ever, is less precise than the potcntiometric end point (*0.05 ml. ml.). There is one class of aminoethylthiosulfates which cannot be acid-titrated in ethylene glycol. The N-acetyl and N-formyl derivatives of aminoethylthiosulfates were neutral salts in ethylene glycol, and hence could not be assayed by titration. These titrations are now performed on a routine basis in laboratory to assay research quantities Of these pounds.
(3) Holmberg, B., Sorbo, B., Nature 183, 832 (1959). (4) Mulligan, B., Swan, J. M., Rev. Pure A p d . f.Xem. 12,82 (1962). (5ilE;if: S. R., ANAL. CHEM. 18, 246
ACKNOWLEDGMENT
Derivatives of 0-aminoethylthiosulfuric acid used in this investigation ere synthesized by ~ 1H.. ~ ~ l lJ.i E. ~ , ~ ~and J. ~C. J ~ ~ i ~ ~ ~ , ~ (6) Saville, . B., Chem. Znd. 1956, p. 660. JOHN C. MACDONALD Boston Laboratory LITERATURE CITED Monsanto Research Corp. ( I ) Cundiff, R. H., Markunas, P. C., Everett, Mass. 02149 ANAL. 28,792 (1956). (2) Fritz, J. S.,“Acid-Base Titrations in WORK supported by the United States Nonaqueous Solvents,” G. Frederick Army Medical Research and Development Smith Chemical Co., Columbus, Ohio, Command, under Contract No. DA1952. 49-193-MD-2 109.
Amperometric Titration of Primary Amides SIR: I n a n earlier paper (d) a method was described for the spectrophotometric titration of primary aliphatic amides with hypobromite in basic solution. Although this procedure is satisfactory for aliphatic amides, it cannot be used for the analysis of aromatic amides because of the instability of the primary reaction product, the corresponding aromatic .Y-bromamide, in basic solution. The decomposition products of the N-bromamide are rapidly oxidized by hypobromite, and no suitable end point can be found. This communication deals with a new titrimetric procedure for amides which can be utilized for the analysis of both aliphatic and aromatic amides. The sample of amide is dissolved in a dioxane-water mixture which is 1M in hydrochloric acid, and titrated with a standard solution of calcium hypochlorite using a n amperometric end point. EXPERIMENTAL
Reagents and Apparatus. An approximately 0.5‘V solution of calcium hypochlorite was prepared by dissolving reagent grade calcium hypochlorite in water, followed by filtration t o remove solid calcium carbonate. T h e solution was standardized iodometrically. KO change in titer was evident over a 1-month period, in contrast to t h a t evident with t h e more dilute solution used previously ( 2 ) . The amides used were of the highest purity obtainable from commercial sources and were used without further purification. The amperometric titration apparatus consisted of a dry cell potential source, a divider, an RCA Model WV-84B microammeter, a Leeds and Yorthrup pH meter, a rotating platinum electrode, and a saturated calomel electrode. Procedure. -4 25-ml. aliquot containing 0.5 mmole of amide in dioxane was diluted to exactly 150 ml. with
aqueous HCl so t h a t the final solution was -1M in HCI and 20’3, dioxane. The applied potential was set to f0.4 volt us. S.C.E., and the calcium hypochlorite titrant was added in 0.5-ml. increments. T h e resulting current readings were taken after t h e addition of each increment of titrant, corrected for dilution, and plotted against volume of titrant. T h e best straight lines were drawn through the experimental points, a n d the intersection of these two straight lines was taken as the end point.
hypochlorous acid may be involved The sequence of reactions suggested is:
+ H+ HOCl + H + HOCl + HC1OC1-
+
Table I. Amperometric Titration of Primary Amides
No. of Amide trials Benzamide 8 o-Toluamide 9 p-Toluamide 5 Acetamide 6 Propionamide 3 n-Butyramide 3
Av . apparent Std. purity, dev., % % 99.6 0.6 100.6 0.9 97.0 0.6 99.6 100.0 100.4
1.3
0.7 0.5
HOCl
H20Cl+
C12
+ H2O
0
I1
R-C-NH2
+ Cl2
4
O
H
I1 / R-C--N \ c1
RESULTS A N D D I S C U S S I O N
The results of the titration of a number of primary amides are given in Table I. Although these results were obtained with a 0.5-mmole sample of amide, as small a sample as 0.05 mmole was titrated with 0.05N calcium hypochlorite with only a slight decrease in accuracy and precision. The time required to complete a titration was about 10 minutes. The reaction utilized in this procedure is the N-chlorination of the primary amide in 1M hydrochloric acid with a chlorinating agent produced by the addition of calcium hypochlorite. The exact nature of the chlorinating agent, however, is not known, and it is probable that both chlorine and protonated
+
+HC1
0
It
R-C-NHa
+ H20C1’ O
+
H
Undoubtedly chlorine is the predominant chlorinating species when hydrochloric acid is used, because the reaction between hypochlorous acid and hydrochloric acid is rapid ( 1 ) . However, similar titrations were performed using I N sulfuric and 1N perchloric acids instead of hydrochloric, and identical titration results were obtained, even though the rate of chlorination was markedly slower than when hydrochloric acid was used. Because the calcium hypochlorite titrant contained a n appreciable amount of free chloride, chlorine is probably still the major chlorinating agent, but the protonated hypochlorous acid may be involved also. The excess chlorinating agent slowly decomposes after the end point of the titration has been passed. The rate of decomposition of 1JI hydrochloric acid is approsimately 1% per minute. A number of methods were attempted to stabilize the chlorinating agent, but none was successful. For example, if VOL. 37,
NO. 9, A U G U S T 1965
1171
