1580
ANALYTICAL CHEMISTRY
thiuram and guanidine types which has been recommended for use in the vulcanization of neoprene rubber Type W (16). ACKNOWLEDGMENT
The authors wish to acknowledge the help of RIarjorie Jean Hannum in preparing this paper. Thanks are expressed to the Firestone Tire and Rubber Co. for permission to publish the results of this investigation. LITER4TURE CITED (1) hlliger. G., and Harrison, S. R., India Rubber World, 123, 181-5 (1950). (2) Bellamy, L. J., Lawrie, J. H., and Press, E. W. S., Trans. I n s f . Rubber Ind.. 23. 15 (19491: 22. 308 (1947). (3) Braus, H., Middieton, F. hI., and Ruchhoft, C. C . , ANLL. CIIEM.,24, 1872 (1952). (4) Brock, M. J., and Hannum, &I.J., Ibid., 27, 1374 (1955). (5) Burchfield, H. P., and Judy, J. X . , Ibid., 19, 786 (1947). (6) Carr, E. L., Smith, G. E. P., J r . , and Alliger, G., J . Oiy. Chern.. 14, 921 (1949). (7) Deal, .4.J. A., Trans. Inst. Rubber Ind.,23, 148 (1947).
(8) Du Fraisse, C., and Houpillart, J.. Rev. gdn. caoutchouc, 19, 207 (1942). (9) Du Fraisse, C., and Jarrijon, A., Rubber Chem. and Technol., 17, 941 (1944). (IO) Endoh, H., J . SOC.Chem. Ind. Japan, Suppl. Binding, 38, 618 (1935). (11) Humphrey, B. .J., ISD. ENG.CHEM.,ANAL. ED., 8, 153 (1936). (12) Jarrijon, A., Rev. gin. caoutchouc, 18, 217 (1941). (13) Koch, H. P., J . Chem. Soc., 1949, 401. (14) Kreps, V., J . Rubber Ind. (U.S.S.R.), 9, 45 (1933). (15) Mann, J., Trans. Inst. Rubber Ind., 27, 232 (1951). and Thompson, D. C., Division of Rubber Chem(16) Murray, R. M., istry, BM.CHEM.SOC.,Louisville, Ky., April 14, 1954; abstract' in India Rubber World, 130, 7 1 (1954). (17) Parker, C. A, and Berriman, J. M., Trans. Inst. Rubber Ind.. 28, 279 (1952). (18) Kailsbach, H. E., Svetlik, J. F., Biard, C. C., and Louthan, R. P., Ind. Eng. Chem., 47, 352 (1955). (19) Shimada, K., J . SOC.Chem. Ind. Japan, Suppl. Binding 36, 82 (1933); 36, 260 (1933). (20) Smith, G. E. P., Jr.. and others, J . Org. Chem., 14, 935 (1949). (21) Vaanderbilt Sews, 13, No. 1 , 24 (1947). RECEIVED for review September 27, 1954. Accepted April 18, 1959. Presented before the Division of Rubber Chemistry at the 126th Neeting of the A h I E R [ r A S C H E Y I C . A L s O C I E T Y . Xew YOrk, N. Y . , 1954.
4-Aminopyridine as Standard in Acidimetry CLAYTON E. VAN HALL and
K. G. STONE
Kedzie Chemical Laboratory, M i c h i g a n State University, East Lansing, M i r h .
4-Aminopyridine is a high melting (161" C.) nitrogen base with a dissociation constant of 1.3 X Methyl red indicator may be used with either O.1Nor 0 . W acid. The free base may be purified by recrystallization from toluene or benzene, or by sublimation at reduced pressure, and may be recovered easily after use. Standardization of acids with 4-aminopyridine yields normalities within 1 part per thousand of those found using sodium carhonate.
