ANALYTICAL CHEMISTRY
724 Table I are sufficient to account for the marked dissimilarity in behavior. Methylamines are ordinarily not adsorbed by mercuric oxide, hut possibly the presence of an ammonia adsorption complex promotes the retention of methylamines. The latter phenomenon does not invalidate the use of yellow mercuric oxide to remove ammonia from amines; however, each lot of mercuric oxide should be pretested for ammonia adsorptive capacity. The results obtained in this investigation contradict those reported by Leone (8)on methylamines. It has been suspected by others that the loss of methylamines and consequently erroneously high ammonia results (obtained by difference) were due to the familiar reaction of the amines with nitrous acid. The occurrence of this reaction was confirmed as described in the experimental portion of this paper, and it is difficult to accept the results described by Leone. -4possible explanation has appeared in recently published work ( 7 ) in which it was found that aliphatic primary amines do not react with nitrous acid below a pH of approximately 3. Leone does not supply data concerning the pH of his reagents and the authors have not had an opportunity to determine if quantitative separation of ammonia from methylamines can be obtained a t such low pH. The precipitation of the ammonia in the cobaltinitrite compley prevents decomposition of the ammonia by the nitrous acid and permits subsequent direct determination of the ammonia in the precipitate. Incomplete precipitation of ammonia occurs unless the solutions are kept cold; the necessity for cooling has been pointed out by Wagner et al. ( 1 2 ) . At temperatures between 1.5' and 18' ammonia losses as high as 5% have been noted and a t 30" the loss has been 15%. In the procedure described here the solubility of the sodium diammonium cobaltinitrite has been decreased by cooling without significantly affecting the solubility of the methylamine salts. Loss of precipitate on washing is eliminated by using a wash solut,ion saturated with the precipitate. When waah solutions not saturated with sodium diammonium cobaltinitrite are used, the recoveries of ammonia are usually between 1.5 and 3% low. As much as 130 mg. of ammonia may he determined quantitatively by the procedure given, but it is advisable that aliquots containing not more than 100 mg. of ammonia be taken for analysis. The ammonia forms a dense, finely divided precipitate and large
.
amounts decrease the speed of filtration. In s a m p k in whlch less than 5% of the total base is ammonia, the relative accuracy of the method decreases rapidly, but it is still useful for most purposes. At the 1% level the uncertainty is 0.25% absolute. Methanol and methylamines do not ordinarily interfere in the determination of ammonia by this method. As much as 70 mg. of monomethylamine, 400 mg. of dimethylamine, or 700 mg. of trimethylamine may be present without causing precipitation of the corresponding methylammonium cobaltinitrites. The determination of ammonia by this procedure requires perhaps 3 hours of an analyst's time and 6 hours' elapsed time. ACKNOWLEDG.MENT
The authors wish to express their appreciation to James Stroupe for the x-ray diffraction studies on mercuric oxide, to David Lents, Henry B. Jones, and TVilliam E. Scanlon for some of the analyses reported here, and to the many members of the Rohni & Haas organization who have aided with advice and suggestions. LITERATURE CITED
Bertheaune. J., Compl. rerid., 150, 1063-6 (1910). Briner, E., and Gandillon, J., H e l c . Chim. dcta,, 14, 1283-307 (193 1).
Egly, R. S.,and Smith, E. I:., Chem. Eng. Progress, 44, ( 5 ) ,38798 (1948).
Erdmann. C. C., J . B i d . Chem., 8, 41-65 (1910). Francois. M.. J . wharm. chim..(6) 25. 517-28 (19071. Fuks, N. A:, and Rapport,' h.A4.8 Doklady. Akad. .brad. S.S.S.R., 60, 1219-21 (1948). Kornblum, N., and Iffland, D. C.. J . Am. Chem. SOC., 71,213743 (1949).
Leone, P., Gazt. chim. ital., 33, 246-52 (1925). Miauch. K. G.. and Savchenko. A. Ya., Org. Chem. I d . U.S.S.R., 7, 24 (1940). Moore, E. K., Highberger, J. H., Kappenhoefer, R., and 0'Flaherty, F., J . Am. Leather Chemist's Sssoc., 26, 341-51 (1931 )
.
