A Polarograph Reading Directly in Percentage Analysis of Lead Arsenate B. P. CALDWELL AND SOLOMON REZNEK’ Polytechnic Institute of Brooklyn, Brooklyn, N. T.
When the conditions can be established so as to obtain a polarographic “wave” or “step” such that the upper and lower levels are essentially parallel to the voltage axis and to each other, a manually operated polarograph may be so manipulated as to read directly in terms of percentage. A polarograph capable of being so operated was constructed. A continuously variable shunt, with coarse and fine adjustments, is required in place of the usual step-
wise shunt. The apparatus consists of three inexpensive radio-type potentiometers, a voltmeter, and a wall-type galvanometer with lamp and scale, together with battery and dropping mercury electrode. The technique of using a manually operated polarograph so as to give direct percentage readings is described and an application to the analysis of commercial lead arsenate insecticides is given.
sorting-out method. The chief usefulness in the met,hod would lie in the determination of relatively minor constituents, present in the order of 1 per cent or less, with a precision equal, in such cases, to most chemical methods,
H I L E automatic polarographs ( 1 ) are convenient and even essential for many types of work, the basic principles of polarography may often be usefully applied by means of relatively simple manually operated devices. These, on the other hand, may range from the precision research instrument of Kolthoff and Lingane (4) to the very simple arrangements of Muller (7) or of Petering and Daniels (9). Neither the automatic nor the manually operated polarographs are designed to read directly in terms of percentage of a given substance. Quantitative determinations are based on (1) the measurement of the wave height of the full wave, as obtained automatically or plotted from manual readings, (2) the increase in current between two voltage settings selected after an examination of the full wave, the “increment” method, or (3) the proportional increase in wave height caused by the addition of a known concentration of a particular substance to a solution containing an unknown concentration of the substance, the “internal standard” method. Since the observed wave heights depend, among other things, on the constants of the galvanometer used, each of these methods requires a calibration curve: a series of standards, or a calculation of some sort to determine the concentration from the wave height. While Kolthoff and Lingane (6) describe a shunt circuit which allows the observed diffusion current to be read directly in microamperes, it is believed that a circuit designed to read directly in terms of percentage of a given substance would further simplify routine analyses. To use such a circuit i t is assumed, as in the increment method, that the diffusion current is directly proportional to the concentration and that the wave form is reasonably close to the ideal-i. e., the two levels are parallel to one another and cover an appreciable voltage interval. The stepwise construction of the Ayrton shunt usually employed precludes exact adjustment of the galvanometer*beam to a predetermined point on the scale. On the other hand, if the shunt consists of a continuously variable resistance, the galvanometer scale should be susceptible of being set to read directly in terms of percentage. The instrument described was so constructed. The application to the analysis of lead arsenate insecticides is intended merely as an illustration of the technique of using the instrument. Normally, the lead content of such a product is determined more precisely by chemical methods. I n this application the polarographic method serves as a rapid Present address, Penna. I
Apparatus The shunt (arranged in an Ayrton connection, so that the total resistance across the galvanometer is constant) consists of two General Radio Company potentiometers arranged in tandem (Figure 1). A fixed resistance is inserted in the galvanometer circuit to bring the total resistance up to the critical damping resistance of the galvanometer employed. A third General Radio Company potentiometer serves to deliver any desired potential from a battery, which may consist of dry cells or a 4- or &volt storage battery. The applied potential, as indicated by the Weston Model 301 voltmeter, V , may be adjusted t o 0.02 volt, which is sufficientlyclose for most applications. In more precise instruments the voltage is determined by a potentiometer balanced against a standard cell. While a lowresistance voltmeter (most cheaper types) may show some deviation from the true reading, this is immaterial in practical work, since standards and unknowns are electrolyzed at the same dial (voltage) settings. The galvanometer was a Leeds Bt Korthrup 2239 D type, with a period of 28 seconds, a sensitivity of 2 X ampere per mm. per meter, and a critical damping resistance of 11,500 ohms,
FIGURE1. DIAGRAM OF CIRCUIT R I . 9000-ohm R2. 2000-ohm 20-ohm Ra. 50-ohm R4.
U. S. Food and Drug Administration, Philadelphia, 187
fixed resistance hlodel 314-A General Radio Co. potentiometer hlodel 214-A General Radio Co. potentiometer hlodel 214-A General Radio Co. potentiometer
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
188
OF GALVANOMETER SCALEIX TERMS OF TABLE I. CALIBRATION PERCENTAGE
Voltmeter Reading Volts 0.4 0.5 0.6C
1.1 1.2 1.3 1.4 1.5 1.6 1.7
a
Blank=
-0.8 0.0 0.0 2.2 2.8 3.5 4.0 4.5 6.1 12
O.Ol’%
Pb
. ..