THE
search for new substances which have the required properties of primary standards is never ending (3). This is particularly true with respect t o bases suitable for the standardization of acids. The most recent suggest,ion, tris(hydros,vmethy1)aniinomethane ( 4 ) , requires a mixed indicat,or which is unsatisfactory in the hands of students ( 8 ) , because inexpeiieric*ed people have trouble with the color change. 4-Aminopyridine satisfies most of the requirements for a primary standard. 4-Aminopyridine was first synthesized by Camps ( 2 ) in 1902. The most widely used synthesis was suggested b!- Koenigs and Greiner (6) and is based on the hydrolytic cleavage of 4-pyridylpyridiniuni dichloride to yield hminopyridine and polynievixed t#arrymat,erial. Techniral grade 4-aminopyridine is also available (Reilly T a r ' and C h e m i d Corp., Indianapolis, Ind.) 4Aminopyridine is a weak, monoprotic base with an equivalent weight of 94.12. Tropsch (R) reported a dissociation constant of 1.3 X 10-5 a t 25" C. from conductivity measurements, and Albert ( 1 ) cited data which lead to a value of 1.6 X l O P . 4Aminopyridine is soluble in water and eth>-lalcohol, and moderately soluble in benzene, t,oluene, and chloroform (1). EXPERIMENTAL
4-Aminopyridine used in this work was prepared by the procedure of Koenigs and Greiner ( 6 ) , and was recrystallized from benzene, ground to a powder, and dried for 2 hours a t 105" C . before use. The melting point, was 161' C. with a heating rate of 2' t o 3" per minute. A 6-liter carboy of approsimately 0 . 1 4 hydrochloric acid solu-
tion was prepared by dilution of reagent grade concentrated arid and was protected against temperature changes. The exact normality was determined by the method of Kolthoff and Sandell ( 7 ) , using Mallinckrodt primary standard grade sodium cwbonate which was treated according to the directions of Kolthoff and Sandell ( 7 ) before use. The acid was also compared against carbonate-free sodium hydroxide solution, which had been standardized against potassium acid phthalate according to the method of Hillebrand, Lundell, Bright, and Hoffman (6). All titrations in this work were made with one volumetric, buret, which was calibrated at 25' C. in the normal wa). 4 1 1 other volumetric equipment used was calibrated if necessary. The concentrations of indicator solutions were those normall! used in analytical work. Indicator corrections were applied for phenolphthalein and bromocresol green used in the carbonate titration, but no correction was necessary with methyl red indicator. The p H titrations were made with a Beckman p H meter, Model H-2, equipped with a glass indicator electrode and :I calomel reference electrode. All distilled water was boiled freshly before use. All samples of 4-aminopyridine were dried a t 105" C. for 2 hours, unless otherwise stated. All weiahings were made with calibrated brass weights and were not corrected to vacuo. STABILITY OF 4-AMINOPYRIDINE
Quantitative experiments to show the stability of 4-aminop7 ridine were carried out in the following manner. Samples of the original recrystallized material (the material after standing 96 days a t room temperature, the material after 56 hours a t 105" C., ana some material kept in a closed bottle for 6 months) were dried 2 hours a t 105' C. and titrated with approximately 0.12Y hydrochloric acid solution. The normality of the acid was calculated assuming that all the samples of 4-aminopyridine were pure, and a value of 0.1020 was found from all samples. It, therefore, must be concluded that 4-aminopyridine is stable under the conditions described. The hygroscopicity of 4-aminopyridine was observed by placing a weighed portion in a weighing bottle, storing the sample and another weighing bottle in a beaker (covered to keep out the dust) a t room temperature, and weighing both the sample and the tare at frequent intervals. The tare was used to determine how much of the change was due to adsorption of moisture on the glass surface. The sample continuously lost weight over a period of 95 days, the total loss being 0.15%. During this time the temperature range was 22" to 31" C., and the relative hu-
V O L U M E 27, NO. 10, O C T O B E R 1 9 5 5
1581
midity range was 50 to 95%. It appears t h a t 4-aminopyridine is not hygroscopic. Since 4-aminopyridine has a definite vapor pressure as evidenced by its steam volatility, samples kept a t 105" C. for drying would be expected to sublime. A sample of 4-amino yridine was kept in an electric oven a t 105' C. for 56 hours. continuous loss of weight amounting to 0.2%per hour was observed. Therefore, drying times must be limited to conserve material. The same experiments described above were carried out with 4-aminopyridine which had been sublimed a t 55" t o 60' C. and 10 mm. of mercury. Sublimation as a means of purification had been suggested by Wibaut, Herzberg, and Schlatmann (IO). The same results were found using the sublimed material as the original material, except that the rates of loss were increased beeause the particle size was decreased.
