RESEARCH
New methods detect titration end points Titrimetry includes cryoscopy and pressuremetry; specific ion electrode monitors fluoride With the vast array of instrumental methods of analysis developed and refined in the past 20 years, it may seem to some chemists that titration has been buried in the welter of these instrumental techniques. But the analyst's old workhorse, titration, still thrives. Evidence of the continued interest in titration as an analytical technique was presented to more than 200 scientists attending the 20th Annual Summer Symposium on Analytical Chemistry, titled "Modern Titrimetry," held at Pomona and Harvey Mudd Colleges (Claremont, Calif.). The symposium was sponsored by Analytical Chemistry and the ACS Division of Analytical Chemistry. Among the highlights at this year's symposium were: • The use of cryoscopic titrations to locate reaction end points and to determine the stoichiometry of new reactions. • The development of pressuremetric titrations for reactions which produce at least one gaseous product. • The development of redox titration methods that stem from an understanding of the mechanism and catalysis of oxidation-reduction reactions. • An application of a fluoride-sensitive electrode in a titrimetric method for fluoride which features extraction of the anion as the tetraphenylantimony salt. The symposium papers illustrated the surging trend toward the use of instrumental methods to determine the titration end point and the rising importance of specific ion electrodes. In his opening remarks, symposium chairman Joseph Jordan of Pennsylvania State University pointed out that the relatively new topic of ion-selective electrodes, a highlight of this year's symposium, was absent from the similar symposium held in 1954 in Minneapolis. (For more on this analytical tool, see C&EN's feature in the June 12 issue, page 146). Cryoscopic titrations. Cryoscopic titrations can be used to locate rapidly and precisely end points for reactions which are understood, Dr. Stanley Bruckenstein of the University of Minnesota said at the symposium. They also permit the chemist to determine the stoichiometry of new reactions, he explained. 40 C&EN JULY 10, 1967
PRESSUREMETRIC. Dr. D. J. Curran (left) and J. L. Driscoll work with instrumentation they developed for pressuremetric titrations used with reactions producing at least one gas. Reactors are in the windowed constant temperature bath
During most titrations, a change in total concentration of solute species occurs as titrant is added to the solution. If this change is different before and after the end point, a plot of the total solute concentration vs. volume of titrant added will consist of two linear portions, provided a negligible change in volume occurs during the titration. The intersection of the two straight lines is the end point. The freezing point of a solution is a direct measure of the total solution concentration. In a cryoscopic titration, the solution to be titrated is placed in a Dewar flask and enough finely divided frozen solvent added to give a 30% (by weight) excess of frozen solvent. The total volume of the mixture is from 100 to 150 ml. By using a simple apparatus consisting of an efficient stirrer and a thermistor Wheatstone bridge, it's possible to continuously monitor the freezing point of a solution while titrant is added with a motor-driven piston buret. Using a recorder to measure the unbalanced e.m.f. of the Wheatstone bridge, Dr. Bruckenstein obtains the temperaturevolume curve. The Minnesota chemist has applied cryoscopic titrations to the study of acid-base equilibria. He has also de-
veloped a general method for cryoscopic titration of metal ions in aqueous solution with ethylenediaminetetraacetic acid ( E D T A ) . These EDTA titrations can be performed in millimolar concentrations. Dr. Bruckenstein has also shown that cryoscopic titration techniques are valuable in studying acid-base titrations of metalEDTA complexes and the formation of mixed ligand complexes. Pressuremetric titrations. Dr. David J. Curran and his coworker, James L. Driscoll, of the University of Massachusetts (Amherst) have developed an apparatus and techniques for detecting the end point of titrations which produce at least one gaseous product. In their titrations, termed pressuremetric titrations, the change in pressure in a closed vessel is measured with a capacitance type pressure transducer system. The two Massachusetts chemists have used volumetric or coulometric generation of the titrant. And with the pressuremetric technique they have studied several redox reactions which produce nitrogen as the gaseous product. These include the iodometric titration of iodate with hydrazine sulfate, the titration of cerium(IV) with azide, and the titration of isonicotinic acid hydrazide with elec-
trolytically generated bromine. In their pressuremetric titrations, they ob tain a plot of the transducer system's output voltage vs. either volume of titrant or time. The plots give in verted L-shaped titration curves. Dr. Curran, Mr. Driscoll, and Jay Kronfeld, also of Massachusetts, have experimented with these titrations at concentrations from 0.01M to 10 _4 M,
catalysis is neither appreciated nor used sufficiently, he believes. This is particularly true with inorganic oxi dation-reduction reactions and coordi nation reactions, he points out. Dr. Schenk and his Wayne State co worker, William E. Bazzelle, are cur rently studying the titration of thal lium (I) with cerium (IV). The reac tion rate greatly varies with the acid
by further titration with cerium (IV). By contrast, the un catalyzed ceri um (IV)-arsenic (III) reaction is very slow in dilute sulfuric acid. But it is still much faster than the uncatalyzed cerium(IV)-thallium(I) reaction, the two Wayne State chemists find. After adding manganese (III) catalyst, they can titrate thallium (I) in the presence of equivalent amounts of arsenic ( III ).
