Titration of Acids in Nonaqueous Solvents Potentiometric Titration of Lithium(I) a n d Sodium(1) in Pyridine W. M. BANICK, Jr., arid R. A. HOLZER Organic Chemicals Division, American Cyanamid Co., Bound Brook, N . J .
b Lithium(1) and sodium(1) can b e quantitatively determined b y titration of their perchlorates with tributylethylammonium hydroxide using pyridine as solvent. The titration can b e followed potentiometrically using a glass indicating-electrode. The behavior of potassium perchlorate was not investigated because of its poor solubility in pyridine. Lithium perchlorate-perchloric acid, lithium perchlorateammonium perchlorate, lithium perchlorate-ammonium p erchlorate-perchloric acid, sodium perchlorate-perchloric acid, sodium perchlorate-benzoic acid, and lithium perchloratebenzoic acid mixtures can be differentiated. The last mixture is unusual in that three inflections are obtained. Mixtures of lithium and sodium perchlorate cannot b e differentiated.
T
on acid-base and rcdox titrat,ions of inorganic compounds in nonaqueous solvents through early 1956 has been reviewed by Kolling (6). Since this review, Cundiff and Markmas ( 4 ) hale described an indirect method which c in be applied to the determination of a p a t number of strictly inorganic salts. The salt is dissolved in water cont'riining an excess of sulfuric acid and the cation is precipitated as the sulfate by the addition of pyridine or acetone. The acid liberated from the salt can then be determined in the presence of the excess sulfuric acid by a differmtiating potentiornrtric titration with tetrabutylammonium hydroxide. Carbonate and hydroxide impurit,ies in the sample do not interfere and no sol~.biiityproblems were encountered. A met'hod similar in principle was describrd by Bruinmet and Hollmeg ( 2 ) for the nonaqupous tit rimet,ric deterniiiiatioii of nickel(II), copper(II), and cobalt(I1) chlorides. ;\n t"x(:ess of an acidic chelate is added LO a solution of the salt. The acid 1ibe:;ated by chelation can be det'ermined by a differentia t h g potentionictric titutioii ill a brnxene-methanol solvent eystein. This invcstigation is cvnccrricd with HE LITERATURE
thc direct titration of mctal cations as acids in nonaqueous solvents. We have found that lithium(1) and sodium(1) can be determined quantitatively by titration of their perchlorates with tributylethylammonium hydroxide using pyridine as sol\-ent. The titration can be followed potentiometrically using a glass indicating-electrode. Many mixtures of protonic acids and lithium or sodium perchlorate can be differentiated potentiometricallv. EXPERIMENTAL
Apparatus. Unless otherwise indicated, all titrations were performed on the Precision-Dow Recordomatic titrator (Model N-2-292), using a glass (Beckman 40498) calomel (Beckman 40463) electrode pair. The sleeve-type calomel was modified by replacing the aqueous electrolyte solution with a saturated solution of potassium chloride in mtthanol ( 3 ) . A 1.ON aqueous solution of tctrabutylammoniuin chloride has becn used as the elcctrolyte solution ( 5 ) . We found this t o be undesirable, since yuatcmary ammonium chlorides react 15 ith mercurous chloride. Potassium chloride is soluble in methanol to the extent of 0.421 gram per 100 ml. of solution ( 8 ) . The sleevetype calomel electrode used in this investigation had an electrolyte flow through the junction of less than 0.05 ml. per hour. For a titration requiring 12 minutes, 0.04 mg. of potassium chloride would be introduced into the titration solvent. For 100 ml. of solvent, this amounts to 0.2 p.p.m. of potassium ion. Harlow (6) has found that this concentration of potassium ion has no apparent effect on the response of the glass electrode. The mercury-film elpctrode was prepared as described by hIorcs ( 7 ) . Reagents. Tributylethylammonium hydroxide, O.LV, in 10: 1 benzenemethanol is preparcd from the purified iodide by the method of Cundiff and Markunas (S), and is standardized potentiometrically against primary standard benzoic acid. Pyridine, a refined grade having a 2" boiling range, was purified by the method of l3anic.k (1). Lithium pcrchlorate, anhydrous reagent grade (G. F. Smith Chemical Co.).