1
DETECTION OF THE EQUIVALENCE POIhT
.4 major requirement for a volumetric standard is that the equivalence point be easily detected. From previous nork which reported a dissociation constant of 1.3 X lo-' (9) for 4aniinopyridine, methyl red indicator appeared to be suitable. Weighed Pamples of 4-aminopyridine were dissolved in freshly boiled distilled water stirred with nitrogen, which had been passed through asrarite. Titrations were made x i t h approximately 0 . 1 S and 0.LV hydrochloric acid solutions. The pH of the solution was measured with a glass-calomel electrode pair after each addition of acid. Two drops of 0.1% methyl red indicator solution were also present. The results of these titrations are shown in Figure 1.
for 2 hours, Fere titrated t o a niethll red end point under nitrogen and under air only. The normalities of the acid calculated from the titrations under both conditions were identical, and the sharpness of the indicator change was not affected by the presence of carbon dioxide from the air. GENERAL RESULTS
X nmiiiiary of the normalities found for a hydrochloric acid solution using a standard sodium hydroxide solution, sodium carhonate, and Paminopyridine is given in Table I. These results were obtained over a period of 6 months, thus do not contain a time bias. The result8 using sodium hydroxide solution and sodium carbonate check well, but the results using 4-aminopyridine are 1 part per thousand lower. However, the sum of the errors is probably 1 part per thousand, SO that 4-aminopyridine appears to be worth while for further investigation in other laboratories.
Table I. Normality of Hj-drochloricAcid Solution NaOH5 0.10210 0.10210
OS.
Na2COa 0.10211 0.10209
08.
0.10214 0.10211 0.10213 0.10214 0,10204 0.10209 0.10210
I
It
-I
us. 4-Aminopyridine Recrystallized Sublimes 0.10201 0.10201 0.10202 O.lOl96 0.10200 0.10196 0.10201 0.10196 0.10200 0.10196 0.10194b 0.10193 0.10195a 0.10198 0.10195b 0.10191b 0.10203b 0.10198b 0.10202 b 0.10196e 0.10203 b 0.10203 b 0.10198b 0.10197e
.4v. 0,10212 0.10211 0.10200 0.10197 Std. dev. 0.000026 0.000032 0.000033 0.000032 a 0.11437N, av. of 10 determinations u8. potassium acid phthalate std. dev. 0.000036. b 4-Aminopyridine recovered from earlier titrations. c 4-Aminopyridine sublimed twice.
AVAILABILITY OF 4-AMINOPYRIDINE
4-ilminopyridine is not availahle as a reagent grade chemical a t this time. I t is available in a tecshniral grade (Reilly Tar and Chemical Corp.), and is not difficult to purify. Three recrystallizations from toluene or benzene, with activated carbon treatment the first time, yield long needlea which after powdering and drying for 2 hours a t 105' C. assay 100.0 =k 0.1% by titration with hydrochloric acid standardized with sodium carbonate. RECOVERY OF 4-AMINOPYRIDINE
\I 0
I
4
I 8
I 12 ML.
I
I
16 20 OF ACID
I
24
I 28
2
Figure 1. Titration of 4-aminopyridine with hydrochloric acid
4-Aminopyridine may be recovei ed readily from titration residue.. The solution is made alkaline to a pH of about 14 and evaporated to dryness. The dry residue is powdered, placed in a Soxhlet extractor, and exhaustively extracted with benzene. The extract is treated a i t h activated carbon, filtered, and redured to a small volume so that crystallization may occur. In a typical case, from 11.2 grams of 4-aminopyridine used in titrations 9.5 grams (85%) of material suitable for use were rerovered. .4s seen in Table I, the recovered material was just as good as the original 4-aniinopyridine. RECOMMENDED PROCEDURE
From Figure 1 it is clear that methyl red is a satisfactory indirator with both 0.lN and 0.5N acids. The p H at the mid-point of both titration curves is 9.1 a t 25" C. from which the dissociation constant is calculated to be 1.3 X 10-6 as previously reported. The question of carbon dioxide interference had to be checked. Samples of Paminopyridine, which had been dried a t 105" C.