the solution being titrated is made 12M in chloride ion, the decomposition rate is diminished to 0.3% per minute, but almost no chlorination of benzamide was noticed after 30 minutes. Bromine was also substituted for chlorine, but no ,IT-bromination was noticed in a reasonable time period. The following technique was then developed to eliminate errors due to the slight instability of the chlorinating agent. A direct titration was carried out, but the first experimental point after the end point was taken with only a ilight excess (usually about 25y0) of hypochlorite. The remaining points, which determined the second segment of the titration curve, were then assumed to be constant if the current decreased less than 1 pa. per minute. This procedure can be used successfully for primary aliphatic amides and
for aromatic amides which are not substituted or which are substituted only with alkyl groups. The presence of other functional groups substituted on the aromatic ring in general causes interference in the determination of the amide group. Electron-withdrawing groups such as nitro and carboxyl reduce the rate of N-chlorination, and also adversely affect the equilibrium constant. Electron-donating groups activate the ring so that chlorination of the ring takes place readily, and the exact stoichiometry of the titration reaction becomes uncertain. In addition, substituted groups such as amino, carbonyl, and hydroxy are easily oxidized by hypochlorous acid in the titration medium. Preliminary experiments indicate that a possible solution to this problem of interference by ring substituents is
exhaustive bromination in acid solution. If the unknown sample is treated w-ith bromine, most of the ring sites which would ordinarily be chlorinated during the titration are brominated. Also, easily oxidizable groups are oxidized by the bromine, and the excess bromine can then be driven off by heating. t-nder these conditions bromination on the amide nitrogen does not take place. LITERATURE CITED
( 1 ) Eigen, SI., Kustin, K., J . A m . Chem. SOC.84, 1355 (1962). ( 2 ) Post, W. R., Reynolds, C. A , , AXAL. CHEM.3 6 , 781 (1964).
WILLIAMR. POST CHARLES rl. REYKOLDS
Department of Chemistry I'niversity of Kansas Lawrence. Kan.
Determination of Adsorbed Ethylene and Propylene Oxides by Distillation and Titration SIR: h modified method for determining adsorbed volatile epoxides that is both simple and rapid has been developed. Such epoxides are not directly titrateable, due to very slow elution into the titrating medium and obscure end points. Mechanical preparation of the sample such as grinding or shrrdding does not materially improve this condition; it provides error through loss of the adsorbed volatiles during preparation and does not eliminate the obscure end points. Since hydrogen bromide reacts virtually instantaneously and the elution is of a physical rather t,han a chemical nature, other reactants-as in the method of Jay (3)-likewise do not make direct titration feasible. Methods using excess reactant with back-titration as in the procedures by Schecter, Wynstra, and Kurkjy (4) and Durbetaki (2) are also inadequate because of the prolonged elution and hence reaction time, and the varied interferences found with the different adsorbing materials. Current methods generally elute the epoxide by aeration and bubbling through a reacting solution; such techniques are time consuming and awkward to handle. In this method, adsorbed epoxides are released from the adsorbing material by distillation in monochlorobenzene, (Bollected in a receiving medium, and titrated with a standard hydrogen bromide solution, using crystal violet as a n indicator, as in the method by Ilurhetaki ( I ) . I h a u s e ethylene and propylene oxides are known to be quite reactive, it was anticipated that a reaction between these epoxides and the adsor1 172
ANALYTICAL CHEMISTRY
bents would take place during the distillation. Recovery studies were made by exposing t'he materials to pure ethylene and propylene oxides, weighing the amount adsorbed, and subsequently determining the epoxides by this procedure. Consistently low values were obtained, and the amount of unrecovered epoxide from each adsorbent followed the expected pattern in terms of individual adsorbent reactivity; in all cases, the value was minimal. Blank distillations and t h a t i o n s were conducted on unexposed samples of the same size to determine the reagent and indicator values and to compensate for any volatile reactant originally in the material. A11 such values thus obtained were found to be consistent and minimal for each adsorbent studied.
Table I.
Epoxide Recovery
Adsorbed Unrecovered Recovered, (mg./gram) (mg./gram) 5 Propylene oxide in gum rubber; 2-gram samples 1.8
5.0
9.6 12 8 19 9 22 1
0.0 0.2 0.2 0 2 0 7 O X
100.0 96.1 97.8 98 6 96 7 96 1
Ethylene oxide in gum rubber; 2-gram samples 0.7 1.2 4.8 5.2 16.1
0.1 0.1 0.4 0.6 1.4
87.5 88.0 92.1 89.4 91.2
(11.8)
1 3
88 9
EXPERIMENTAL
Apparatus. The apparatus used were as follows: a 250-ml. roundbottom flask wit'h 24/40 ground joint; a 300-mm. West condenser with 24/40 ground joints; a 75" connecting tube with 24/40 joints; a 105" adapter tube with 24/40 joint; a 75-ml. collecting flask; and a 250-ml. heating mantel with voltage regulator. Procedure. Place a n accurately weighed 2-gram sample of the adsorbent in the distilling flask with 2-3 glass beads and add 100 ml. of monochlorobenzene; apply apiezon "&" wax to the glass joints. Add 15 ml. of glacial acetic acid to the receiving flask, and connect the apparatus with the tip of the condenser adapter tube covered by the glacial acetic acid in the receiving flask. Set the voltage regulator a t 75 volts and distill unt'il one half of the monochlorobenzene has
&gram samples. Propylene oxide in tygon; 2-gram samples 1.4 12.9 14.5 17.0 24.8 39.3 52.3 63.4
0 1 0 2 0 4
100.0 102,7 99.4 100.4 99.4 99.0 101.5 100.4
Propylene oxide in polyethylene: 2-gram samples 1.0 1.1 3.5 3.8
0.0 0.0 0.0 0.0
100.0 100.0 100.0 100.0
