67
V O L U M E 28, NO. 1, J A N U A R Y 1 9 5 6 Whatever the true mechanism of the thermal processes may be, the thermograms are manifestations of molecular composition. The differential thermographic coutours serve not merely t o characterize or identifl- substances, but eventually t o yield important information pertaining t o the relation between molecular Composition and chemical property. ACKNOWLEDGMENT
The author is indebted to the Sorthern Utilization Research iI,, Kolloid-2. 130, 31-9 (1953). (21) Told, M.J., AN.~L.CHEM.,21, 683-8 (1949). (22) Whitehead, W. L., and Breger. I. A , , Science 111,279-81 (1950). (23) Wolff, I. A., Gundrum, L. J., and Rist, C. E., J . A m . C'hem. Soc. 72, 5188-91 (1950). R E C E I V E D f o r review February 28, 1955. Accepted October 22, 1953. Division of Carbohydrate Chemistry, 126th-Meeting. ACS, New York, Septemher 1964. Contribution No. 298, Cheniistry Division, Science Serl-ice.
Thermometric Titration of Stannic Chloride S. T. ZENCHELSKY, JAMES PERIALE', and J. C. COBB2 School o f Chemistry, Rutgers University, New Brunswick,
Thermometric titration is presented as a method for the determination of Lewis acids in organic solvents. Stannic chloride can be titrated with dioxane in benzene or carbon tetrachloride with an accuracy of within about 1% and in nitrobenzene or chloroform with lesser accuracy. Calorimetric measurements have been used to explain the effect of solvent on the titration, and the useful concentration ranges for these titrations have been presented.
N
UMEROUS reports on the use of thermometric titrations for aqueous solutions may be found in the literature.
Linde, Rogers, and Hume ( 3 ) have presented a comprehensive list of references on the subject, together with a discussion of the principles involved and a n indication of the general applicability of the method. I n contrast, very few applications to nonaqueous systems have been made (6, 7). This f a c t is surprising in view of the scarcity of end-point detection methods: Neither electroinetric techniques nor indicators are generally applicable t o nonaqueous solutions. Furthermore the lower specific heats of many organic solvents, as compared with water, introduce a favorable sensitivity factor. These considerations suggested the applicability of thermometric titrations for the determination of Lewis acids in connection with the authors' studies of acid-base reactions in aprotic solvents. T h e successful titration of aluminum chloride by this method has been reported ( 7 ) , although experimental details regarding the concentration range and effect of solvent are lacking. Rice, Zuffanti, and Luder ( 5 ) have investigated, qualitatively, the behavior of indicators in solutions of Lewis acids and bases, but their general suitability has not been demonstrated quantitatively. Conductometric titrations, as used by La Mer and Present address, Shell Chemical Corp., Union, S. J. Present address, Lamont Geological Laboratory, Palisades, N. T.
N . J. Dovines ( 2 ) for Bronsted acids in benzene, gave unsatisfactory results. Stannic chloride was chosen for this investigation because of its solubility in the aprotic solvents employed. Thus the reactions are homogeneous, in contrast with those involving aluminum chloride. Dioxane was selected as the base because its stoichiometry appeared simpler than those of some other bases studied. Four common solvenh were used : benzene, carbon tetrachloride, chloroform, and nitrobenzene. The stoichiometry was studied as a function of acid and base concentrations, and the effect of the solvent was invest.igated. Calorimetric meaaurenients were made in order to explain the results and indicate how these data can be used t o indicate the suitability of solvents. EXPERIMENTAL
Apparatus. The uniform feed buret was similar to Lingane's ( 4 ) , and the temperature measurement and recording system for thermometric titration was like t h a t of Linde, Rogers, and Hume ( 3 ) . A Western Electric 14A thermistor may be used where greater sensitivity is desirable, but it requires the use of a highinput-impedance recorder. For calorimetric measurements, the thermistor vias calibrated against a standard thermometer using a Kheatstone bridge t o measure resistance. The calibration was performed a t several points over the narrow working temperature range, and resistances were converted t o temperatures by means of standard procedures (I). Reagents. STAXXICCHLORIDE, Baker's analyzed , anhvdrous. " reagent. DIOXANE,technical grade, a-as refluxed with hj-drochloric acid, neutralized with potmsium hydroxide, dried over calcium chloride, then refluxed and distilled over sodium. BESZESE, thiophene free, was shaken with sulfuric acid, washed with mater, dried over calcium chloride, then refluxed and distilled over sodium. CARBONTETRACHLORIDE, technical grade, was refluxed and distilled over calcium chloride. CHLOROFORJI, technical grade, was refluxed and distilled over calcium chloride. SITROBESZESE, reagent grade, was distilled under vacuum.
