Sodium Borohydride Reduction and Polarographic Determination of Tin DENNIS H. EVANS Department o f Chemistry, Harvard University, Cambridge, Mass;
Sn(lV) is quantitatively reduced to Sn(ll) b y sodium borohydride in boiling 1M sodium hydroxide. The reduction i s incomplete in 5M sodium hydroxide and some metallic tin i s formed when the sodium hydroxide concentration i s less than 0.2M. Sn(lV) can b e determined i n the presence of CI-, As(V), As(lll), Bi(lll), Cu(ll), Pb(ll), and Zn(ll) b y prereducing Sn(lV) to Sn(ll) with borohydride, then determining Sn(ll) via its cathodic polarographic wave. Gelatin shifts the halfwave potential of both the anodic and cathodic waves of Sn(ll) in 1M sodium hydroxide. The half-wave potentials at low gelatin concentrations approach -0.89 volt and - 1.1 5 volts vs. S.C.E. for the anodic and cathodic waves, respectively.
R
of tin compounds by sodium borohydride has been studied mainly in acid solutions where stannane and distannane can be formed (8, 18). This communication reports the characteristics of t,he reduction in alkaline solutions. Condit’ions are established which permit the quantitative reduction of Sn(1V) to Sn(I1) in 1W NaOH by sodium borohydride. Because Sn(1V) is not reduced a t the dropping mercury electrode in alkaline solutions ( I O ) , the polarographic determination of tin in such media is difficult. The tin must first be prereduced to Sn(I1) which has bot,h an anodic and a cathodic wave in alkaline solutions ( I O ) . I n the proposed method for tin analysis, sodium borohydride is used to effect the prereduction. X a n y polarographic methods of tin analysis have been reported ( 1 , 7 , 12, f 4 ) , most of which use acid-supporting electrolytes. EDUCTION
EXPERIMENTAL
Apparatus. T h e polarograph was a Leeds and Northrup Electro-Chemograph with an auviliary polarizing unit. .ill current measurements were made with the damping set a t “galvanometer equivalent.” For the polarographic determination of tin a
special apparatus which served both as a reduction vessel and polarographic cell was devised. This apparatus was comprised of a round.bottomed 100-ml. three-necked flask fitted with a 14/20 T stopper and a 110-mm. straight condenser in the outer joints. The center neck was fitted with a 14/20 to 10/30 adapter and a 10/30 stopper. A 4-mm. 0.d. nitrogen inlet tube was inPerted through the condenser. -1fter boiling under reflux, the test solution was cooled and the 14/20 stopper was replaced by a reference electrode probe sealed in a 14/20 inner joint. The reference electrode was a 70-cm., length of 1-mm. diameter silver wire which u-as coiled around the glass probe. The silver wire was amalgamated by dipping in mercury. The center adapter is just large enough to accept the 6-mm. o.d. capillary of the dropping mercury electrode. A mercury stand tube with automatic m-determiner (11) was used. The capillary used in this work had a drop time of 3.93 seconds and an m-value of 1.54 mg. per second at - 1.50 volts us. S.C.E. and an heft of 48.2 em. in a solution of 1mM stannite in 1 M NaOH and 0.1JI sodium borohydride plus 0.005% gelatin. Reagents. Analytical reagent grade chemicals were used u-henever possible. Standard Sn(1V) solutions were prepared determinately by dissolving 0.05-mole metallic tin in 25 ml. of 1 2 M HCl. Chlorine gas was bubbled through to aid t h e dissolution, after which 60 ml. of 12J1 HC1 were added and the solution mas heated just, t o the boiling point to expel chlorine. T h e cooled solution was diluted to 1 liter. Two grades of sodium borohydride (Metal Hydrides, Inc., Beverly, Mass.) were used. The lower grade (98%) dissolves in 1JI XaOH leaving a faint white turbidity which disappears after boiling. The analytical grade (99fojo) produces a clear solution. Solutions 0.1Jf in either grade of sodium borohydride and l.li in S a O H produce an anodic polarographic wave a t --0.65 volt us. S.C.E.This wave occurs a t the same potential as the second wave observed by 1Iarshall and Widing (16) in the first report of the polarography of sodium borohydride. Gardiner and Collat ( 5 ) have shown t,hat t,his m v e is due to the oxidation of an