The reagent was dried in vacuo a t 150" C. for 4 hours. A negative chloride test was obtained on the dried reagent. A Karl Fischer determination gave 0.067% water. Sodium perchlorate, hydrated reagent grade (G. F. Smith Chemical Co.). The hydrated material, 25 grams, was dissolved in 50 ml. of acetone. The solution was filtered. The filtrate was diluted with 50 ml. of chloroform to precipitate the sodium perchlorate which was collected by filtration, washed with chloroform, and dried a t 40" C. for 10 minutes. It was then dried in vacuo a t 150" C. for 4 hours. A negative test for chloride was obtained on the dried material. A Karl Fischer determination gave o.03?i7c water. Perchloric acid, reagent grade, 707c',. Ammonium perchlorate, reagent grade (G. F. Smith Chemical Co.). Lithium benzoate is prepared by the reaction of 0.20 mole of anhydrous lithium hydroxide with a 5% excess of benzoic acid in 200 ml. of boiling 1: 1 mixture of ethanol and water. The solution was evaporated to a volume of 50 ml. and cooled. The lithium benzoate was precipitated by the addition of 50 ml. of acetone, collected by filtration, and washed with acetone. The product was dried a t room temperature. o-Nitroaniline solution (0.3%) is prepared by dissolving 75 mg. of the reagent (Eastman Kodak White Label) in 25 ml. of pyridine. Titrations. Dissolve 25 mmoles of lithium perchlorate or sodium perchlorate in exactly 250 ml. of pyridine. Pipet a volume containing 1 to 1.5 mmoles of the lithium perchlorate or 0.5 to 1.0 mmoles of the sodium perchlorate into a titration flask and dilute to 100 ml. with pyridine. Titrate on the Precision-Dow Recordomatic Titrator with tributylethylammonium hydroxide. Take the inflection point in the titration curve as the end point. Subtract the solvent blank from the total volume. The end point for the lithium perchlorate titration can be determined visually using 4 drops of 0.3% o-nitroaniline indicator solution. The end point is characterized by a yellow t o red-orange color change. net hfmoles of lithium or sodium 1111. of titrant X N of titrant. Mixtures were titrated in a similar nianner. =i
VOL. 35,
NO. IO, SEPTEMBER 1963
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1,OOO-
Table 1. Analysis of Lithium and Sodium Perchlorates"
900800 700
Li CI 04
~
-
'Im l
100
ml. 0. IN Bu,EtNOH
Figure 1 . Titration of lithium and sodium perchlorate in pyridine RESULTS A N D DISCUSSION
Titration of Lithium and Sodium Perchlorate. The potentiometric titration curves for t h e lithium and sodium perchlorate are shown in Figure 1. Titrations with a Beckman bluetip glass electrode gave titration curves practically identical t o those with the general-purpose glass electrode. Quantitative analytical data obtained on the sodium and lithium perchlorate are shown in Table I. The data indicate that the reaction of these salts with the tributylethylanimoniuni hydroxide (Equation 1) is stoichiometric. MClOi
+ BuJEtSOH ;VOH + BuiEtXClOp +
(1)
where JI = Sa(1) or Li(1). Although the titrant \vab preimcd very carefully, it did contain 0.5 mole 70 carbonate (99.5 mol(, % hydroxide). The presence of the carbonate could
,oat 700 800
600 -
ml. O.IN BuaEtNOH
Figure 2. Titration of perchloric acidlithium perchlorate and perchloric acidsodium perchlorate mixtures in pyridine
1414
ANALYTICAL CHEMISTRY
Compound Purity,* % Lithium perchlorate 99.6 & 0.14 Lithium perchlorate 99.3 & 0. %Ic Sodium perchlorate 99.6 f 0.49 a Based on five determinations each. * Precision index given is the standard deviation of a single value. c Visual end point.