Rosin, G., "Reagent Chemicals and Standards." Znd'ed., S e w York, D. Van Nostrand Co., 1946. Wagner, C. D., Brown, R. H., and Peters, E. D., J . Ani. Chem. SOC.,69, 2611-14 (1947).
Weber, F. C.. and Wilson, J. B., J . B i d . Chem.. 35, 385-410 (1918).
RECEIVED October 10. 1950.
Carbon on Cracking Catalyst Determination by Combustion and Conductometry CECIL H. HALEI AND MARGIE N. HALE' Esso Laboratories, Baton Rouge, La.
WORK
recently reported by Schmitkons ( 4 )revealed that in the determination of carbon on cracking catalyst by combustion, more or less unburned coke may be entrapped. The amount of carbon entrapped depends on the combustion temperature, nature of the catalyst, and nature and quantity of coke. Schmitkons pointed out that in borderline cases of entrapment, a slow, continuing evolution of carbon dioxide, or "straggling," occurs. As an aid in the selection of proper burning times for individual types of catalysts, a method for following the progress of the combustion has been devised. It is based upon the change in conductivity of a solution of sodium hydroxide when carbon dioxide is absorbed (2). The replacement of highly conducting hy1
Present address, Southwestern Analytical Chemicals, 1107 West Gibgon
St., Austin, Tex.
droxide ions by less mobile carbonate ions results in a marked increase in the resistance of the solution. COz
+ 20H- +COa-- + H20
Because the conductivity of the solution can be measured in a fex seconds, the progress of the combustion can he followed closely. Presumably, a recording conductivity bridge would furnish a continuous record of the reaction. EXPERIMENTAL
Apparatus. The combustion apparatus consisted of an 800mm. y t z tube packed according to Lescher (3), and heated with t ree split-type heavy-duty furnaces. The first and second, furnaces were 125 mm. long and were operated a t 1100" and 1600 F.,respectively. The third furnace was 250 mm. long and wad kept a t 1600" F. The carbon dioxide formed by the combustion
e
725
V O L U M E 23, NO. 5, M A Y 1951 Considerable variation in the burning rates of coke deposited on cracking catalysts has recently been observed. Carbon is frequently entrapped during the combustion, the amount depending upon the conditions of burning. An analytical method that would follow the progress of the combustion for use in the study of burning rates was devised, based on the decrease in conductivity of aqueous sodium hydroxide as the carbon dioxide produced by the combustion is absorbed. As the conductivity can be measured at intervals of a few seconds, the progrees of the combustion can be followed closely. The conductometric method gave results for total carbon comparable to those obtained by the conventional gravimetric method, and has proved useful i r i the comparison of burning rates of catalysts and in the selection of optimum conditions of combustion for a particular catalyst.
Instead of a dehydrant bulb before the absorption cell, a but)bler was inserted to saturate the combustion gases and, thus, prevent changes in the water content of the absorbing solution. The bubbler contained a dilute solution of sulfuric acid of approximately the same ionic strength as the sodium hydroxide in the allsorption cell. Resistances were measured with an Industrial Instruments conductivity bridge. Model RC-BC.
u.
W
5 o,/l
004
008
012
016
020
024
NORMALITY OF N A O H
was absorbed in the cell illustrated in Figure 1. The cell is designed to hold 75 to 100 ml. and a fritted plate is used to disperse combustion gases through the absorbing solution. A stopcock i3 provided a t the bottom of the cell to allow removal of used solutions and a spherical joint inlet permits convenient addition of fresh solutions. Platinized platinum electrodes are located just below the fritted scrubber and are consequently unaffected by bubbles of scrubbing gases. The cell is water-jacketed t o maintain a constant temperature. The cell constant was 2.94 reciprocal cni.
