Evaporation Error in Volume Fractionation Chromatography CH.4RLES I)I.4DER, ..igricirltural Chemistry Research, Oklahoma A . and M . College, Stillwater, Okla., AND GEORGE MhDER, JR., Dewe.y, Okla.
s VOLUME fractionation chromatography, evaporation of the
1 eluent upon leaving a column is a source of error that may
cause inaccurate and irreproducible results. The significance of the evaporation error varies with the vapor pressure of the eluent, temperature and its variation during collection, the rate of flow, antl :tny change of the eluent. SIGNIFICAYCE
Previous fractionating mechanisms as thoseof Brimlej and Snou ( I ) , Gibson ( S ) , Grant and Stitch ( 4 ) , Mader and LIader ( 5 ) , and Stein and Moore (6) allow evaporation of the eluent. The importance of evaporation error in using such a fractionating mechanism was investigated. The method used was the partition chromatography of organic acids with increasing butanol in chloroform, described by Donaldson, Tulane, and Liarshall ( 3 ) . Collected 5-mI. fractions of eluent, when almost pure chloroforni.
Figure 1. Jlerctiry Pipet Switch
were found t o represent actually from 6 to 8 mi. of eluent, varying with rate of f l o ~ and temperature, As the butanol conceritration increased, the collected 5-ml. fractions more closely represented the actual volume of eluent. Less evaporation occurred because butanol has a lower vapor pressure than chloroform. The eluent loss from evaporation was sometimes as high a3 one third the total passed through the column. It is apparent that unless the evaporation error is eliminated, wcurate or reproducaible results will not be obtainablc This may explain sonic of the divergent data in the literature. ELIMINATION
The elimination of evaporation necessitated the re lwign of the pipet switch of fraction collectors presently in operatioil. The pipet switches were used with the collection mer-lianism of 1Iader antl L3ader ( 5 ) . The pipet switch of Grant and Stitch ( 4 ) ,wit,ha merrury s\\-itc.h controlled by the static pressure of the liquid above i t , \\-as retiesigned t o prevent evaporation by the addition of a female ground-glass joiiit fitted on 3 male ground-glass joint itt the Iwttom of the column and by the addition of a sidc iwm above the liquid level. This side arm was connected by tubing t o a suction flask containing the solvent as shown in Figurc I . This arrangement saturates the air which passes into the pipet witch when it empties and prevents evaporation of the eluent. This piFct switch arrangement is simple, inexpensive, and easy
U 'Sal.'
'ICM.t
,
Figure 3.
'6 CM!
Figure 2.
'I cM.b
Capacity Pipet Switch
Circuit Diagram of Capacity Controller
CI, Cz. 0-100 mmfd. comp. type eondenser Ca. 47-mmfd. condenser G . O.OIi-rnfd., 600-volt condenser Ca. 0.1-mfd., 600-volt condenser Cs. 8 mfd 450-volt condenber Cy, Ca. 10 L f d . , 450-volt condenser CP. 20-mfd 450-volt condenser LI. Potter 2nd Brumfield Type L M5 5000-ohm relay LI. Oscillator coil, 2.5-millihenry, radio-frequency choke, center tapped RI. 4.?-megohm, carbon resistance Rz. 1-megohm 1-watt resistance Rg. Malloy No.'M2MPX Pot, 2000 o h m Rt. 47.000-ohm. 2-watt resistance Rs. 470,000-ohm, 2-watt resistance Re. 5000-ohm, 10-watt resistance Ri. 10,000-ohm, 10-watt resistance Rs. 2000-ohm. 10-watt resistance Ro. 20,000-ohm, 10-watt resistance Rio. 40 000-ohm, 10-watt resistance VI. VaZuum tube 6SQ7, numbers are for bottom view Vz. Vacuum tube 2050, numbers are for bottom view Va. Vacuum tube 6 x 4 , numbers are for bottom view T. Merit P-6010 power transformer M . Leads t o collection mechanism I . Coaxial cable t o inner plate of pipet switch 0. Coaxial cable t o outer plate of pipet switch
1556
V O L U M E 2 5 , NO. 10, O C T O B E R 1 9 5 3
1557
to construct. However, contact of the eluent with a mercury surface is often undesirable. Readjustment of the mercury contart was found to be necessary with 5-ml. fractions when the eluent density changed. With larger fractions (20 ml. or greater) the mercury differential should be such that one adjustment will tw sufficient unless the density variation is very large. The pipet sxitch of Gibson (S), activated by a change in capacitance, is a more universally adaptable although somewhat more expensive form of a pipet switch. This capacity pipet switch o as redesigned to eliminate evaporation by the addition of glass joints, side arm, and suction flask in the same manner as the pipet first described (Figure 2). For greater sensitivity, a plate of copper or platinum foil instead of the single wire (3) was use 1 for the inside electrode. The outside plate n’as of copper foil. The resulting increased surface area of the electrode and the improved circuit increased the sensitivity to the extent
that liquids as nonpolar as chloroform will activate the switch. Any increase of polarity thereafter will activate the switch a t a lower liquid level. The improved, more sensitive circuit developed for this pipet switch is shown in Figure 3. The components cost about $40. Similar capacity controllers are sold commercially for about $200. LITERATURE CITED
(1) Briniley, R. C., and Snow, A., J . Sci. Instr. and Phys. zn Ind.,
26, 73 (1949). (2) Donaldson, K. U., Tulane, V. J., and Marshall, L. &I., AKAI.. CHEM.,24, 185 (1952). ( 3 ) Gibson, A. R., Chemistry & Industry, 1951, 185. (4) Grant, R. X.,and Stitch, S. R., Ibid., 1951, 230. (5) Mader, Charles, and llader, George, Jr., ANAL.CHEM.,25, 1423
(1953). (6) Stein, W. H., and Moore, S., J . BioE. Chem., 176, 337 (1948).
