Table Ill. Effect of Manganese
Zinc Found, Cyanide-
7
Metallic Ions LTsual Added, y hydrate Zincon lln’+ Zn++ modification method 10 0 4 2 12 4 50 10 34 0 34 6 50 0 3 8a 18 4 50 10 13 50 0 1 Ob 18 4 50 10 10 4b .. 0 5 gram of sodium ascorbate added to sample. b 0.5 gram of sodium ascorbate added to sanmle. Reference solution for uhotometric *measurements prepared sime as sample but 3 ml. of n-ater was added in place of the 3 ml. of chloral hydrate solution.
Table IV.
Recovery Tests on Various Types of Water
Zinc, P.P.M. Sample Present Cooling water 0 50 0 16 Potable water 0 05 Effluent from 0 70 metal finishing plant Plating waste 0 00 vater 0 00
ReAdded covrred 1 00 1 46 1 00 1 18 1 00 1 02 1 00
1 70
0 10 1 00
0 10 1 00
Chromate0 02 treatedproc- 0 02 ess nater
0 10 l 00
0 11 l 00
Treated com- 0.00 bined plat- 0 00 ing and sewage n-aste
0 10 1 00
0.09 1 00
only cadmium interferes in the cyanidechloral hydrate modification. Zinc can be determined accurately by the cyanide -chloral hydrate modification in the presence of many heavy metal ions even though the concentration of these ions is many times grcater than that of zinc. I n contrast, serious errors are produced with most metals in the usual Zincon method. I n the presence of ferric ion the color intensity of the sample is less than that of the blank, producing lorn results. Aluminum in the concentration tested did not interfere with either procedure. Cadmium produces high results Tvvith either procedure but the error is less with the cyanide-chloral hydrate modification. Table I11 shows that manganese interferes seriously with both the usual Zincon method and its modification. Hydroxylamine hydrochloride and salts of polyphosphates, ferrocyanide, ferricyanide, fluoride, persulfate, iodine, and ascorbate !?-ere tried in an attempt to eliminate the effect of manganese. Only sodium ascorbate decreased the error caused by nianganesc. The influence of manganese can be decreased further if the reference solution for photometric nieasurement is prepared by treating another aliquot of the sample with 0.5 gram of soclium ascorbate, followed bj. the reagents in the procedure, but replacing the 3 ml. of chloral hydrate solution with 3 ml. of distilled water. This technique compensates for interfering substances because the value of the reference solution not only includes the color of the Zincon reagent but also the effect of interfering metals not complexed with cyanide. Conditions of the test are the same for both sample and reference solution up to the point at
which chloral hydrate is added to the sample to release the zinc which then combines lyith Zincon. This special reference solution is particularly recommended for color and turbidity in the sample. All steps in the procedure can be completed within 5 minutes. APPLICATION OF METHOD Recovery tests were run OII various types of m t e r samples to check the accuracy of this method. Sodium ascorbate and the special reference solution discussed previously were used. Table IV shows a maximum recovery error of 4% for a 1 p.p.in. zinc addition and 10% for a 0.1 p.p.m. zinc addition. Zinc can he determined rapidly and accurately in various types of water by the Zincon method using the proposed ascorbate-cyanide-chloral hydrate modification. The method should also find use for the analysis of materials other than water. LITERATURE CITED
(1) Ani. Public Health Assoc., New \-o;k, “Standard Methods for the Esaminatlon of Water and Sewage,” 10th ed., p. 212, 1955. (2) Hatch: G. B., Corrosion 11, 461t (1955). (3) Kinnunen, J., Merikanto, B., Chemist Analyst 41,76 (1952). (4) hicCal1, J. T., Davis, G. K., Stears, T. W., AS-AL.CHEM.30, 1345 (1958). (5) Maier, R. H., Kuykendall, J. R., Chemist Analyst 47,4(1958). (6) Pribil, R.?Collection Czechoslou. ('bent Communs. 18, 783 (1953). ( 7 ) Rush, R. M., Yoe, J. H., ANAL.CHEM. 26, 1345 (1984). RECEIVEDfor review October 14, 1958. hccepted March 4, 1959. Division of Water, Sexage, and Sanitation Chemistry, 134th Meeting, .4CS, Chicago, Ill., September 1958.