0.0 0.0 86.0 93.5 98.2 100.2 101.1 103.5 108.0
Scale Readings0.01% O,Ol5% Pbb Pb
...
0.0 0.0
...
0.020% Pb
...
0.025% Pb
...
...
0.0 0.2
...
0.0 0.2
0.0 0.5
l02:5 104.0 105.3 106.7 110.2
148:5 153.5 155.1 157.0 158.8
198 201 205 208 211
244 248 251 255 259
... ...
... ...
20 cc. of foundation solution diluted t o 50 cc. 104.0 a t 1.4 volts t o allow for blank. Readings between 0.6 and 1.1 volts omitted t o condense table.
b Solution set t o read o
With a fixed resistance of 9000 ohms, the total resistance of the shunt circuit was 11,020 ohms, thus leaving the ga!vanometer slightly overdamped. The galvanometer was used with a Leeds & Northrup lamp and scale, No. 2100. A small (10- to 15-cc.) beaker was used as the electrolysis cell. No attempt was made to protect the contents from air, since the sulfite present was sufficient to take care of any oxygen diffusing into the solution for a considerable period. A large calomel electrode of a simple type ( 3 ) served as an anode; the dropping mercury cathode consisted of a 10-cm. piece of “marine barometer” tubing (Corning) +ached to a 60;cm. length of heavy-walled rubber tubing previously washed with alkali, acid, and water to remove talc and surface sulfur. With the particular capillary and head of mercury used, the drop rate in distilled water, under zero voltage, was 4.0 seconds. An on-off single-pole snap switch serves to disconnect the battery.
Procedure To employ the instrument so that a reading is obtained directly in terms of percentage, the following procedure is employed : The approximate range of concentrations a t which a most nearly ideal step is obtained must first be ascertained. At too low concentrations the residual current, due to traces of oxygen, impurities, “condenser current”, etc. (4, approaches the diffusion current of the ion being determined. At too high concentrations the characteristics of the curve become such as to make i t less useful for quantitative comparisons. The slope becomes flatter, so that the step occupies too great a voltage range and the upper plateau is no longer parallel to the lower. In the present case, for example, i t was found that concentrations of lead up to about 0.01 per cent were suitable. The step, beginning to rise above 0.6 volt, flattened out at 1.4 volts, and did not rise appreciably until about 1.9 volts. While even this is greater than the usual range of the polarographic step, i t is satisfactory where a single constituent is being determined in the absence of interfering substances. A detailed plot of the current voltage curve for 0.01 per cent lead showed an apparent half-wave potential of about -0.9 volt as compared with -0.755 reported by Muller and Petras (8) and -0.82 by Hohn (9). The difference is largely accounted for by the relatively large i R drop. The galvanometer deflection of 100 scale divisions, a t an approximate ampere. sensitivity of amounts to a current of 2 x Assuming a solution resistance of 5000 ohms (6) the iR drop is of the order of 0.1 volt, which is to be subtracted from the applied voltage. If half-wave potentials are desired they are determined from the voltmeter readings by making correction for iR drop, and anode potential, if necessary. In actual analysis the voltmeter readings are convenient dial settings to indicate that the same applied voltage has been used for standard and unknown.
