Potentiometric Differentiation of Certain Inorganic Cations CHARLES W. PIFER, ERNEST G. WOLLISH, rnd MORTON SCHMALL N. 1.
Products Control Laboratory, Hoffmann-La Roche, h c . , Nutley,
Nonaqueous titrations have proved useful for the differentiation of organic bases of varied strength. It was thought feasible to extend such titrations to certain inorganic cations having the common acetate anion. The acetates of ammonium, lithium, potassium, and sodium were titrated potentiometrically as bases i n an acetic acid-chloroform solvent. I t was possible to differentiate quantitatively between potassium and sodium acetates in the presence of each other. The differential titration of a mixture of inorganic and organic bases under similar experimental conditions was studied.
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inorganic cations, such as sodiuni, potassium, and lithium, in the presence of each other by various techniques. These methods include flame photometry, spectroscopy, ion exchange chromatography ( I ) , and precipitations. Potentiometric titrations in water have not permitted a distinction between these cations. -in excellent review of the theory concerning different systems of acids and bases has been presented by Hall ( 2 ) . Kolthoff and Willman ( 3 ) have studied the acid strength of inorganic cations and the b a i c strength of inorganic acetates in acetic acid, using conductometric and colorimetric methods for the determination of the end point. Investigations in this laboratory have shown (4)that many inorganic cations can be titrated with perchloric acid in nonaqueous medium. I n the present work, a procedure was developed for determining the relative basicity of the acetates of
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Titration of Lithium and Potassium Acetate6
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Figure 1. Titration of Sodium and Potassium Acetates 0 A curve. 0 B c u r v e
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Titration of Ammonium and Sodium Acetates
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ANALYTICAL CHEMISTRY 15
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Figure 4. Titration of Lithium and Ammonium Acetates
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certain inorganic cations by differential potentiometric titration when dissolved in an acetic acid-chloroform solvent.. The titrant used was 0.01N perchloric acid in p-dioxane.
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Titration of Ammonium-Potassium and Lithium- Sodium Acetates
The results of these experiments (Figures 1 to 4) show that both potassium and ammonium acetates have greater basic strength than either sodium or lithium acetate, when titrated in an acetic acid-chloroform medium. When attempts were made to differentiate between ammonium and potassium acetates, only one potentiometric inflection was obtained indicating that these salts are of nearly the same basic strength in this solvent system (Figure 5). Cations of inorganic acetates, which were studied, are listed below in groups I and 11. It was possible to differentiate the cations of group I, from any of those in group 11. However, the cations listed in group I1 could not be differentially determined in the presence of each other.
EXPERIMENTAL
The first system studied concerned the differentiation between sodium and potassium as their acetate salts. Solutions of 750 mg. of potassium acetate and 600 mg. of sodium acetate in 100 ml. of glacial acetic acid were prepared. Two milliliters of each solution were pipetted into a 150-ml. beaker and 100 ml. of chloroform was added. The solution was titrated potentiometrically with O.OIN perchloric acid in pdioxane using a glass-calomel (sleeve-type) electrode system. Two distinct inflections were obtained, as shown in curve A , Figure 1. 800. The first potentiometric break was due to the potassium acetate and the second to the sodium salt. These POTASSIUM ACETATE breaks were identified by keeping the quantity of the sodium salt constant 700. while increasing the amount of the v) 4( n-BUTYLAMINE potassium salt and repeating the titrac 2 tion (curve B ) . The end points ob0 > served corresponded to the calculated J quantities of titrant. J When a solution of potassium and I 600. lithium acetates in a similar solvent mixture was titrated, curves similar to those for sodium and potassium salts were obtained (Figure 2). The potentiometric titration of a solution 500. containing 8 mg. of ammonium acetate and 12 mg. of sodium acetate in a mixture of 4 ml. of glacial acetic acid and 100 ml. of chloroform showed two die1'0 I '5 2'0 0 tinct breaks (Figure 3). Titration ML 0.01 N H C L O 4 I N p-DIOXANE curves of ammonium and lithium acetates are presented in Figure 4. Figure 6 . Titration of Organic and Inorganic Bases