using solution volumes rrom l o to zo ml. In the higher concentration re gion, they have generally attained a precision and accuracy of a few tenths of a per cent. Further studies indicate that titra tions at 10 - 5 M are possible, Dr. Curran says. This places sensitivity of the method in the microgram re gion. Redox methods. If a chemist un derstands the mechanism and cataly sis of oxidation-reduction reactions, he can develop new redox titration meth ods, Dr. George H. Schenk of Wayne State University (Detroit, Mich.) said at the symposium. This understand ing can also assist the chemist in over coming interferences in older meth ods, he has shown. The modern highly instrumented ti tration is superior for routine analysis of a large number of samples. But it is not always preferable to a simple titration for a "one-shot" analysis or for a few samples, Dr. Schenk empha sizes. In analyzing one or a few samples, an analyst must still rely on his knowl edge of chemistry, particularly mask ing, catalysis, mechanisms, and kinetic information. The use of masking is well understood, Dr. Schenk says. But kinetic information and the use of
solvent used. For example, in warm 3M hydrochloric acid, the reaction is fast enough to use for a titration. But in 6M nitric acid, it is quite slow, with a half-life of several hours. The reac tion is even slower in 0.12 to 0.25M sulfuric acid—less than 4% reaction in 70 hours at 25° C. After adding manganese(III) catalyst solution, however, Mr. Bazzelle finds that the reaction is rapid enough to be followed by a pho tometric titration. An increase in absorbance at 425 τημ from unreacted yellow cerium (IV), added in excess, signals the end point. Knowing the rate laws and rate con stants of other cerium (IV)-metal ion reactions in sulfuric acid, it is possible to predict successful analysis of mix tures of thallium (I) and other metal ions in this solvent, Dr. Schenk says. For example, mixtures of thallium (I) either with arsenic ( I I I ) , iron ( I I ) , chromium ( I I I ) , or with mercury (I) may lend themselves to this type of analysis. The cerium ( IV ) -iron ( II ) reaction is very fast in dilute sulfuric acid. If the catalyst is omitted, iron (II) can be oxidized quantitatively before thal lium (I) reacts measurably with the cerium (IV) titrant. After manganese(III) catalyst is added, thallium (I) in the same solution can be determined
Thallium (I) can also be titrated with cerium (IV) in dilute sulfuric acid in the presence of chromium( I I I ) , Mr. Bazzelle has shown. He finds that the rate of the cerium (IV)chromium (III) reaction limits the amount of chromium (III) that can be tolerated (0.1 millimole). By adding cerium ( III ), however, he can slow the cerium ( IV ) -chromium ( III ) reaction enough so that more than three times as much chromium ( III ) can be pres ent without interfering with the thal lium (I) titration. Ion-selective electrodes. An appli cation of specific ion electrodes in the titration of fluoride was revealed by Dr. Michael D. Morris of Pennsylvania State University. In this application, studied with graduate student James B. Orenberg, tetraphenylantimony flu oride is extracted from water into chloroform. The two phases are placed in the same beaker. Portions of tetraphenylantimony sulfate are added, and the two phases are stirred for 30 to 40 seconds to allow phase equilibration. Between additions of titrant, the fluoride activity in the aqueous phase is measured potentiometrically with an Orion fluoride-sen sitive electrode. The fluoride extraction is more effi cient if the aqueous phase contains a JULY 10, 1967 C&EN
41
high concentration (0.1M) of sodium sulfate, Dr. Morris says. The sodium sulfate prevents emulsification and promotes rapid phase separation. The titration is reliable for fluoride concentrations from 1 Χ 10 - 3 to 50 X 10" 3 M, the two Penn State chemists find. It's necessary, how ever, to remove halide, nitrate, nitrite, sulfite, and thiocyanate ions before the extraction, they point out. If these ions aren't removed, they are ex tracted along with fluoride, and posi tive end-point errors and small titra tion curve slopes at the end point are obtained. The Penn State pair has thus devel oped a simple, rapid fluoride titration by taking advantage of the extraction chemistry of organometallic com pounds. They point out that the prob lem of nitrate interference can be solved if the ion is removed by ex tracting it with tetraphenylarsonium ion, which does not extract fluoride. The extraction properties of