Vol. 14, No. 2
At lead concentrations around 0.01 per cent i t is necessary to remove dissolved oxygen, normally present in solution to the extent of about 8 p. p. m. This was conveniently accomplished by the use of podium sulfite and by working in alkaline solution to prevent precipitation of lead sulfite. Alkali, sulfite, and Methocel (Dow brand of methyl cellulose, grade M-361, here used as a “maximum” suppressor) were incorporated in the foundation solution. By using the same dilution of this solution throughout, the composition was kept constant except for the concentration of the material being determined. Temperature control was not found necessary, other than to allow the solutions, previously made to volume in volumetric flasks, to stand at room temperature in the vicinity of the polarograph together with the standards. As a reading takes but a few minutes by the method described, the samples and standards may all be run within a time interval during which the room temperature is not likely to change appreciably. If a change of more than 0.5’ C. is anticipated, some sort of bath for temperature control may be necessary. Samples (0.500 gram) are dissolved in 50 cc. of 5 per cent sodium hydroxide with the aid of heat if necessary, and made to 500 cc. To a 5-cc. aliquot are added 20 cc. of foundation solution (10 cc. of 5 per cent sodium hydroxide, 2.5 cc. of 1 per cent Methocel, and 2.5 cc. of 10 per cent sodium sulfite made to 20 cc.) and the sample is made t o 50 cc. The final dilution thus contains 0.01 per cent of sample, 1 per cent of sodium hydroxide, 0.05 per cent of Methocel, and 0.5 per cent of sodium sulfite. The proportions of the constituents of the foundation solution may be allowed to vary somewhat, but the volume of solution added to sample and standard must be accurately measured (pipetted), so that the concentrations are the same in samples and standards. The foundation solution is stable except for the sulfite, which should be prepared daily. Five to 10 cc. of a standard containing 0.01 per cent of lead, prepared bv addition of foundation solution to a suitable volume bf standard lead nitrate solution, are poured into the beaker serving as electrolysis cell, the galvanometer scale is set at 0 with the voltmeter reading 0.5 volt, and the coarse and fine shunts are manipulated so that the 0.01 per cent solution gives a reading of approximately 100 on the scale a t a voltage setting of 1.4 volts. The standard solution is removed, the cathode and agar-salt bridge are connected to the anode washed with water, and a beaker containing foundation solution, diluted in the same manner (20 to 50 cc.) is introduced. With the shunt settings as before, the displacement of the galvanometer due to the residual current in the range 0.5 to 1.4 volts is determined. The 0.01 per cent lead standard is re-introduced, after rinsing the electrodes by immersion in a beaker containing the solution to be measured, the galvanometer is set a t 0 at 0.5 volt as before, and the voltmeter is again set a t 1.4 volts. The fine adjustment is then manipulated so that the reading at 1.4 volts is 100, plus the correction due to the residual current (Table I, column 4). Subsequent readings are then directly in terms of percentage, after subtracting the scale divisions equivalent to the residual current.
a
TABLE11. LEADCONTENTOF LEADARSENATEINSECTICIDES P b Content -Scale Readings-0.5 1.3 1.4 b y Chemical volt volts volts Analysiao 70.1 0.0 74.2 74.8 64.3 0.0 64.0 59.8 63.5 64.0 59.6 0.0 59.4 0.0 62.0 63.2 58.5 0.0 60.9 61.3 93 , Gravimetric chromate method. b From scale reading a t 1.4 volts by subtracting residual current. Sample 23 25 33 72
1.5 volts 75.1 64.9 64.2 63.8 61.7
P b Content from Scale Readingb 70.8 60.3 60.0 59.2 57.3
4.0 units t o correct f o r
K i t h shunt settings undisturbed, the sample solutions were measured with the results shown in Table 11. The samples consisted of lead arsenate and basic lead arsenate insecticides, previously analyzed for lead content by chemical assay by W. J. Kirby, Insecticide Division, Agricultural Marketing Service. After subtracting the blank (4 scale divisions a t 1.4volts) the readings are compared in terms of lead content with the chemical analysis.
ANALYTICAL EDITION
February 15, 1942
The zero setting is checked at the end of each reading t o compensate for galvanometer drift. Another variation t o be allowed for is the oscillation of the galvanometer beam due t o the changing size of the mercury drops. With a long-period galvanometer this amounts t o about one scale division under the above conditions, but the amplitude of swing is very constant and i t is satisfactory t o take either the maximum, the minimum, or the middle of the swing for each reading, so long as a consistent practice is followed. It is seen (Table I) that after setting the shunt so that the reading at 1.4 volts in column 4 represents 0.01 per cent of lead, after subtracting the blank, the subsequent readings at the same voltage are directly in terms of percentage u p t o concentrations of about 0.025 per cent when the proportionality between wave height and concentration begins t o fall off. Within this range the precision is within 2.5 per cent and somewhat better than this at the lower concentrations.
189
Literature Cited Heyrovskj., J., Phil. Mag., 45,303 (1923). (2) Hohn, H., "Chemische Analysen mit dem Polarographen", Berlin, Julius Springer, 1937. (3) Kolthoff, I. M., "pH and Electro-Titrations", p. 94, New York, John Wiley & Sons, 1931. (4) Kolthoff, I. M., and Lingane, J. J., Chem. Rep., 24, 1 (1939). ( 5 ) Kolthoff, I. M., and Lingane, J. J., "Polarography", NewYork, Interscience Publishers, 1941. (6) Muller, 0. H., Chem. Rev.,24, 67 (1939). (7) Muller, 0.H., J . Chem. Education, 18, 111 (1941). (8) Muller, R. H., and Petras, J. F., J . Am. Chem. Soc., 60, 2990 (1938). (9) Petering, H. G.,and Daniels, F., Ibid., 60, 2796 (1938). (1)
PRESENTED before t h e Division of Analytical and Micro Chemistry a t t h e 102nd Meeting of t h e AUERICANCHBMICALSOCIETY,Atlantic City, N. J . From a dissertation submitted by S. Reznek t o t h e graduate faculty of the Polytechnic Institute of Brooklyn in partial fulfillment of t h e requirementu for t h e degree of doctor of philosophy, June, 1940.