intermediate formed in the hydro1 of borohydride. The main anodic wave for borohydride occurs a t -0.32 volt us. S.C.E. in this medium ( 1 7 ) . Procedure. A known volume of standard or test tin solution was transferred to a volumetric flask and diluted with 1M KaOH. Then enough solid sodium borohydride was added to provide concentrations ranging from 0.1 to 1M and the solution was made up to the mark with 1 M NaOH. The solution was transferred to the reduction vessel and boiled for 15 minutes under total reflux. I t was then cooled to 25.0’ i 0.1’ C. while being swept continuously with nitrogen. The solution was made 0.005% in gelatin, the reference electrode and dropping mercury electrode were inserted, the nitrogen stream was diverted over the surface of the test solution, and the polarogram was recorded. The diffusion current was measured 0.3 volt cathodic from the half-wave potential for the reduction of stanniteLe., itt -1.5 volts us. S.C.E. ( I O ) . The silver amalgam reference electrode is depolarized by the osidation of Sn(I1) to Sn(1V). The major precaution which must be observed is the scrupulous removal of oxygen. The common practice of scavenging dissolved oxygen from alkaline solutions by adding sodium sulfite was not satisfactory. When the borohydride reduction of Sn(1V) was carried out in 1 X NaOH and 0.1Ji S a 2 S 0 3 and all manipulations were performed in the presence of air, the diffusion current for Sn(I1) was 30y0low because of air osidation. Even when the reduction was carried out in a deaerated vessel and the cooled solution was pumped under nitrogen into a previously deaerated polarographic H-cell, low results were obtained when the tin concentration was below 0.5mJZ. For these reasons the apparatus and procedure described above were devised so that transfer of the solution after reduction would not be necessary. RESULTS AND DISCUSSION
Reduction of Sn(1V). K h e n a 1 m M solution of Sn(1V) in 1.11 IZ’aOH and 0.1M sodium borohydride i2 prepared, no reaction occurh. Ai polaro\IOL. 36, NO. 13, DECEMBER 1964
2435
gram shows only the anodic waves of sodium borohydride and no cathodic waves because Sn(1V) is not, reducible a t the dropping mercury electrode in t'his medium ( I O ) . However, if the solution is heated to about 60' C., Sn(1V) is reduced to Sn(I1). Sn(I1) was detected by its two polarogralihic wayes, an anodic wave resulting from its oxidat,ion to Sn(1V) and a rathodic wave correPponding to its reduction to tin amalgam ( I O ) . Fifteen minutes' boiling is sufficient to ensure that all Sn(1S') is reduced to Sn(I1). The reduction terminates a t the +2 oxidation state. Yo metallic tin is formcd. Table I summarizes diffusion current data obtained with solutions of various initial Sn(1V) concentrations. The Sn( 11) concentrations were calculated from the known diffusion current constant for Sn(I1) in 1J1 KaOH, 3.45 Fa. second l'*,~m.lfnig.*j3 (fO),the observed diffusion current corrected for residual currcnt and the capillary characteristics, m and t. The residual current was obtained by measuring the current in deaerated, a t -1.5 volts vs. S.C.E.
