LITERATURE CITED
Table II.
Band
I 1.9 1.7 1.8 8.2
AV.
100
Parenamine Developed at
25" C.
Cm. from Point of Application I1 I11 IV 5.1 12.9 3.4 4.9 12.9 3.3 12.9 3.4 5.0 58.5 15.5 22.7
as a rigorous test of the ability of the paper t o separate closely related substances and because separation and identification of amino acids in complex mixtures normally require several working days. The three basic amino acids are readily identified. The order of separation, however, does not follow the order of p K for the amino group nor the sequence found b y Moore and Stein (9) but rather the sequence obtained by Hirs, Noore, and Stein (S), in \vhich lysine has a greater R, value than histidine. As there is almost complete overlap of tyrosine and phenylalanine, the aromatic group can be identified, but the components
V 19.2 19.6 19.4 88.4
cannot be distinguished. Leucine has the smallest Rf value in the neutral and acidic amino acid group, so that R, values greater than that for leucine identify this third group.
Dean, R . B., Dixon, K . J., ANAL. CHEX 23, 636 (1951). Hale, D. K., Chem. R. I n d . (Londoii) 1955, 1147. Him. C. H.. &Toore.s..Stein. \v. H.. J.'BioZ. Chem. 195,669 (1952). ' Hoffpauir, C. L., Guthrie, J. D., Ibid., 178, 207 (1949). Hoffpauir, C. L., Guthrie, J. D., Tezttle Research J . 20, 617 (1950). Kember. K . F.. Kells. R. A . , LVatw-e Lautsch,
w:>LTaneclie,
G., Broser,
W,, 2. Suturforsch. 8b, 232 (1953).
Lederer. ll,, ,4nal. C h ~ m .Ictu 12, 122 (1955). Noore, S., Stein, W. H.. J . Biol. Chem. 192, 663 (1951).
ACKNOWLEDGMENT
The author xishes to express his gratitude for advice and encouragement to Frederick C. Xachod, SterlingM3nthrop Research Institute, and Robert A. Osteryoung, Rensselaer Polytechnic Institute. Thanks are also expressed to Dita Froelich for her careful preparation of the figure.
Partridge, S. M. Ibid.. 48. 313 (1951). r-r, I aL L L l U l j C ,
U.
_*I., Y
.
' L " . L L J ,
AL.
L
.)
Pepper, K. IT., Ibid., 46, 334 (1950). (15) Partridge, S. M., \Irestall, R. G., Ibid., 44, 418 (1949).
RECEIVEDfor revien- 1 I a y 22, 1957. Accepted October 16, 1957,
Potentiometric Titration of Halide Mixtures A. J. MARTIN Polychemicals Department, Experimental Station,
b
To satisfy the need for accurate analyses of halide mixtures in mole ratios as high as 50 to 1, potentiometric titration with silver nitrate in dilute nitric acid was studied. The end point detection is based on the intersection of straight lines. The inaccuracies in individual halide analyses resulting from mixed salt formation are reproducible and predictable, and thus subject to correction. The correction factors for intersection-point data are calculated by statistical analysis using the Doolittle solution of partial regressions. The corrected inflection-point analyses show a standard deviation about 1.5 times greater than that of intersection-point data. Limits of detection of one halide in the presence of large amounts of another are determined for this method.
P
titration O f iodide, bromide, chloride, and some mixtures of these halides with silver nitrate was first reported in 1893 by Behrend ( I ) . I n 1920 Liebich pin-pointed the two problems which h a w prevented wide application of the titration. He reported that halide mixtures could be resolved only if the mole ratios were OTEKTIOMETRIC
E. 1.
du Pont de Nemours & Co., Inc., Wilmington, Del.
greater than 1 to 10 (4, 5, 9). At mole ratios less than 1 t o 10, the end points were difficult t o locate precisely. He observed that direct titration to inflection points gave positive errors for iodide and bromide, and a negative error for chloride (IO). These effects were attributed t o mixed salt formation-Le., the formation of solid solutions of the silver halides (7, I S , I S ) . Many improvements and modifications have been made to the direct titration of halide mixtures (2, 6,6, 9,11, la), but none has eliminated the main problem, that of inaccuracy in individual halide determinations in halide miutures. Levy described the use of an additive correction factor in microtitrations ( 8 ) , but the author found additive correction inapplicable to macrolerel titrations.
loned less than 0.005 mmole of chloride to diffuse through its tip in 4 hours. Materials. Standard solutions of potassium iodide, potassium bromide, and sodium chloride n ere made u p by weight from the pure dried salts t o be exactly 0.1OOO.Y. The silver nitrate solution was prepared from reagent grade silver nitrate and the concentration ma5 adjusted to 0.1.Y. The solution was accurately standardized by the Volhard titration of pure sodium chloride. Procedure. Known amounts of t h e halide solutions were pipetted into a 400-ml. beaker and diluted t o 150 nil. with distilled 1T-ater. S e x t . approximately 1.5J1 nitric acid solution was added dropnise until a p H of about 1 was attained with indicating paper. Too high a concentration of nitric acid was avoided as it might oxidize the iodide ion to iodine.