Titration of Ketimines in Glacial Acetic Acid P. L. PlCKARDl and F. A. IDDINGS University of Oklahoma, Norman, Okla.
b
The titration of ketimines in acetic acid solution b y perchloric acid has been developed to replace the more tedious and time-consuming Dumas and Kjeldahl methods. Accuracy and precision are within 1% when either a potentiometric or visual end point i s used for individual samples containing as little as 0.06 meq. of ketimine. The potentiometric end point used a platinum-glass electrode system in conjunction with a Beckman Model H pH meter. The familiar S-shaped titration curves are obtained. The violet to blue color change of crystal violet occurred a t the potentiometric
1228
ANALYTICAL CHEMISTRY
end point in the titrations. Few impurities other than amines or ammonia interfered.
K
have been analyzed for yield and purity by micro-Kjeldah1 and micro-Dumas determinations of nitrogen content (6,6-8). Although some modifications n ere made (6), the analyses were inconvenient for occasional samples and time-consuming for even a fen samples. A more convenient method requiring less time per sample and less exhaustive techniques TTBS desired. This paper is thr result LTIJIINES
CJf a study of the feasibility of titration
of ketimines by perchloric acid in glacial acetic acid solutions (referred to here as acetic acid). Because of the instability of many of t,he ketimines in water solutioiis, a nonaqueous solvent for the titrations was desirable. Acetic acid was chosen for its leveling &cct upon weak organic bases and esccllcnt solvent, properties (9)’
1 Present address, Chemical Division, Celanese Corp. of America, Clarkwood,
Tes.
EXPERIMENTAL
Solutions. -40.1S solution of perchloric acid in acctic acid n a s prepared and standardized against potassium acid phthalate by t h e potentiometric end point described by Seaman and Allen (10). The 0.1-V perchloric acid solution was diluted n i t h acetic acid containing 10 nil. of acctic anhydride per liter of solution to obtain 0.01S acid, and standardized like the 0.LV solution. A solution of 1 weight yo crystal violet (gentian violet) in acctic acid was prepared for use as a visual indicator. One or two drops mere used with each saniple of ketimine for end point detection after the proper color change of the indicator was chosen (Figure 1). d solution of 0.6 gram of reagent grade pyridine in 100 ml. of acetic acid was prepared to check the performance of the electrodes. Apparatus. A Becknian Model H p H meter equipped with a platinumglass electrode system was used in t h e potentiometric determinations. T h e glass electrode (Beckman 1190-80) a a s prepared bv soaking in acetic acid before use and betn e r n titrations. T h e platinum electrode x-as prepared b y sealing a piece of 26-gage platinum n i r e into t h e end of a piece of borosilicate glass tubing. Electiical contact mas made n i t h t h e platinum wire through a pool of mercury in the bottom of the glass tube. A section of about 2 cm. of the platinum wire extending through the glass to the outside was melted into a small, shiny sphere a t the tip of the glass. The platinum electrod(. was connected to the p H meter through the reference cell input. Thcl micro-Dumas nitrogen detrrminations nere made in tlie apparatus described by Sicderl and Niederl (4). Procedure. Ketimine samples were tieighed by difference into small glass stoppered bottles containing a few milliliters of acetic acid. T h e mixture was then either rinsed into a volumetric flask and diluted t o volume ~ ~ i acetic t h acid for aliquot samples 01 iinsed into t h e titration vessel and t i t r a t d in 50-ml. beakers containing 10 ml. or more of solution with one or two drops of the crystal violet indicator. Stray currents set up by magnetic stirrers necessitated agitation of the sainplcs by hand for the potentiometric titrations. RESULTS
Aliquot portions of the acetic acid solution of pyridine \\-ere titrated to determine the shape of t h r curve produced by the platinum-glass electrode system and t o indicate the reproducibility for identical titrations. The Sshaped curve shown in Figure 1 was obtainrd. The vertical portion of the curve corresponded to a change of over 100 niv. for an aliquot containing about 0.4 meq. of pyridine and its position on the abscissa n as reproducible within thc error of reading thc buret. The
Table I. Analysis of Individual Samples of Diphenyl Ketimine
1
-1201
Added, Meq. 1.546
Found, Added, Found, Meq. Meq. Meq. 1.540 0.0617 0.0614a 0.2271 0.2248 0.1254 O.125la 0.5901 0.5864 0.1149 0.1149a Titrated using 0.01N perchloric acid with semimicroburet. 0
t40
Table II. Titration of Aliquot Samples of Diphenyl Ketimine Containing 0.309 Meq. of Ketimine
1 I
t
I 2
3
4
ML. al
5
N
6
7
8
HCIq
Figure 1 . Sample titration curve showing the position of crystal violet end point
Table 111.