Table I. Completeness of Reduction of Sn(lV) to Sn(ll) by Sodium Borohydride
1M XaOH, 0 1M NaBH4, 15 minutes' boiling, zd measured at -1.5 volts us. S.C.E., 0.0057, gelatin Sn(IV) Sn(I1) taken, found, Difference, mdf (2 mL?-f 0 100 0 099 -1 0 0 495 0 491 -0 8 0 991 0 995 +0 4 4 98 5 07 $1 8 9 96 10 03 +0 7 Av. = f O . 9
Table II. Determination of Tin in the Presence of Other Elements
1M SaOH, 0.2M XaBH4, 15 minutes'
Tin taken, mM 0 9915 0 991 0 991 0 991' 0 991 0 991 0 991
boiling, 0.0057' Other element, mM CI-, 25 As(V), 100 Zn(II), 10 Pb(II), 4 As(III), 100 As(III), 10 Sb(III), 20
gelatin Tin found, Error, mM o/o 0 995 +0 4 0 990 -0 1 1 00lb +1 0 0 979' -1 2 0 0 987 -0 4 0 807 -19 Ob
0 9(Ed Bi(III), 10 0 WP Cu(II), 100 0 1I1 NaBH4 30 minutes' boiling
0 962 0 989
-3 3 -0 6
1 0.11 8aBH4 Supporting electrolyte 0 5M sodium tartrate, 10.11 NaOH Diffusion current constant for the reduction of Sn(I1) = L 68 d
2436
ANALYTICAL CHEMISTRY
boiled, 1-11 N a 0 H with 0.1.11 sodium borohydride plus 0.005% gelatin. The close agreement between t,he concmtrations of tin taken and those found serves to confirm the previously deterniined diffusion current constant of Sn(I1) in 1:11 NaOH and to demonstrate the quantitat,ive character of the reduction of Sn(IV) to Sn(I I) by sodium borohtdride Polarographic Determination of Tin. T h e quantitative reduction of Sn(1V) to Sn(I1) can be made the basis of a polarographic method of tin analysis. I k a u s c Sn(1V) is not reducible a t the dropping mercury electrode in alkaline solutions, it must be prereduced to Sn(I1). The d a t a of Table I demonstrate that sodium borohydride reduction of Sn(1V) followed by polarographic anal stitutes an accurate metho analysis in the concentration range of to 10-2A11. The precision diminishes a t lower concentrations because the residual current (0.23 pa, at - 1.5 volts us. S.C.E.) is then responsible for the major part of the observed current. met,hod can be adapted to the of mixtures of Sn(IT') and n alkaline solution. First the Sn(I1) concentrat,ion is determined by polarographic anal solution. Then the total tin concentration is measured by the borohydride reduction procedure. Acidic tin solutions may be analyzed after t,he addition of sufficient, KaOH to neutralize the acid and to make the solution 1-11in base. To test the general applicability of the method, solutions of known tin concentration were analyzed in the presence of large escesses of ot,her elemen&. The data are summarized in Table 11. The elements tested can be divided into two groups: those which are not reduced by borohydride and those which are. I n the first group As(V) and C1- can be tolerated without modifying the procedure. When Zn(I1) is present in a 10-fold molar excess, a longer reduction time is required to ensure complete reduction of Sn(1V). When a 100-fold molar excess of Zn(1I) was used [50mM Zn02-*, 0.5mM Sn(OH)G-2],the reduction of Sn(1V) was only 87y0 complete after boiling 1 hour in lJ1 sodium borohydride. I n these polarograms, the zinc wave [Ei.2 = -1.53 volts LIS. S.C.E. (IO)] follows the Sn(I1) reduction wave. Lead tends to interfere in two ways. Because it is not reduced by borohydride and because its polarographic wave precedes that of Sn(II), large quantities will interfere by obscuring the Sn(I1) wave. Second, the presence of lead, like zinc, inhibits the reduction of t,in. However, tin can be determined in the presence of a fourfold molar excess of lead when the reduction is prolonged to