The following procedure is designed for the standardization of 0 . l N acid, but by increasing the amount of 4-aminopyridine may be used as well for 0.5LVacid. Weigh 0.4- 0.05-gram samples of 4-aminopyridine dried for 2 hours a t 105" C. into 250-ml. flasks and dissolve in 100 ml. of freshly boiled distilled water. Add two drops of 0.1% methyl red indicator solution and titrate to the first change away from yellow. The milliequivalent weight of Paminopyridine is
*
ANALYTICAL CHEMISTRY
1582 0.09412. The indicator blank is normally negligible, but it should be checked. SU*MMMARY
4-Aminopyridine is a solid, monoprotic base which is suitable as a standard for acidimetry when methyl red is used as the indicator in the titration. LITERATURE CITED
(1) Albert, A., J . Chem. SOC.,1951, 1376. (2) Camps, R., Chem. Zentr., 7 3 11, 647-9 (1902). (3) Farr, H. V., Butler, A. Q.,and Tuthill, S. M., ASAL. CHEM.,23, 1534-7 (1951). ( 4 ) Fossum, J. H.. Rfarkunas, P. C., and Riddick, J. A., Ibid.. 23, 491-3 (1951).
(5) Hillebrand, W. F., Lundell, G. E. F.,Bright, H. A., and Hoffman, J. I., “Applied lnorganic Analysis,” 2nd ed., pp, 180-1, Wiley, S e w York, 1953. (6) Koenigs, E., and Greiner, H., Ber. deut. chem. Ges., 64, 1049-56 (1931). (7) Kolthoff, I. >I., and Sandell, E. B., “Textbook of Quantitative Inorganic dnalysis,” 3rd ed., pp. 5 2 2 4 , hlacmillan, Yew York, 1952. (8) Stone, K. G., unpublished results. (9) Tropsch, H., Monatsh. Chem., 35, 775-9 (1914).
(IO) Wibaut, J. P., Heraberg, S.,and Schlatmann, J., Rec. trav. chim., 73, 140-2 (1954). RECEIVED for review February 10, 1956. Accepted June 24, 1955. Abstracted from M.S. thesis submitted b y Clayton E. Van Hall in J u n e 1954 to Graduate Faculty of Michigan State University. Presented before the Division of Analytical Chemistry, 127th Meeting, ACS, Cincinnati, Ohio.
Complexometric Titration of Indium KUANG LU CHENG’ Department of Chemistry, University of Connecticut, Storrs, Conn.
This investigation was undertaken to develop a rapid and accurate method for the titration of small amounts of indium. Indium may be titrated directly with (ethylenedinitri1o)tetraacetic acid using l-(2-pyridylazo)-2-naphthol as an indicator at pH 2.3 to 2.5 or pH 7 to 8 with an accuracy within 0.5%. At pH 2.3 to 2.5, alkali metals, alkaline earth metals, aluminum, and manganese do not interfere. A t pH 7 to 8, copper, zinc, cadmium, nickel, silver, mercury, and other metals which form very stable complexes with cyanide do not interfere if cyanide is added. Iron may be masked by the addition of fluoride. The common anions such as chloride, sulfate, nitrate, perchlorate, fluoride, tartrate, and citrate do not interfere. Bismuth, lead, gallium, and tin interfere.