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V O L U M E 26, NO. 1, J A N U A R Y 1 9 5 4 Group I Ammonium Potassium
Group I1 Barium Cadmium Calcium Lead Lithium 3Iagnesium
Sickel Silver Sodium Strontium Zinc
An interesting aspect of these experiments is the fact that ammonium acetate behaves as a base of strength comparable to potassium acetate in acetic acid-chloroform solution, while in water ammonium hydroxide is a much weaker base than potassium, sodium, or lithium hydroxide. I n another series of experiments, alkali metal salts were differentially titrated in the presence of organic bases. In one example, a solution containing 15 mg. of potassium acetate, 15 mg. of pyridine, and 4 mg. of n-butylamine was prepared in 10 ml. of glacial acetic acid and 100 ml. of chloroform. This mixture was titrated poteiitiometrically with 0.01.Y perchloric acid in p-dioxane (Figure 6 ) . I t is evident from this graph that potassium acetate is a stronger base than either n-butylamine or pyridine. When ammonium acetate was qubstituted for potassium acetate, a similar curve was obtained. Hon-ever, when cations of group I1 were used instead of thow of gioup I, the basicity of their acetate silt‘ was
found equal t o that of n-but?-laniine but stronger than pyridine (Figure 6). CONCLUSIONS
The experiments described have shown that ammonium and potassium acetates behave as stronger bases than other inorganic acetates studied in the nonaqueous solvent system of acetic acid-chloroform. Thua, a differential titration of sodium acetates and potassium acetates was accomplished. ACKNON LEDGMENT
The authors are indebted to E. G. E. Shafer for his valuable suggestions, and to Esther Critelli for preparing the graphs. LITERATURE CITED
(1) Beukenkamp, J., and Riemann, K , 111, ANAL. CHEW.,22, 582 (1950). (2) Hall, N. F., J Chem. Educ., 17, 125 (1940). (3) Kolthoff, I. hl., and Willman, &4., J . A m . Chem. Soc., 56, 1014 (1934). 14) Pifer, C. W., and Wolhsh, E. G., ANAL.CHEM.,24, 519 (1952) RECEIYED for review July 27, 1953. Accepted October 20, 1953. Presented a t the Pittsburgh Conference on Analytical Chemistry and Applied SpectrosC O P > , March 1-5, 1953
Analytical As peck of Reactions of 2- (2-P y ridy I)-benzimidazole And 2-(2=Pyridyl)-imidazoline with Iron( II) JOSEPH
L. W A L T E R
and
H E N R Y FREISER
Department o f Chemistry, University o f Pittsburgh, Pittsburgh, P a .
The compounds, 2(2-pyridyl)-benzimidazole and 2(2-pyridyl)-imidazoline, of interest because of their structural similarity to 2,2’-bipyridine and o-phenanthroline, were prepared and their reactions with metal cations tested. Both were found to give color reactions with copper(I), copper(II), cobalt(II), iron(II), and iron(II1). Most notable were those with iron(II), with which deep red-purple colored complexes were formed. These complexes were studied spectrophotometrically and found to have maximum absorbances at 490 mp for 2-(2-pyridyl)-benzimidazole at pH 5.7 and 560 mp for 2-(2-pyridyl)-imidazoline at pH 9.0. The molar extinction coefficients for the 2-(2-pyridyl)-benzimidazole and 2-(2-pyridyl)imidazoline are 3800, and 7800, respectively. The iron complexes could also be extracted with isoamyl alcohol, thus increasing the sensitivity of the color reaction.
I N T H E course of a study of analytical reagents of the type
where X = 0, S, XH and I’ = C-OH, C-SH, and l\j (7’-9), 2-(2-pyridyl)-benzimidazole(I) and 2-(Z-pyridyl)-imidazoline(II) were prepared and tested. These compounds were of interest because of their structural similarity to bipyridine and o-phenanthroline, reagents for iron. Also, because certain substituted phenanthrolines and diquinolyl did not exhibit reaction w-ith iron, ostensibly because of steric hindrance of the groups adjacent to
the nitrogens, the question arose whether the benz- portion of the benzimidazole would exert sufficient hindrance to prevent complex formation. I n this, comparison with the imidazoline with no steric hindrance should be revealing.
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111 2-( 2-Pyridyl)-benaoxazole(lIII)\\-as also prepared for purposes of comparison because it was felt that the benz- portion of the benzoxazole would offer less hindrance to coordination reactions of the nitrogen than would occur in the corresponding benzimidazole nitrogen ( 8 ) . The reactions of 2-(2-pyrid> 1)-benzimidazole and 2-(2-pyridyl)imidazoline with metal ions were found to be quite similar to those of bipyridine and o-phenanthroline in that a red complex with iron(I1) was formed ieadily. The colored complex was readily extractable with isoamj 1 alcohol. That the analogous benzoxazole(II1) did not give any reaction with iron(I1) was attributed to the lower basicity of the oxazole nitrogen as compared to the imidazole nitrogen. REAGENTS AND APPARATUS
Preparation of Z-(t-Pyridyl)ybenzimidazole. The procedure outlined here is essentially that of Leko and Vlajinats (3).