30 niinut,es and 111 sodium borohydride is used. In the second group, A\s(III) i b reduced to a dark brown precipitate of elemental arsenic. -1 100-fold molar excess of As(1II) interferes because the reduction of the arsenic is incomplete and no Sn(1V) is reduced. This might possibly be remedied by a longer reduction time and higher borohydride concentration. Arsenic can be easily oxidized to the +ti state where it does not interfere. h 10-fold inolar excess of As(II1) can be tolerated. The polarographic measurements were made with the finely divided arsenic precipitate in susiiension. Although the supenbion was opaque, the polarographic wave of Sn(I1) was not noticeably affected. Sb(II1) is reduced to elemental antimony when 1 J I S a O H solutions are boiled \vit,h sodium borohydride. When an analysis of tin \vas carried out in the presence of a 20-fold molar excess of Sb (111), the polarographic diffusion current obtained was 19y0lo~v(Table 11). This current could have resulted from the reduction of Sn(I1) or unreduced Sb (111) because their half-wave potentials are almost equal. However: after 30 minutes' boiling no polarographic \Taw at all was obtained. I t was concluded that the tin had been taken down with the black antimony precipitate because none could be found in the supernatant, solution. Thus., Sb(II1) constitutes a serious interference in tin anal) method. Sb(V) is also reduced to a black precipit'ate so it would also probably interfere. Most, other metallic elements are quite insoluble in alkali so they would not be found normally in alkaline tin solutions. I t might be thought' t'hat acidic samples of these elements with tin could be analyzed by neut'ralization, removal of the precipitated oxides or hydrous oxides, and analysis of t,he solution for tin. However, tin is extensively coprecipitated with metallic oxides so such a separation would be impractical. Instead, these elements were retained in solution with a 0.551 sodium tartrate, 1.011 NaOH supporting electrolyte. The diffusion current constant for Sn(I1) in this media was 2.68. This was determined by a borohydride reduction of 1mM Sn(1V) using the same procedure used to obtain the data in Table I. This compares with a diffusion current constant of 2.86 obtained for Sn(I1) in 0.5.11 sodium tartrate, 0.1.11 XaOH (13). When tin was determined in the presence of Bi(III), the bismuth-tartrat'e complex was reduced to a black precipitate which coagulated leaving a clear supernatant in which the Sn(I1) was determined polarographically. I n the presence of copper another complication appeared. The deep blue tartrate
complex of copper was reduc6d to a reddish-brown precipitate by borohydride. Cnlike the red cuprous oxide precipitate obtained by the reduction of the copper tartrate complex by sucrose, this precipitate did not redissolve in hydrochloric acid. It was thus probably mostly metallic copper. The metallic copper precipitate catalyzes the decomposition of sodium borohydride. After 15 minutes' boiling of 100 nil. of a solution initially 1mM in Sn(IV) with 0.1M Cu(II), 0,lM sodium borohydride, and 1. O M KaOH, the borohydride was completely destroyed and Sn(1V) was not reduced. T o circumvent this problem, the copper was first reduced in a separate flask by boiling for 15 minutes with borohydride. The precipitated copper was then separated by decantation and washed with water and 1.0111 KaOH; the qolution of Sn(1V) was treated as before with the result shown in Table 11. Many other element; could be retained in solution by tartrate complexation and Sn(1V) might be determined in their pre5ence by analogous procedures. The data of Table I1 demonstrate that tin can be determined in the presence of a variety of elements. However, the number of procedural modifications required is almost equal to the number of elements tested, a circumstance which detracts from the general analytical promise of the proposed method. The method may find specific application for tin analysis in samples where the other constituent elements are limited, but its general applicability is inferior to other polarographic methods ( 1 , 7 , 11,1 4 ) . General Features of Sodium Borohydride Reduction of Tin. T h e reduction of Sn(IV) to Sn(I1) by borohydride is quantitative in 1M N a O H . This reduction does not proceed further producing metallic tin, in spite of the potentials of t h e following couples :
BOzE"
+ 6 H 2 0 + 8e = BH,- + 8 0 H =
-1.24 volts
+
US. N.H.E.
