1119
V O L U M E 27, NO. 7, J U L Y 1 9 5 5 Table 11.
Coulometric Determination of Copper
(Volume of solution was generally 100 to 150 ml., and of mercury cathode was 30 to 35 rnl.) Supporting ECu s . Electrolyte, S.C.E., hleq. hleq. Error, 1 41 Volts n Taken Found 72 +0.29 2 9 476 9 503 0 50 -0.28 9 476 9 451 -0.02 7 103 7 105 -0.07 4 73‘1 1 733 +0.03 4 737 4 736 2 352 +O.l6 2 349 -0.04 1 4183 1 4178 0 7073 10.04 0 7070 -0.09 0 4672 0 1668 t0.07 0 2373 0 237.5 - 0 15 0 07546 0 07534 4 734 - 0 08 HCl -0 10 1 4 738 1 1745 1 1748 + O 03 9 198 +O 13 - 0 50 2 9 476 2 348 -0 04 2 349 “3 - 0 75 2 7 105 7 089 - 0 10 NHL I 0 7070 0 7068 -0 03 HClOa -0 50 2 18 96 18 96 +O 2 349 2 356 - 0 13 3Iean - 0 01 I O 09
Table 111. Extrapolation to Zero Current (Electrolysis of 6.482 meq. of Cu’+ in I50 ml. of 1M HCI E = 0.46 volt 1’s.S.C.E.; l i ~ g= 35 ml.) Register Elapsed Reading Extrapolated Time, ( 1 Count = Counts per Reading 0.001 Meq.) Minute hh. 0 29230,Z 20 35364 46 165 22 26 3.5529 24 35529 (24/22)(165) 88.0 11.6 = 35709 0 30 36617.0 12.4 35617 0 4- ( 1 2 . 4 / 1 1 . 6 ) 45.5 6.0 (88.0) = 35711.1 35 3,j662.0 0.4 35662.0 (6.4/6.0)(45 6 ) 22.8 2.9 = 35710.5 40 33684.8 3.3 35684.8 ( 3 . 3 / 2 , 9 )( 2 2 . 8 ) 11.6 1.6 = 35710.7 4.3 3X98,3 1.7 35598 3 (1.7/1.6)~11.5~ = 35710..1 3Iean extrapolated value 35710 4 I0 . 5 Expected final reading 33712
+
+ + +
Error,
%
-0.023
of this time, and washing, drying, and weighing of a solid electrode were entirely eliminated. Moreover, the total time required could be decreased considerablJ by employing an extrapolation method for estimating the last few per cent of the quantity of electricity consumed in an electrolysis. This technique, n hose theoretical foundation is identical with that of the MarSevin and Bahcr procedure ( 1 3 ) , is illustrated by
the data in Table 111, which were secured during the electrolysis of a 1 M hydrochloric acid solution containing 6.482 meq of cupric copper. With the 1-ohm input resistor used, a total of 6482 counts should have been recorded. Sineteen and a half minutes after the beginning of the electrolysis-the time was chosen solely for symmetry in constructing the table, and, as is not true of the MacSevin and Baker technique, is of no importance in the calculations-when about 5% of the original amount of copper remained undeposited, the register was read and again was read exactly 1 minute later. During this 1-minute period 46 counts were recorded. This is practically equal to the instantaneous counting rate a t a register reading equal to the mean of the values at 19.5 and 20.5 minutes, or 35364. The measurements were repeated a t 24.5 and 25.5 minutes, and the counting rate then was found to be 24 counts per minute a t a register reading of 35529. Consequently 35529 minus 35364 ( = 165) counts were required to decrease the counting rate (which is proportional to the electrolysis current) from 46 to 24 counts per minute. T o decrease the counting rate to zero, an additional (24/22) X 165 ( = 180) counts would be required. From this the final register reading would be expected to be 35529 180 (=35709), which corresponds to a total of 6479 counts and to an error of only -0.05%. The remaining data were treated in exactly the same way, with the results shown in the last column of Table 111. Results of this technique are accurate to within a few counts with a saving u p to one half of the total time required for an analysis, and a controlled potential coulometric analysis may be carried out within about 0.5 hour and with an average precision of j=O.l%.
+
LITER4TURE CITED (1) Buzeell, A., a n d S t u r t e v a n t , J. >I., Rev. Sci. Instr., 19, 688 (19 4 8 ) . (2) Diehl, H., “Electrochemical Analysis with G r a d e d C a t h o d e P o t e n t i a l C o n t r o l , ” p. 29, G. F. S m i t h C h e m i c a l Co., Columbus, Ohio. (3) Hickling, A., T r a n s . F a r a d a y Soc , 38, 27 (1942). (4) Lingane, J. J., “Electroanalytical C h e m i s t r y , ” Interscience, N e w Y o r k , 1953. (5) I b i d . , pp. 192-5. ( 6 ) Ibzd., pp. 263-4. (7) I b i d . , pp. 269-72. ( 8 ) Ibid.. DD. 329-31. i9) Ibid., 355. (10) I b i d . , p. 359. ( 1 1 ) L i n g a n e , J. J., J . Am. C‘hem. Soc., 6 7 , 1916 (1945). (12) Lingane, ,J. J., Swain, C. G., a n d Fields, M., I h i d . , 65, 1348 (1943). (13) M a c S e v i n , W. M., a n d B a k e r , B., . ~ N A L .CHEX.,24, 986 (1952). (14) Meites, L., I b i d . , 24, 1618 (1952). (15) P a g e , J. A . , Ph.D. thesis, H a r v a r d University, 1954. (161 PzebellBdy, L . , a n d Soniogyi, Z., Z . anal. Chem., 112, 313 (1938j.