Micromethod for Identification of Volatile Liquids Vapor Pressure, Boiling Point, and Olefin Content of Cyclobutane and cis-2-Butene SIDNEY W. BENSON, Mallinckrodt Chemical Laboratories, Harvard University, Cambridge, Mass.
A
PREVIOUS paper described a micromethod for identi-
fying hydrocarbons by their physical properties ( I ) , using the vapor pressure of the material as one of the properties for identification. I n the apparatus vapor pressure measurements were limited to a small range of vapor pressures, from 1.0 t o 30.0 mm. of mercury, because Of the small size of the sample. Since then a n apparatus has been devised t o measure vapor pressures u p to the boiling point on samples as small as 3 cc. of gas at normal temperature and pressure. This apparatus has been incorporated into the previous system, with some minor changes. As a further aid in the identification of liquids, an adaptation has been made of a bromination titration to determine the olefin content.
Apparatus The vapor pressure apparatus constitutes the only essentially new addition. The assembly is shown in Figure 1. A capillary manometer is connected to a bulb through capillary tubing, all of 2-mm. diameter. The total volume is estimated at 1.8 cc., so that a t the boiling point a gas sample of 3 cc. at S . T. P. would be only 60 per cent in the vapor state. The manometer has the left end joined to a three-way stopcock, so that it can be pumped out with a vacuum pump and the absolute pressure read directly, or it can be opened to the air and the pressure read against the barometric pressure. This method enables one to use a manometer of only half the barometric length, and still cover a range of from 0 to 1000 mm. The manometer is connected to the system through a three-way stopcock and a Toepler pump. The sample can be distilled into bulb C1 (Figure l ) , using liquid nitrogen, or it can be pumped into the bulb with the Toepler pump. Once it is in the bulb the leveling bulb is raised so that the mercury acts as a cut-off to keep the system free of stopcock grease in which the hydrocarbons dissolve at the higher pressures. Measurements made on cis2-butene using the stopcock showed that as much as 8 per cent of the hydrocarbon dissolved in the stopcock grease in the course of a determination. Dewar flasks containing freezing mixtures at various temperatures are placed around bulb C1 containing the sample and the vapor pressure is read. Because of the small volume of the
system, equilibrium is established very quickly, usually in 5 to 7 minutes, and it is possible to make a complete set of readings up to the boiling point in about 45 minutes. Freezing mixtures used were absolute methyl alcohol (-96" C.), solid carbon dioxide in acetone (-78.5' C.), chloroform (-67" c.1, m-xylene (-55" c.),cyclohexanone (-45" C.), mercury (-38.9' C.), carbon tetrachloride (-22' C.), ice-salt (-100 c.),ice ( 0 . 0 ~c.),benzene (5.5" C.), ethylene bromide (10.0~ C.), and water from 10" C. to room temperature, These baths keep their temperatures to within 3" C. over a period of a
TABLE I. DEXSITIES AND MOLECULAR WEIGHTS Compound Ethylene Cyclobutane
cis-2-Butene
Temperature
c.
.Molecular Weight
Density G./cc. 0.540
-78.5 -96.0 -80.0
0 , 5 3 4 (8)
28.0 2 8 . 0 (theoretical)
...
-78.5 -67.0 -46.0 -37.0 -26.0 0.0 0.0
0,802 0.788 0.761 0.731 0.715 0.698 0 . 7 0 3 (4)
55.6 56.0 (theoretical)
-78.5 -78.5
0.724 0.711a
56.0 56.0 (theoretical)
0.560
... ... ... ... ...
Extrapolated from data of C o f b and Maass (3).
a
TABLE 11. VAPOR t
c.
-78.5 -66.6
-48.0 -38.9 -22.5 -11.0 0.0 2.7 3.5 5.5
PRESSURES O F
CiS-2-BUTENE
P (Author)
P (Kistiakowsky)
Mm.
Mm.
6.9 16.1 60.9 107.5 234 413 659 730 752 813
6.5 20.1 62.0 106.8 251 420 658 730 751 816
Deviation
%
+- 2 06. 0. 2 1.8 +- 0.7 6.8 +- 001 ...027 +- 00.3. 1