tetraphenylphosphonium salts are quite similar to those of the corresponding tetraphenylarsonium salts, but quite different from those of the antimony salts. Tetraphenylphosphonium sul fate could have been used to remove nitrate, but it is more expensive and a little less efficient than the arsonium sulfate, he explains. Tetraphenylarsonium cation seems to be a good extractive titrant for chlorate, perchlorate, or nitrate, Dr. Morris finds. He and Mr. Orenberg are beginning to study the titrations of these anions. Certain organometallic cations of nontransition metals—generally of the R n M+ type—form precipitates with various ions. Thus these cations can be used as selective titrants for certain anions. For example, tetraphenylar sonium chloride has been used recently for the analysis of hexafluorophosphate and other complex anions. And tetraphenylantimony sulfate is useful as a titrant for perchlorate, perrhenate, and pertechnate. Most organometallic cations are polarographically reducible. Amperometry with the dropping mercury elec trode therefore can be a convenient way to detect the end point in titra tions with these reagents, Dr. Morris points out. For example, he and Mr. Orenberg have followed the titration of perchlorate with tetraphenylantimony sulfate amperometrically. Dr. Morris and Joseph DiGregorio, another Penn State graduate student, are currently studying amperometric titrations of nitrate with diphenylthallium fluoride. The availability of ni trate-sensitive electrodes makes potentiometric titration of nitrate with diarylthallium cations an attractive pos sibility, Dr. Morris says. 42 C&EN JULY 10, 1967
Lithium salt leads to stable epoxyamine that shows unexpected ring expansion Chemists at Wayne State University, Detroit, Mich., have successfully iso lated and characterized epoxyamines, a new class of chemical compounds. The concept of the epoxyamine func tional group had been previously pro posed, but scientists have thought compounds containing it occur mainly as intermediates in other chemical re actions. Now, however, Dr. Calvin L. Ste vens of WSU's chemistry department and his coworkers, P. Madhavan Pillai and T. R. Potts, have synthesized a stable, crystalline, epoxyamine, 2 - ( l aziridinyl) - 2 -phenyl-l-oxaspiro[2.5]octane [/. Am. Chem. Soc, 89, 3084 (1967)]. The synthesis route used will probably lead to a generalized re action scheme, Dr. Stevens points out. Dimethyl and cyclopentyl analogs have been prepared, although charac terization studies on these materials have not been published. The apparent inaccessibility of this class of compounds was thought to be due to high reactivity of the epoxy amine group and its tendency to rear range. Epoxyamines rearrange read ily, but in a way contrary to that which would normally be expected, the WSU workers find. For example, the epoxyamines isomerize to aminoketones. In the Wayne experiments, the pro totype material was prepared by react ing a-bromocyclohexyl phenyl ketone with the lithium salt of ethylenimine in ether at room temperature. The product was isolated by distillation
and purified by recrystallization from pentane. The corresponding epoxy amine, formed in 7 5 % yield, had a boiling point of 90° to 95° C. and a melting point of 20° to 22° C. The in frared spectrum shows no hydroxyl or carbonyl absorptions, but has strong peaks at 1025 and 1045 cm.- 1 The nuclear magnetic resonance spectrum is consistent with the epoxyamine structure, showing aromatic protons from τ = 2.45 to 2.85 and aliphatic protons from τ = 7.8 to 9.0 in a 5:14 ratio. Some of the reactions of the epoxy amine compound furnish added confir mation of the structure and, at the same time, point to the versatility of epoxyamine as a functional group. For example, acid hydrolysis of the compound converts it quantitatively to the known ketone, a-hydroxycyclohexyl phenyl ketone. Reduction with sodium borohydride in methanol gives [ 1-a- ( 1-aziridinyl ) benzyl] cyclohexanol, whose hydrogénation yields (l-a:-N-ethylaminobenzyl)cyclohexanol. This amino alcohol can also be obtained by direct hydrogénation of the epoxyamine. Of particular interest is the rearrangement which the epoxyamine undergoes. When refluxed in o-dichlorobenzene, the rearrangement with ring expansion occurs to give 2-(1-aziridinyl ) -2-phenylcycloheptanone. According to previous ideas on epoxyamine rearrangements, a- ( 1-aziridinyl) cyclohexyl phenyl ketone would have been the product.
α-Bromoketone gives rise to the new epoxyamine which rearranges to form a seven-membered ring