p:
RECEIVED for review October 1 1 , 1964. .4ccepted April 18, 195.5. Presented before the Division of Analytical Chemistry a t the 126th Meeting of the A h i ~ ~ r c CHEMICAL . 4 ~ QOCIETT, S e w York. September 1954. Contribution 1255 from Department of Clieniistry, Tale Uni\-ersity, N e x Haven, Conn.
High Frequency Combustion-Volumetric Determination of Carbon in Metals EDWARD L. SIMONS, JOHN E. FAGEL, JR., EARL W. BALIS, and LEONARD P. PEPKOWITZ G e n e r a l Electric Co., Schenectady,
N. Y.
An examination has been made of the variables present in the high frequency combustion-volumetric method for the determination of carbon in metals, and methods of controlling or correcting for them are discussed. The experimental work was done with the commercially available Lindberg equipment. The standard deviation for a single determination has been established as one division of the Lindberg gas buret scale, which corresponds to 0.005% carbon for a 1-gram sample when the gas is measured at 20” C. and 760 m m . of mercury.
T
HE convenience of high frequency induction heating devices for the determination of carbon in metals arises from the fact that they permit rapid combustion of the sample in a total volume of oxygen (about 500 cc.) which is small in comparison with the volumes used in more conventional combustion equipment. It therefore becomes possible to substitute a rapid volumetric anal) sis of the combustion products for the gravimetric determination of carbon dioxide generally employed. A previous publication (6) described a precision determination of carbon in metals using a modified Lindberg high frequency
1120
ANALYTICAL CHEMISTRY
combustion furnace and volumetric carbon determinator ( 5 ) . Replicate analyses on seven standard steels were reported with a mean error of 0.004% carbon and a lower limit of precise measurement of 0.0201, carbon with a 1-gram sample. [The standard deviation estimated from the data is 0.006% carbon ( I ) . ] During continued use of similar Lindberg equipment in other laboratories of this company errors were frequently observed which were larger than those expected on the basis of the original work. This led to a re-examination of the method, and after careful consideration had been given t o the measurement of and correction for certain variables in the process it became possible to operate the equipment consistently with a standard deviation of 0.00570 carbon. A4ccurate volumetric gas analysis requires adequate control over the variables of temperature and pressure, or application of corrections where control is not feasible. What constitutes "adequate control" will be determined both by the accuracy desired and by the volume t o be measured. In the Lindberg volumetric carbon determinator the amount of carbon diovide in the combustion products is calculated from the difference betxveen two gas volumes (both about 500 cc.), one measured before and the other after absorption of the carbon dioxide by a potassium hydroxide solution. As will be shown in subsequent calculations, 0.005% carbon, the desired standard deviation, is equivalent to 0.10 cc. of gas. The experimental problem, therefore, is that of measuring the difference between tTvo gas volumes of about 500 cc. with a standard deviation of 1 part in 5000. Translating this into its equivalent in temperature and pressure control, a change of 0.10 cc. in the gas volume can be produced by a variation of only 0.05" K. between initial and final buret readings in the neighborhood of 300" K., or by a pressure variation of slightly more than 0.1 mm. of mercury in the neighborhood of 1 atmosphere. Such variations have been observed during the operation of the equipment in this laboratory. In a conventional Orsat gas analysis apparatus the total volume measured is 100 cc. or less, so that a proportionally larger relative error in the measurement can be tolerated. Furthermore, the geometry of the Orsat buret permits more effective thermostating of the gas by the water jacket than is posqible with the Lindberg buret. Finally, for precision gas analysis the Orsat buret is readily fitted with a compensator to eliminate the effects of barometric and temperature fluctuations VARIABLES IN GAS VOLUME >IEASURERlE\T
The variables involved in the operation of the Lindberg volumetric carbon determinator may be discussed in terms of Figures 1 and 2. Temperature. The thermometer originally mounted in the buret has been replaced by one graduated in 0.1' K. inrrements, H . It is read immediately after each measurement of gas volume. Pressure. Three factors can produce a pressure variation. BAROMETRIC F ~ u c ~ u . 4 ~ 1 o sThe s . barometer is read after each measurement of gas volume if there is any indication of rapid changes in this variable. TEMPERATURE FLUCTUATIONS. h temperature change of 0.1" K. will change the vapor pressure of the leveling fluid by slightly more than 0.1 mm. of mercury. LACK OF INTERNAL PRESSURE EQUILIBRIUM. This is the major source of pressure variation, and, fortunately, is the one sourre which can be eliminated. The gas is returned from the potassium hydroxide absorption vessel t o the buret by applying pressure t o the atmospheric side of the potassium hydroxide vessel through the opening, K (6). The inertial effect of this gas transfer is to produce, a t the moment of seating the potassium hydroxide check valve, I (or a t the moment of bringing the potassium hydroxide t o a constant level mark below the check valve), a pressure gradient in the system. If the stopcock, C, is closed immediately u on seating the check valve, the pressure of the gas thus isolatefin the buret will be slightly greater than that in the manifold between valve and stopcock.