68
ANALYTICAL CHEMISTRY
Procedure. For thermometric tit,rations, concentrated solutions of stannic chloride in the appropriate solvents were prepared immediately before use. These solut,ions were stable with respect to atmospheric moisture, in contrast with pure stannic chloride. This fact was verified experimentally; nevertheless precautions were taken to exclude moisture. Portions of these solutions were delivered by means of a weight buret t o 80-ml. samples of solvent, contained in the titration vessel. When constant temperature was attained, these solutions were titrated with a solution of dioxane in the same solvent. Since solution preparation and titration were performed at the same temperature, no volumetric errors could result from changes in solution density. Concentrations of dioxane were chosen so that all titrations required between 6 and 8 ml. of titrant, which was delivered with a n accuracy of within 0.3%. Calorimetric measurement,s were made by combining the various liquids, after permitting them, and the Dewar flask, t o reach ambient temperature. For the measurement of AH? and A H , , the integral heats of dilution of pure st,annic chloride and of purr. dioxane, respectively, the acid and base, respectively; were added from a calibrated pipet t o 100 ml. of the solvent. T o obtain A H a , the heat of reaction between stannic chloride solution and pure dioxane, the base was added in slight excess of the stoichiometric amount to the resultant solution from the determination of AH2. Temperature-time curves were plotted and temperature differences were read within a n accuracy of 0.01" C. Calculations of AH mere based on the heat capacity of the solvent, rather than that of the solution. This leads to but a small error since the solutions are dilute, approximately O.IM. The heat capacity of the calorimet,er itself was neglected, although this also leads to an error, which may approach 0.2 kcal. per mole (8). These errors, together with t h a t involved in the addition of one liquid t o the other, limit the over-all accuracy t o about 0.5 kcal. per mole. RESULTS AND DISCUSSION
.$-ill t,it,ration curves show only one end-point break regardless of the solvent used, and their appearance is conventional (3). T h e quantity of heat evolved during the titrat,ion depends upon the solvent used. It increases in t h e follom-ing order: chloroform < nitrobenzene < benzene, carbon tetrachloride. I n all solvents, except chloroform, a dense white precipitate appears during the titration as the reartion product. The results are summarized in Table I .
fortuitous, as the end-point break is not sharp and the extrapolation is inaccurate. Both the titration curves and the calorimetric observations indicate that the reaction in chloroform is more complicated than those which take place in other solvents. A slow reaction seems to be superimposed on the more rapid one. This slow reaction appears to be the crystallization of the product, since on standing a precipitate gradually appears. The same experimental results are observed when using very high purity chloroform, so t h a t the difference in behavior between chloroform and the other solvents is not attributable t o the presence of impurities.
Table IT. Solvent
Benzene
Stoichiometry of Stannic Chloride-Dioxane Titrations
Carbon Sol \.en t Benzene Tetrachloride Nitrobenzene Chloroform SnCh concn. range' 3-15 3-33 6-24 6-QOb Mean molar ratioc 0.99 1.01 1.04 0.46 % std. d e r . 1 4 0.8 5 0 1.3 No. of detns. 17 6 7 5 a values indicated are niillimoles of SnCl, per 100 ml. solvent. These represent the useful range. b No upper limit >vas reached, hence this represents only t!ie highest value investigated. This ratio is calciilated f r o n i the end-point volumes and solution concentrations.
Carbon Tetrachloride
Xitrobeneene Cliloroform
16.2 1.5 9 19.9 24 5 - AH2 -0.5 0.1 fi.7 18 9 -AH& 16.7 1.; s 13.2 5 6 -AH4e-f 0.2 0 1 0.4 2.2 Dielectric constant 2.3 2 2 35 4.5 Dipole moment 0 0 4.2 1 1 a Mean temperature over which heat d a t a is taken. Variation. i ~ 1 . C 0 .~, corresponds t o maximum variation between different runs. During any one run, it is constant to within +0.Ol3 C. b - A H ) = -4H3 - IHz. It is calciilated froni the measured values of - AH3 and - 4Hz. A11 A H values are given in kcal. - AHz is the measured integral heat of dillition of pure SnClr prr mole SnCli. The final concentration is 8 . 5 X 10-2 mole per liter a t 2 2 . 0 LE -AH,;
1.00
c.
-IHq is the measured heat of reaction. per mole of SnCI:, of pure gioxane with the solution of SnClr used to obtain - A H * . Dioxane is added in v e r y slight excess of molar ratio of unity. e - A H 4 is the mezsured integral heat of dilution of pure dioxane, per mole of dioxane. The final concentration is S.5 X 10-2 mole per liter a t 22.0 i 1.00 c. I 411 measured values are the inean of a t least two determinations with a inean deviation no greater than 0.3 kcal. per mole.
The factors n-hich govern the quantity of heat evolved during the titration may be explained in terms of the calorimetric data vhich are presented in Tahle 11. Generally, the titration reaction ma)- be n-ritten: Stannic chloride (solution)
Table I.
Calorimetric Data
Temperature. 22.0 =z 1.0' C.a
+ dioxane (solution) = product + solvent - AH:
(1)
d i e r e - A H 5 represents the heat evolved per mole of stannic chloride titrated. This may ke analyzed in terms of the following reactions: Stannic chloride
+ solvent
=
stannic chloride (solution) - A H ?
