vicinit,y of the end point is reached. As the approach t o the end point potential is signaled by the silver indicator electrode, t'he anticipator circuit initiates a series of momentary off and on operations, stopping the titration a t the exact equivalence potential setting and thereby avoiding possible overshooting of the end point. Materials. The silver anode consists of a 3-nini. diameter wire of pure (99.9 %) silver (Baker R- Co., Inc.) of which a 6-inch, spiral-shaped length is exposed t o the electrolysis solution. The silver wire leading out of the solution is enclosed in a glass tube and is sealed at, the lower end n-ith dpiezon wax (IT-100, James C. Biddle Co.). A coarsr frit'ted glass tube comprises t h r cathode cwmpartment. The silver indicator consists of a I/*-inch length (exposed) of 3-mni. diameter silver iT-iresealed in a glass tube n i t h Apiezon wax. A new silver anode should be installed in the cell usually after 200 to 250 determinations. The U-shaped agar bridge connecting the saturated calomel electrodi. vessel with the electrolysis cell is filled with 47, agar-agar in 0.1S sodium nitrate. I t is usable for several months, provided the ends of the tube are kept immersed in solution. il solution of approximately 0.11- sodium nitrate is used in the vessel containing the saturated calomel electrode. The solution for testing tlie perforniance of the titrator consists of 140 ml. of specially prepared nitrattl-nitric- acid reagent to which is added 10 nil. of standard 0.1-Y hydrorhloric acid and 300 nil. of 3A alcohol (5 gallons commercially pure methanol added to 100 gallons of 190 proof ethyl alcohol). The
+
\