A
C O M l I 0 4 method for the determination of indium in-
volves weighing the precipitate as indium sesquioxide after ignition of the hydroxide a t llOOo to 1200’ C. An aniperometric method ( 7 ) and a flame photometric method (6) have been proposed. Recently, Flaschka and others (2-4) have reported the titration of indium by (ethylenedinitri1o)tetraacetic arid (ethylenediaminetetraacetic acid) using eriorhrome black T as the indicator and a lead solution for back-titration at p H 10. I n the course of an investigation of the reaction of metals n i t h 1-(2-pyridylazo)-2-naphthol( 1 ) it appeared that this azo dye might be used as an indicator in the complexometric titration of indium. This paper describes a simple and quick method for the direct titration of indium by (ethylenedinitri1o)tetrpacetic acid using 1-(2-pvridylazo)-2-naphtholas the indicator. The titration is carried out a t pH 2.3 to 2.5 using acetic acid as a buffer. Alkali metals, alkaline earth metals, aluminum, and manganev(I1) do not interfere with the titration, The heavy metals which form strong complexes with cyanide do not interfere if the titration is made in the presence of cyanide a t p H 7 to 8. Jentzsch and others ( 5 ) developed a simple scheme for the separation of indium. When this separation is followed by a complexometric titration, the result is a rapid and widely applicable analytical method for indium. REAGENTS
(Ethylenedinitri1o)tetraacetic acid solution, 0.01M. Approximately 3.72 grams of the disodium salt of (ethylenedinitri1o)tetra1
Present address, Westinghouse Electric Corp., E a s t Pittsburgh, P a
acetic acid were dissolved in water and diluted to 1 liter. This solution waa then standardized against the standard indium solution using the procedure recommended in this paper. Standard indium solution, 0.01M. An accurate weight of 1.1 to 1.2 grams of indium metal (99.95%) in a small amount of hydrochloric acid was heated on a hot plate and diluted to 1 liter with water. The pure indium metal was obtained from the Indium Co. of America. Indicator solution, 0.01%. Approximately 0.001 gram of 1-(2pyridylazo)-2-naphthol was dissolved in 10 ml. of methanol. The indicator solution is very stable. The preparation of the azo dye has been described ( 1 ) . Sodium hydroxide, 1 s . All other chemicals used were of reagent grade. PROCEDURE
Titration at pH 2.3 to 2.5. The solution containing 0.05 to 0.2 millimole of indium in a 200-ml. beaker was diluted to approximately 50 ml. and neutralized with 1.V sodium hydroxide until a slight white precipitate was formed. Two milliliters of glacial acetic acid were added to dissolve the precipitate. The solution was titrated with (ethylenedinitri1o)tetraacetic acid solution after addition of 2 drops of the indicator solution. The end point from red to pure yellow was very sharp. Titration at pH 7 to 8. I n the presence of copper, zinc, nickel, and other metals which form strong complexes with cyanide, the titration of indium in alkaline medium using cyanide to mask the interference is recommended. The solution containing 0.05 to 0.2 millimole of indium in a 200-ml. beaker n-as diluted to approximately 50 ml. and adjusted to p H 7 to 8 using acetic acid and ammonium acetate after the addition of a suitable amount of potassium cyanide and approximately 1 gram of potassium sodium tartrate. The solution was titrated with (ethylenedinitri1o)tetraacetic acid solution after addition of 2 drops of the indicator solution. The end point was also from red to pure yellow. The calculation may readily be made according to a 1 to 1 ratio of the indium-(ethvlenedinitri1o)tetraacetic acid complex DISCUSSION
Effect of pH. Flaschka and Amin (3, 4) used eriochrome black T as the indicator, which requires a p H of 10 for detecting the end point. I t was found, by the author, that indium was still strongly complexed by (ethJ-1enedinitrilo)tetraacetic arid a t p H 2 and that indium could be titrated by (ethylenedinitri1o)tetraacptic acid a t a wide range of pH between 2 to 10 using 1-(2pyridylazo)-2-naphthol as an indicator. KO sharp end point was obtained when the solution was adjusted to p H 1.5 or below because the indium-indicator complex is not stable at that low pH. I t was also impossible to titrate indium at very high p H because the formation of indium hydroxide caused a slow end point and because the indicator itself is an acid-base indicator; it changes