RECEIVED May
1 1 , 1953.
Accepted June 15, 1953.
Electrolytic Determination of Copper with an Isolated Anode DUNCAN G . FOSTER Swarthmore College, Swarthmore, Pa.
IN
THE electrolysis of copper by controlled cathode potential methods, a frequent difficultyarisesout of thereoxidation of the cuprous ion a t the anode, substantially slowing the procedure. Oxidation of deposited copper from circulation of liberated oxygen also occurs, causing results by this method to run 0.4 to 0.5% high. In 1947 Diehl(1) reported a series of experiments in which he attempted to obviate these difficulties by placing the anode inside a porous cup. His results were promising, but he was unable to obtain adequate stirring with the motor-and-propellor type stirrer which he used, and abandoned the method as calling for apparatus too complicated to be worth while. The writer has succeeded in obtaining satisfactory results with this method by the simple expedient of using a magnetic stirrer of the type now widely sold [as was also done by Lingane (311, and by the considerable simplification of the apparatus which this allowed. The apparatus is shown diagrammatically in Figure 1, and details are fully described in t h e legend a t t a c h e d . The potentiostat u s e d h a s been described elsewhere (a).
I
Z Y
A . 26-mm. o.d., borosilicate glass tubing, 30 c m . long B. Lucite cell cover C. 250-ml. electrolytic beaker D . Rubber collars E. Alundum thimble: Norton Co., R.A. 84, m e d i u m porosity, cut to length of anode F. Platinum gauze cathode, 35 X 50 mm. G. Platinum gauze anode. 15 X 50 mm. H . Magnetic stirring bar. 1 inch I . 250-ml. leveling bulb J . Saturated eafomel electrode K . 4-mm. diameter soft glass tube with anode sealed into lower end
Table 1. Duplicate Analyses of Brasses Samplea Pb content, % Cu found, %
Mean Manufacturer’s ralue
1 0.90
2 0.94
3 5.32
4 5.88
5 6 7 7 . 8 3 11.10 19.36
60.07 70.85 78.01 76.36 82.14 80.96 69.18 59.96 70.82 78.02 76.43 82.14 81.12 69.22 60.02 70.84 78.02 76.40 82.14 81.00 69.20 60.00 70.78 78.01
76.39 82.16
81.00 69.04
PROCEDURE
A stock solution of an anolyte was made up, consisting of 135 ml. of concentrated hydrochloric acid and 14 grams of hydrazine dihydrochloride per liter. The brass samples were dissolved in 15 ml. of 2 t o 1 hydrochloric acid with the aid of 5 ml. of 30% hydrogen peroxide (4). After boiling out the excess hydrogen peroxide, the solution was cooled, and 1 or 2 grams of urea were added. Anolyte was introduced into the leveling bulb, the weighed cathode was clamped in place, and the level of liquid in the vertical tube adjusted to approximately 25 cm. above the top of the cathode. The beaker containing the solution of brass was then set in place over the electrodes and supported by the magnetic stirring motor. The solution was diluted until its level was about ‘ / z inch below the top of the cathode. With the control set a t -0.22 volt us. the saturated calomel electrode (S.C.E.), the stirrer was started, and the current was raised until the controls operated. This took place a t a current of from 4 to 8 amp., depending on the concentration of salts in the solution. The instrument was left to operate automatically until the current had dropped t o 2 amperes, which took from 5 to 8 minutes. During this time there mas very little deposition of copper, and the color of the solution gradually disappeared. The “up” control was then turned off, and the control potential was set a t -0.30 volt us. S.C.E. The instrument was left to reach this value of its own accord, after which the “up” control was turned on again and the electrolysis continued until the current had fallen t o below 10 ma. and had become essentially constant. RESULTS
Table I shows a series of duplicate determinations of copper in brasses varying in lead content from less than 1% to nearly 20%. Half-gram samples mere taken. Results are both precise and accurate within about 2 parts per thousand or better, for the samples studied. Oxidation appears to be negligible as shown by the light pink color of all