EXPERIMENTAL
Apparatus. The potentiometric d a t a were obtained using a Leeds and Northrup &lode1 7655 hydrogen ion potentiometer or a Beckman Model H-2 p H meter operated on the p H scale. The silver indicator electrode was a wire, 1.5 mm. in diameter, held in a rubber stopper and soldered to a n electrode cable. A Beckman No. 4970 calomel electrode served as a reference electrode. The calomel electrode al-
RESULTS AND CONCLUSIONS
Curve I of Figure 1 shows the iodide and bromide inflection points obtained when a solution containing 0.5 mmole each of iodide and bromide and 5.0 mmoles of chloride mas titrated. Curves I11 and IV show the first derivative curves of the two inflection points. (These two curves have different units on the ordinate to bring the heights to VOL. 30, NO. 2, FEBRUARY 1958
233
-0.20
1' . ' *
I
C.URVE
_I
4 z c W
g
1
'
-1
$ I
04
05
,
I
0.6
07
I
,x 08
09
I
10
-02
I
II
-0 I
MILLIMOLES O F SILVER NITRATE
Figure 1. Titration curve and first derivative curves for iodide and bromide in a halide mixture (0.500 mmole iodide, 0.500 bromide, and 5.000 chloride.
+ 0.08 volt)
Curve II displaced
g
m
12
48
50
52
54
56
58
60
\ c, o
I
v c
MILLIMOLES OF SILVER NITRATE
Figure 2. Titration curve and first derivative curve for a halide mixture (5.000 mmoles iodide, 0.500 bromide, and 0.500 chloride)
convenient levels.) Lack of symmetry The concentration of silver ion can in the iodide and bromide inflection be calculated as points causes the first derivative curves + - KsPAgI to be distorted, and quantitative inter[Ag 1 - _ _ (2) [I-] pretation is difficult. If the midpoint By combining Equations 1 and 2, we (point X ) of the first derivative curve (curve IV) w r e accepted as the end get Equation 3, which describes the concentration of silver ion a t any point rather than the volume a t which AE/A volunie is a t a maximum (point iodide inflection point in the presence of bromide. Y ) , this bromide analysis would be in error by about 10%. X is the approximate point usually chosen for the end point by visual inspection of the The potential of the silver-calomel original titration curve or from all but electrode couple as used in the titration the most carefully drawn derivative is given by Equation 4. curves. TITRATING TO PREDETERMINED E = +0.552 0.0591 log [Ag+] (4) POTENTIAL. Another method of end Therefore, the equation for the point detection has been suggested: potential a t the iodide inflection point t,hat of titrating to a predetermined in the presence of bromide can be potential (11, 12). This technique is obtained by combining Equations 3 and subject to several errors: The apparatus 4. This is shown in simplified form as must be very accurately standardized; Equation 5 . the presence of some salts is known to affect the potential of the silver elecE-Do
c1 0.171 0.221 NDn 0.315 0.371 9.743 9.864
condition--e.$., total volunie of titratcd iolution-must be reproduced prccisc,ly. Constarits for other coiiditioiis can be readily detcmiiiiird. LITERATURE CITED
(4) Iiolthoff, I. AI., Fiirman, K . H;: “Potentiometric Titrntionr, Kilev. S e w Tork. 1926. (5) Zbid., p i . 165-9. ’ (6) Zbid., pp. 148-50, 168-9. [ i )Kuster, F. W., 2. anory. t h e m . 19, 81 (1899). . (8) Levy, R., Compt. rend. 235, 882
(1952 ). \
( 1 ) Behrend, R.,2. p h y p i k . Chert!. 11, 466 ( 1893).
121 Dutoit. P.. von \I-eisse. G.. J . c h i ! ~ i . phys: 9 , ’ 5 i 8 (19ll).‘ ( 3 ) Goulden, C. H,,, ‘f?Iethoda of Statistical Alnalysis, 2nd ed.; Kiley, \
I
Se\V
York, 1!) 1