0
E
Figure 1. Diagram of modified Lindberg volumetric carbon determinator '1
B
c.
D
E. F. G.
H. I. .I K. L.
.If.
s.
Dust filter and sulfur dioxide absorbent
Bnhhlrr
i&&ay buret stopcock is buret .aduated portion of gas buret reling bulb tnd bulb ierinonieter, 0.1' C'. ciaduations Check valve Potassium hydroxide absorption pipet Opening of potassium hydroxide absorption pipet Stopcock Tube filled with alternate layers of Ascarite and indicating Drierite Tube through which pressure can be applied t o seat the potassium hydroxide pipet check valve
This effect may be eliminated by adding to the volumetric system a stopcock, L, at the atmospheric side of the potassium hydroxide vessel. After the gas has been transferred to the buret and the check valve seated, stopcock L is closed and stopcock C kept open. This Kill maintain the check valve in position and also permit internal pressure equilibrium to be established between the manifold and buret. K i t h stopcock L closed the attainment of equilibrium may be facilitated by slowly raising and lox-ering the leveling bulb a fen- times until a constant buret level is attained. After equilibration, stopcock C is closed and the volume in the buret measured. each determination the manifold is brought back t o atmospheric pressure by appropriate manipulation of stopcock C while stopcock L remains closed. Water Vapor Equilibration. The vapor pressure of the potassium hydroxide absorption solution is less than that of the buret leveling fluid by about 4 mm. of mercury (2). If the buret has been thoroughly dri-filmed (General Electric silicone product, Dri-film 9987) a prohibitively long time is required for the attainment of water vapor equilibrium after the gas is returned to the buret from the potassium hydroxide absorption vessel (6). Calculations recently published by Stein and Reid ( 7 )show that the gas in a buret )\-hose sides are not ITetted by the leveling fluid does not become saturated with water vapor even after 4 hours, whereas 2 to 3 seconds suffice if the buret walls are covered by a film of liquid. If the buret has been scrupulously cleaned, smooth drainage and reproducible buret readings can be achieved, and the buret can be kept in this condition if the liquid level is always kept just below the buret stopcock, C, when the unit is not in use. The leveling bulb itself should also be kept covered t o prevent dirt contamination. Wlth these precautions it has been possible t o maintain, without further cleaning, excellent drainage characteristics in the present buret, which was thoroughly cleaned in December 1951. Drainage. With a clean buret reproducible readings can be obtained with a drainage period of 30 seconds. Gas Solubility. The leveling fluid used (acidified 20% aqueous sodium sulfate) is one in u-hich oxygen and carbon dioxide have
V O L U M E 27, N O , 7! J U L Y 1 9 5 5
1121
very low solubilities. S o difference in readings of gas volumes was noted between runs using fresh leveling solution and solution through which oxygen had been bubbled for 1 hour.
If, however, the final temporaturc and pressure differ from those a t which the gas was collected, the observed AV is not proportional to ncol. It must be corrected m follows: AV,,,,.
The correction of a measured gas volume far changes in temperature or pressure is ordinarily a simple and straightforward gas law calculation. In the Lindberg volumetrio carbon determinator, however, only part of thc buret is graduated, and that part in an arbitrary scale showing per cent carbon. Thus, no direct measurement is made of either the final or initial volumes of gas. Before the ga8 law corrections of Equations 3 and 4 b e low can be applied, additional data and calculation are necessary.
=
AV0ia, f AVi
(3)
in which AVJ is the change in Vi which would result upon chituging gas temperature and pressure back to their original values:
AVi
=
( V ~ h s. (Vi)nbs.
(:)
(E)
(4)
In order t o apply t,hcse corroetions the following data are needed: I. p.--vapor pressure of leveling solution a t any temperature. This may bo estimated with sufficient accuracy for these edmlat.ions by interpolation of data available in the literature on the vapor pressures of water ( 5 ) and of sodium sulfate solutions and sulfuric acid solutions (t). 2. Relationship between AV &ii read from the buret in units of per cent carbon and AV in units of cubic centimet.ers. When the carbon dioxide obtained by burning 1gram of a 1% carbon sample is measured a t a barometric pressure of 760 mm. of mercury (0" C.) and a temperat.ure of 20" C. the (AV), = 1.000. AV-. for the above conditions e m be obtainod from Equation I; it is 20.46 cc. 20.46 CC. = 1.000% 1.00 cc. = 0.049% 3. Values for V