Stannic chloride (solution)
+ dioxane =
product
Dioxane
+ solvent
=
+ solvent --AH3
diosane (ao!iition) -AH4
(2) (3) (4)
and by adding Equations 2 and 3 , I niercuric ion which, in turn, is reduced to free rnercur?. The analjsis i4 concluded h\ an iodometric measurement of the mercurj. The niethod is applicable to the determination of lirtuall? an) concentration of aldehj d e in the presence of most alcohols, acids, esters, acetals, ketones, ethers, organic chlorides, and epoxides. Reaction conditions and puritj data are presented for 12 aldehjdes which can he determined b! the method.
A
COJ2MO9 prohleni iii organic malysis is the quantitative
analytical resolution of mixtures containing both the aldehyde and ketone functions. Various hydroxylamine hydrochloride proredures have been developed t o determine the total c:trbonyl value of such a mixture, but they are not specific for :tldehydeP. Several methods have been proposed for the determination of aldehydes by oxidation with silver compounds, such as the procedures of Alitchell and Smith ( I O ) and Siggia ( 1 2 ) . A n excellent review of the field of carbonyl analysis has t)c,en prepared by llitchell (9). tteriipted the determination Previous investigators who ha\ o f aldehydes by some mercurimetric procedure have recomniended the method primarily for the estimtttion of formaldehyde (1, 2, 12). I n addition, Ihiigault :nid Gros ( 3 ) have reported the determination of furfur;il, benzaldehyde, and piperonal, and Gosrrsnii, Das-Guptx, aiid Ray (j),Goswami and Das-Purkaystha ( 6 ) , and Goswami and Shaha ( 7 ) have estimated sugars with various degrees of success using empirical factors. These investigators all employed an alkaline solution of potassium mercuric iodide, Ii?Hg14, as an oxidizing agent, In the reaction, aldehyde is oxidized t o the corresponding acid whereas mercuric ion is reduced to free mercury:
ItCHO
+ K,FIgI, + 2IiOH+RCOOH
+ H g ” + 4KI + HZO
Both isolation ant1 nonisolation methods have been proposed for tmhedetermination of the free mercury. I n the authors’ opinion it is best t o acidify the reaction mixture and react the free mercury with a measured excess of iodine. Thc amount of iodine consumed is a stoichiometric function of the free mercury
which, in turnt is a measure of the aldehyde originally present. .4gar is employed as a protective colloid t o maintain the free mercury in a finrly divided state, thus promoting its reaction Ivith iodine. The name, ”n~ercuralreagent,” has becn coined to differeiitiate the reagent from other potassium mercuric iodide preparations such as Nessler’s reagent. “Ylercural” signifies x mercuric oxidation of aldehydcs. 1tE:iGESTS
Mercural Reagent. To 1830 nil. of distilled water contxi~ied in a 1-gallon jug add 150 grams of reagent grade potassium chloride, 240 grams of U. S. Pharmacopeia grade mercuric chloride (mercury bichloride), 642 grams of reagent grade potassium iodide, and 1000 nll. of an aqueous 40% by weight, potassium hydroxide solution. Shake the contents after each addition t o ensure complete solution. This reagent is stable and does iiot deteriorate on standing. The slight amount of yellow or brown precipitate which may foim is assumed t o be due t o ammonium ion in the reagent?, h o w v e r , it is not detrimental t o the dfectiveiiess of the reagent, Agar Solution, 0.1%. Add 3.0 grams of Difco Barto-Agar to 300 nil. of boiling distilled water. Continue heating with ne(::+ sional swirling until the solid has dissolved and the resulting solution is essentiall. clear. Cool and dilute t o 3 liters with additional distilled water. .4dd 0.1 gram of mercuric iodide as a preservative arid shake vigorously for a few seconds. Acetic Acid, analytical reagent grade Iodine, approximately 0 . l S Starch Indicator, 1 .OY0solution Standard 0.1NSodium Thiosulfate Methanol, commercial grade, Carbide and Carbon Chemicals co. SAMPLING
Unless direct satnplc addition is specified, introduce t h r sample into a tared 50-ml. volumetric flask containing 30 nil. of the required solvent (methanol which has been neutralized t o hromothymol blue iritlirator, or distilled water) using a hypodermic syringe fitted with a 3-inch needle and chilled if necessary t o facilitate tranpfer. Stopper the flask and swirl t o effect solution. An acetaldehyde dilution must he allowed to stand for approximately 15 minutes, with occasional venting t o the atmosphere t o reach equilibrium before recording the gross weight. The gross weight of dilutions of other aldehydes may be determined immediately. Dilute to t,he mark with additional solvent and mix thoroughly. .i 5-mL aliquot of this dilution should contain not more than 3.0 meq. of aldehyde. Fill the pipet by pressure t o avoid loss of aldehyde. If the sample is weighed directly into the reagent, care must