explain the very small negative bias. If very high accuracy is desired, the titrant could be standardized against highly-purified lithium perchlorate. -1 perceptible precipitate appears in the solution after 0.3 to 0.4 mmoles of the lithium(1) or sodium(1) ha5 been titrated. S o discontinuity appears in the titration curve as a result of the precipitation. A comparison of the acidity of the two cations is not possible without a h o d edge of the relative response of the glass electrode to both sodium(1) and lithium(1). The upper potential limit in the titration of the sodium perchlorate is much lower than that for lithium perchlorate (Figure 1). This could result from a greater sensitivity of the glass electrode toward sodium(1) than toward lithium(1). o-Sitroaniline gives a sharp color change from yellow to orange or redorange which corresponds to the potentiometric end point in the titration of lithium perchlorate. The color change in the titration of sodium perchlorate occurs before the potentiometric end point. This beha1 ior suggests that the iodium hydroxide is more highly dissociated than lithium hydroxide. Effect of Methanol and Water. Increaqing the titer value from 5 t o 10 nil. of 0,l.V tributj lethylamnioniuni hldroside reduces the e.m.f. span for the titration of lithium and sodium lmchlorate by approxiniately 90 mv. The additional methanol introduced with the larger titer value has no noticeable effect on the e.1ii.f. value a t half-neutralization. -4ddition of 1% water to the solvent reduces thc e.m.f. span for the titration of sodium perchlorate by 65 mv. and for lithium perchlorate by 200 mv. The reduction in e.m.f. span associated \\ ith thc addition of 37, n ater to the d v e n t is 120 and 350 niv. for >odium perchlorate and lithium perchlorate, respectively. I n the presence of water, the e.m.f. value at half-neutralization for both sodium and lithium perchlorate qhifts t o more positive values. Use of Other Solvents. Lithium as good a n inflection in acetone as in pyridine. The inflcction for sodium perchlorate, howewr, is barely perceptible. The potentiometric inflcction for both compound5
900
700
1
2
3
ml. O.IN Figure 3. dine
4
5
6
7
Bu3EtNOH
Titration of mixtures in pyri-
Curve 1. Perchloric acid-ammonium perchlorate-lithium perchlorate Curve 2. Ammonium perchlorale-lilhium perchlorate
is barely perceptible in 10: 1 benzenemethanol. Titration of Mixtures. Perchloric acid-lithium perchlorate and perchloric acid-sodium perchlorate mixtures can be differentiated. Typical titration curves are shown in Figure 2 . This nonaqueous titrimetric method should be useful i n the determination of lithium or sodium after the wetashing of a sample with perchloric acid. Ammonium perchlorate-lithium perchlorate and perchloric acid-ammonium perchlorate-lithium perchlorate mixtures can be differentiated. Typical titration curves are shown in Figure 3. 1,000
900
v)
L c O
zus e500 I
400
300 200
I
I
,
'
'
'
'
'
ml. O.IN Bu,EtNOH
Figure 4. Titration of mixtures in pyridine Curve 1. Curve 2.
0.20 mmole iifhium perchlorate and 0.20 mmole lithium benzoate 0.20 mmole lithium perchlorate and
0.50 mmole benzoic acid Curve 3.
0.50 mmole lithium perchlorate and 0.20 mmole benzoic acid
Mixtures of lithium and sodium perchlorate could not be differentiated. The method can be dsed, however, to determine total lithium plus sodium. A mixture of lithium salts (lithium perchlorate-lithium benzoate) can also be differentiated as shown in Figure 4. The titration behavior of a mixture of lithium perchlorate and benzoic acid is unusual. When the benzoic acidlithium perchlorate mole ratio is less than or greater than one, the titration curve has three inflevtions (see Figure 4). .4n equimolar mixture of lithium perchlorate and benzic acid gives but two inflections. The first inflection in all three cases probably represents completion of the reaction
+
IAiC104 C‘6HjCOOH f Bu8Et?JOH C&COOLi BuaEtSCIOl HzO (2) -.f
+
+
When the benzoic xid-lithium perchlorate mole ratio is greater than one, the second inflection i!i due to the excess benzoic acid; when the ratio is less than one, the second inflection represents the titration of the excess lithium perchlorate. The third inflection (or the sccond inflection in the case of the equimolar mixture) represents the titration of the lithium benzoate. The titration curvw obtained for a mixture of sodium perchlorate and benzoic acid using two different indicating electrodes are shown in Figure 5. The volume of titrant to the first inflection is equivalent t o the amount of sodium perchlorate present and probably r e p r e s e h completion of the reaction.
1,000
t
900
8001
7001 6
I
4
4500 lL%
a -300 300
-200
2 00
- 100 Ordinate for Curw I 1
Ordinoto for Curve2
2 3 4 5 A. O.IN BU3EtNOH
6
7
Figure 5. Titration of sodium perchlorate-benzoic acid mixtures in pyridine Curve 1 ,
0.1 5 mmole sodium perchlorate and 0.50 mmole benzoic acid; glass electrode
Curve 2.