Sn(OH)6-2 2e = HSn0230HH20 E o = -0.93 volt US. X.H.E. HSnOZ-
E"
+
(20)
+
(3)
+ HzO + 2e = Sn + 3 0 H =
-0.91 volt
US.
X.H.E.
(3)
The reduction of Sn(IV) to Sn(II), though thermodynamically favorable, is a very slow process a t room temperature. I t proceeds a t a significant rate only when the solution is heated. The reduction of Sn(I1) to the metal must be an even slower reaction. The rates of these reactions depend upon the concentration of hydroxide ion. Though the reduction of 1mM Sn(IV) is complete after 15 minutes' boiling in
-!
dnadls W o n
-I
+-+---&-1.2
EM ~ 0 1 1 sVI. S&E
Ek
YOltS V I .
-1.3
S.C.E.
Figure 1. Effect of gelatin on halfwave potentials of Sn(II) waves in 1M N a O H x = d a t a of Llngone ( 1 0 ) for 0.01 % gelatin
1 M XaOH, 0.1M sodium borohydride, less than 10% of the Sn(IV) is reduced to Sn(I1) in 5-lf NaOH after the same treatment. On the other hand, a t lower alkalinities the reduction of Sn(1I) to the niet'al can become significant. Solutions of 1 m X Sn(IV) and 0.1N sodium borohydride were prepared with various concentrations of sodium hydroxide. The solutions were boiled for 15 minutes. ,4 0.5M NaOH solution remained clear but a 0 . 2 N XaOH solution developed a dark brown t,urbidity and with 0.111 NaOH a large quantity of gray precipitate formed and coagulated into a ball. The solution was cooled, the precipitate was removed, washed and dried, and it was then pressed into thin plates which had a metallic luster and high electrical conductivity. The precipitate is t,hought to be mostly metallic tin. The supernatant cont'ains more dissolved tin because slow acidificat'ion produces much more precipitate. I n general, when alkaline solutions of tin which contain borohydride are slowly acidified a t room temperat'ure, the tin is reduced to the metal. However, when such solutions are added to strongly acidic solutions, good yields of stannane can be obtained (?, 1 7 ) . Thus, Sn(1V) can be successfully reduced to Sn(1I) only over a narrow range of sodium hydroxide concentrations. The reduction is incomplete when t,he sodium hydroxide concentration is too large, and some of the Sn(I1) is reduced t,o metallic tin when the alkalinity is too low. -4 sodium hydroxide concentration of 1.V is optimal for the procedure used in this investigation. Another reaction of interest is the disproport,ionation of Sn(I1) to give Sn(IV) and metallic tin. It is commonly stated that, Sn(I1) is unst'able with respect to disproportionation in alkaline solutions (3, 9 , 19). Yet in this study, solutions of Sn(I1) in 1-11 XaOH were stable a t both room temperature and the boiling point. I n fact, the proposed method depends on the stability of Sn(I1).