Found, Found, Meq. Dev. hleq. -0 001 0 306 0 310 -0 005 0 307 0 304 -0 007 0 305 0 302 -0 002 0 305 0 307 -Av. deviation -0 003 Standard deviation 0 004
Del -0 003 -0 002 -0 004 -0 004
Comparison of Titration and Dumas Analysis of Two N e w Ketimines
% K (Dumas) Ethyl 2-pyridyl ketimine 2-Thienyl 5-acridyl ketimine
Calcd.
Found
20.89
20.81
9.72
initial and final potentials were not reproducibile, varying RE. much as 20 mv. Samples of diphenyl ketimine (boiling point 127’ C. at 3.5 mni., di0 1.0849, n’,” 1.6191), shown to be relatively pure b y a comparison of physical constants n-ith the literature values (boiling point 128’ C. a t 4 mni., dig 1.0547, ng 1.6191) of analyzed material ( 3 ) , were titrated and found to give titration curves similar to that in Figure 1. The vertical section of the krtimine curve was relatively greater than the pyridine curve. Table I sho.l\-s the analysis of separate, weighed samples of diphenyl ketimine titrated potentiometrically. Table I1 shows titration of a series of aliquot samples of the diphenyl ketimine. By using either larger samples or the more accurately read semimicroburet, the 1% average error may be reduced, as shown by euainination of the analyses in Table I. A sample of 2-propyl-m-tolyl ketiniine, which had been stored in a sealed glass capsule, found to contain 8.74% nitrogen by Pickard and T’aughaii (S), using the Dumas method, gave 8.70% basic nitrogen by titration in acetic acid (calculated 8.70% X). Ethyl 2-pyridyl ketimine and 2thienyl5-acridyl ketiniine were prepared by Tolbert (M),who reported the analytical data presented in Table 111. Only one break in the potentiometric titration curve is obtained, although the compounds contain basic groups in addition to the ketimine nitrogen. Because visual detection of the end
9 66
Neut. Equiv. Calcd. FOUI~ 67.10 67.20 06.12 96 .50
point in the titration vas more desirabk than the slower plotting of potentiometric data, a n indicator was needed. Crystal violet was chosen because itq broad color change from violet to bluc to green to yellow n-ith increasing acidity has been well studied and the eo101 change corresponding t o the potentiometric end point may be chosen ( I 10). The large circle on the curve in Figure 1 represents the point on the titration curve for ketimines a t which the crystal violet changes from violet to a brilliant blue. This color change is the easiest to see and more nearly represents the potentiomctric end point than any othcr color change. T o obtain the same color each time, a sample of pyridine was treated with 0.1N perchloric acid until the color matched the blue color deqcribed above. The blue standard mas very stable and was used for several months. Use of incandescent lighting of thc titration vessel rather than fluorescent lighting facilitated observation of the violet to blue color change. By using either the visual or pot