0.17 mmole sodium perchlorate and 0.50 mmole benzoic acid; mercuryfilm electrode
+
NaC104 f C&COOH BurEtXOH + CeHjCOOXa Bu3EthTC104 HLI (3)
+
+
The total volume of titrant to the second inflection is equivalent to the amount of benzoic acid present. The titration curve obtained with the mercury-film electrode is well defined with no irregularities. The relatively sharp second inflection in which the e.m.f.
approaches the limit for this particular electrode-solvent-titrant combination indicates that the sodium benzoate is nonacidic. Irregularities appear in the titration curve obtained with the glass electrode. The e.m.f. drop after the first inflection and the unsymmetrical nature of the second inflection could be explained by sudden changes in the sodium ion permeability of the glass electrode. The irregularities resemble those reported by Harlow ( 5 ) for the titration of phenol in pyridine with a quaternary ammonium hydroxide containing small amounts of sodium ion. Jt7e have found that the irregularities arising from these small amounts of sodium ion in the titrant or the sample can be eliniinated by the addition of small amounts of water to the solvent. This 15-ork ~t-illbe reported in the near future. LITERATURE CITED
(11 Banick, W. M., Jr., AKAL.CHEJI.34, 296 (1962). (2) Brummet, B. D., Hollweg, R. M., Ibid., 28,448 (1956). (3) Cundiff, R. H., Markunas, P. C., Ibid., p. 792. (4) Cundiff, R. H., Markunas, P. C., Anal. C h i n . Acta 21, 68 (1959). ( 5 ) Harlow, G. b.,~ A L CHEW . 34, 148 (1962). (6) Kolling, 0 . IT., J . C h e m Ed. 34, 170 (1957). ( 7 ) %IOrOS, s. A,, A N A L . CHEM. 34, 1584 (1962). (8) Seidell, A,, “Solubilities of Inorganic and Xetal Organic Compounds,” 3rd ed., p. 777, Van Nostrand, New York, 1940.
RECEIVED for review February 15, 1063. Accepted June 25, 1903.
Direct Analysis of Oxygen-1 8 in Organic Compounds C. GARDNER SWAIN, GEN-ICHI TSUCHIHASHI, and LYNN J. TAYLOR Department of Chemistry and laboratory for Nuclear Science, Massachusetts Institute o f Technology, Cambridge, Mass.
b By repeated scanning of the molecular-weight regions of inass spectra and careful measurement of peak intensities, it is possible to determine the oxygen18 content of many organic compounds without the need for chemical conversions and with a precision sufficient for most tracer and isotope-effect experiments, provided the iracer enrichment is about 5%.
A
LTHOUGH SEVERAL methods
have been suggested for analysis of isotopic composition of the oxygen in organic c~ompountts, most involve the construction of relatively complex apparatus and none appears to be general :iiid completely satidfactory (19). Doer-
ing and Dorfman (6) adapted the Unterzaucher procedure for quantitative oxygen analysis (21, I , 10) to the determination of oxygen-18 content and reported good results with a variety of compounds. However, more recent work (2, 3 ) indicates that the accuracy of the method is limited by the need to apply a correction for the reaction of hot carbon with silica. Anbar, Dostrovsky, and coworkers ( 2 ) reported that they have made many attempts to develop a general method for isotopic oxygen analysis, but without success. New procedures have been suggested and applied in particular cases (1.3, 5 . 7 , Q), but their general applicability has yet to be demonstrated.
In view of thit hituntion, n e have examined the possibility of determining oxygen-18 content by direct massspectrometric analysib. This technique has already been applied in case? where it was desired to dcterrniiie the isotopic composition of a particular oxygen atom (11, 2 2 ) . Such a method would avoid most of the difficulties of chemical conversion processes, such as incomplete conversion and dilution effects. Contamination by oxygen-containing impurities such as witer, carbon dioside, or oxygen should bt. less serious if a method is ba.ed on direct analysis than if chemical conversion is employed (8). We have developed a procedure which appetlrs to be suitable for :tnalysis VOL. 35, NO. IO, SEPTEMBER 1963
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