Actually, reports of the disporportionation of Sn(I1) pert'ain to hydroxide ion concentrations greater than 2.5X (2, 4)and/or to equilibration times of several months (4). Saturated solutions of Sn(l1) in 5mJZ to 1JI NaOH were stable for 20 days a t 25" C. (6). The report, of Foerster and Dolch ( 4 ) that Sn(I1) disproportionates on long standing in 111 S a O H seems untenable because their reported Sn(I1) concentrations exceed the solubility (6). Thus, it is more accurate to say that Sn(I1) slowly disproport,ionates in solutions several molar in hydroxide ion. Disproportionation in 1JI YaOH has not been established. Because of the st,oichiometry of t,he disproportionation reaction, the equilibrium favors Sn(l1) at low total tin concentrations such as those used in this investigation. This suggests that metallic tin itself might be a useful reductant for Sn(IV) in alkaline solutions, provided that the reaction betxeen metallic tin and water was slow.. However, when deaerated 1 m J l Sn(IV) in 111 S a O H wac: equilibrated with metallic tin a t room temperat,ure, no detectable Sn(I1) was formed in 1 hour and the Sn(l1) concentration after boiling for 15 minutes, 3 X 10-5X, was no greater than that obtained when 1M S a O H was boiled with metallic Sn for the same length of time. Therefore, whichever reaction is thermodynaniically favored under these conditions, the disproportionation or the reverse process, that, reaction iq slow even a t the boiling point. Effect of Gelatin on Half-Wave Potentials. I n addition to the supression of maxima, gelatin can also exert other influences on polarographic waves. Half-nave potentials of both the anodic and cathodic waves of Sn(I1) in lJ1 N a O H were changed when the gelatin Concentration was increased, the anodic wave becoming less negative (more anodic) and the cathodic wave becoming more negative. A Sn(I1) solution was prepared by dissolving enough SnClz.2H20 in deaerated 1 M HCI to make a 0.0500M solution of Sn(I1). Two niillilit,ers of this solution were transferred to 100 ml. of deaeratrd 1-11 IiaOH. This solution was pumped under nitrogen into a standard polarographic H-cell. Polarograins were then recorded after successively larger quantities of gelatin were added. The inaxima of both waves were just suppressed by 1.5 X lOp4Yc gelatin. The data are presented in Figure 1 where ElIzis plotted z's. the logarithm of the gelatin concentration. So t,heoretical significance is attached to this form of presentation. I t is used simply for the sake of convenience. The drop time at the highest gelatin concentration was only 7ycless than the VOL. 36, NO. 13, DECEMBER 1964
2437
drop time a t the lowest concentration so that the shift in El,e caused by a change in drop time is negligible compared to the observed effect. At low gelatin concentrations, the half-wave potentials approach -0.89 volt us. S.C.E. for the anodic wave and -1.15 volts us. S.C.E. for the cathodic wave. These values should much more reliably reprebent the true half-wave potentials than those reported by Lingane (IO) who used 0.01% gelatin. The diffusion current constant of both the anodic and cathodic waves decreases slightly with increasing gelatin concentration. This is of no significance analytically because a constant gelatin concentration would normally be used. The decrease is 47, between gelatin concentrations of 5 X and O . O l ~ o . Thus, this system should be placed in group IT.’ of the classification system of Meites and Meites (16)
regarding the effects of gelatin-i.e., the half-wave potentials are shifted and the diffusion current constant is decreased by gelatin. LITERATURE CITED
(1) Allsopp, W. E., Damerell, T.’. R., ANAL.CHEM.21, 677 (1949). ( 2 ) Ditte, A., Ann. Chim. et Phya. 27, [ 5 ] , 145 (1882). ( 3 ) Ephraim, F., “Inorganic Chemistry,” 5th ed., p. 469, Interscience, Yew York, 1948. ( 4 ) Foerster, F., Dolch, M., Z. Elektrochem. 16, 599 (1910). ( 5 ) Gardiner, J. A., Collat, J. W., J . Am. Chem. SOC.,86, 3165 (1964). ( 6 ) Garrett, A . B., Heiks, R . E., J . Am. Chem. Soc. 63, 562 (1941). ( 7 ) Geyer, R., Geissler, X , 2. Anal. Chem. 201, 1 (1964). ( 8 ) J$y, W. L., J . Am. Chem. SOC.83, 33s (1961). ( 9 ) .La:jmer, W. SI., “Oxidation Potentials, 2nd ed., p 148-50, PrenticeHall, Englewood 8iiffs, N . J., 1952.
(10) Lingane, J. J., IND.ENG. CHEM., ANAL.ED. 15, 583 (1943). (11) Z b d , 16, 329 (1944). (12) Lingane, J. J., Zbzd., 18, 429 (1946) (13) Lingane, J . J., J . A m . Chem. SOC. 65, 866 (1943). (14) Love, D. L., Sun, S. C., ANAL.CHEM. 27, 1557 (1955). (15) hlarshall, E . D., Widing, R. A , , AECD-2914, June 1950. (16) Meites, L., Meites, T., J . A m . Chem. SOC.73, 177 (1951). (17) Pecsok, R. L., Ibid.,75, 2862 (1953). (18) Schaeffer, G. W., Emilius, Sister M., Ibid., 76, 1203 (1954). (19) Sidgwick, N . V., “The Chemical Elements and Their Compounds,” 1‘01. 1, p. 621, Oxford University Press, London. 1950. (20) Stockmayer, W. H., Rice, D. W., Stephenson, C. C., J . A m . Chem. SOC. 77, 1980 (1955).
RECEIVEDfor review July 31, 1964. Accepted September 24, 1964. Division of Analytical Chemistry, 148th Meeting, ACS, Chicago, Ill., August 1964.
Auto matic Ion Exclusion-Parti tion Chromatography of Acids G. A. HARLOW and D. H. MORMAN Shell Development Co ,, Emeryville, Calif.
b A method has been developed for the automatic separation and determination of acids by ion exclusionpartition chromatography. Acids are separated on a column of a sulfonic acid ion exchange resin using only water as the eluent. The effluent is titrated automatically and the volume of titrant required is recorded as a function of time. The resulting integral curve indicates the quantity and possible identity of the individual acids. No pretreatment or regeneration of the column is required. Emergence data for over 50 acids of widely differing structure are presented relative to acetic acid. Apparatus which can be assembled from commercially available components is described.
D
ESPITE
THE
RECENT
ADVANCES
in nonaqueous titrimetry, many acid mixtures are still encountered which cannot be resolved by this means. Chromatographic methods have proved to be useful in separating many such mixtures. However, liquid chromatographic methods have not been widely used because the procedures which have been published usually involve the tedious collection and analysis of numerous individual fractions, a technique which is not very satisfactory for either routine or exploratory use. 2438
ANALYTICAL CHEMISTRY
This paper describes a method based on ion exclusion and partition chromatography, followed by automatic titration, which has proved to be very useful during the last four years in the resolution of mixtures of water soluble acids. Data are provided on the separability of various acids, and a procedure and automatic apparatus are described to make such separations convenient. Wheaton and Bauman (11) first described ion exclusion separations. They also reported the application of the techniques of ion exclusion chromatography to the separation of nonionic materials (10). Simpson and Wheaton ( 7 ) examined the effects of varying the column parameters on column efficiency. However, ion exclusion has been described principally as a unit operation (1, 6, 9, 11) for process work. Analytical applications have been alluded to, but little information has been published to suggest what separations can be achieved by ion esclusion and by the same techniques applied to partition chromatography other than the deionization of nonelectrolytes. Wheaton and 13auman (11) reported complete or partial resolution of the various pairs of acetic, chloroacetic, dichloroacetic, trichloroacetic, and hydrochloric acids, excepting the pair acetic-chloroacetic which were not separated despite their great difference in ionization constants. Reichenberg
(4, however, subsequently found that acetic acid could be almost completely separated from n-butyric acid although their pK’s are almost identical. Thompson and Morris (8) investigated the separation of salts from amino acids by ion exclusion and extended the technique to the separation of amino acids from one another. Buchanan and Markiw (2) recently described the separations of a number of amino acids by what they called water elution chromatography on ion exchange resins. The method described here consists of separating the acids on a sulfonated polystyrene ion exchange resin in the hydrogen form using only water as the eluent. The column effluent, is titrated automatically and the volume of titrant required is recorded as a function of time. The resulting integral curve gives the amount of each acid present and the emergence time indicates its possible identity. The method has a number of attractive features to reconinlend it and compares favorably with gas chromatography in some respects. The procedure is extremely simple since it consists of passing the acids through a short column of ion exchange resin using only water as the eluent. S o pretreatment of the resin is required since it is used in the acid form as supplied, and the resin may be used repeatedly without regeneration since, ordinarily, no ion exchange