45/50 GLASS
PL ELECTRODE Figure 1. Cell for Conductometric Determination of Carbon
Figure 2. Effect of Sodium Hydroxide Concentration on Calibration Factor in Conductometric Determination of Carbon
Concentration of Sodium Hydroxide. The effect of the UOIIcentration of sodium hydroxide on the sensitivity of the method, as measured by the change in resistance per milligram of carbon burned, is shown in Figure 2. These data were obtained by burning samples of a standard catalyst, t'he carbon content of which was accurately known, and absorbing the carbon dioxide in various concentrations of sodium hydroxide. As xould be expected, the sensitivity is much greater for low concentrations of sodium hydroxide. To provide an adequate escess of hydroxide ions, small samples were taken for analysis. As a compromise between maximum sensitivity and reasonably sized samples to ensure against depletion of sodium hydroxide, a concentration of 0.04 M and a sample weight of 0.2 to 0.3 gram wew chosen. Depletion of the sodium hydroxide should not excerd 50%.
Solutions of barium hydroxide and sodium carbonate were also investigated as absorbents but were found to be less satisfactory than sodium hydroxide. Excessive foaming was observed with the barium hydroxide, as also reported by Bennet, Harley, and Fowler ( 1 ) . Calibration. It is necessary to calibrate each batch of sodium hydroxide because the relationship between change in resistanr9 and milligrams of carbon burned is dependent upon the concentration. In these experiments, 3-gallon quantities of sodium h!,droxide were prepared and stored in borosilicate glass bottle.;, protected from the atmosphere by Ascarite absorbers. -4used cracking catalyst, the carbon content of which had beerr established as 3.50% by several hundred gravimetric analyses, was employed as a standard for calibration purposes. By meaiurement of the increase in resistance observed when knowii weights of this standard sample were burned, the relationship. milligrams of carbon per ohm change in resistance, wm established for each fresh batch of sodium hydroxide. This value was checked a t intervals of several days and was found to be constant for weeks. The relationship between milligrams of carbon burned and change in resistance of the sodium hydroxide was a straight line in the range of 0 to 100 mg. of carbon. A typical calibration factor found for a batch of approximately 0.04 N sodium hydroside was 0.0884 mg. of carbon per ohm change in resistance.
ANALYTICAL CHEMISTRY
126
Procedure. Pipet i 5 ml. of the calibrated sodium hydroxide into the absorption cell with oxygen flowing through the combustion tube and cell a t about 200 ml. per minute. Weigh 0.2 to 0.3 gram of catalyst into a porcelain boat. After measuring the resistance of the sodium hydroxide solution, remove the stopper from the front end of the combustion tube, and, by means of a wire, push the sample boat into the center of the first or second furnace, depending upon whether the combustion is to be made a t 1100" or 1600" F. Replace the stopper and ignite the sample at
shown in Table I and graphically in Figure 4 according to the method described by Youden (6). Assuming the errors by the standard gravimetric method to be small as compared to those by the proposed conductometric method, the -lope of the line was calculated by Youden's system of computations to be 1.016 * 0.011, and the intercept \\as calculated to be 0 This indicates the absence of a blank and that the conductometric method has a tendency to give 1 . 6 5 higher results than the gravimetric method. The standard deviation ( 5 ) of a single analysis was calculated to be *0.08wc carbon.
Table I. Comparison of Conductometric and Gravimetric Methods for Determination of Carbon on Catalysts Wt. % Carbon Sample
Gravimetric
Conductonletric
0.06 0.06 0.25 0.22 0,58 1.18 1.14 1.13 1.88 1.94 2.67 2.59 3.08 3.18 3.18 3.57 3.56 4.07 4.06 4.50 4.49 5.22
0.05 0.10 0.25 0 23 0.53 1.12 1.09 1.03 1.96 2.07 2.87 2.78 3.04 3.17 3.21 3.77 3.64 4.07 4.04 4.52
1
2 3
4 5
6 7 8
9 10 11
12
5.30 WT
the desired temperature, measuring the resistance of the sodium hydroxide solution a t frequent intervals. When the resistance becomes constant, the total amount of carbon burned, or the carbon burned after any given length of time, is found by multiplying the corresponding change in resistance by the calibration factor of the sodium hydroxide. Burning rate curves are obtained by plotting the amount of carbon burned against the length of time of combustion. BURRING RATE
Inasmuch as measurements can be made a t intervals of only a few seconds, it is possible to follow the progress of the combustion clojely. Figure 3 illustrates the burning rate of the carbon on
'70
C A R B O N BY GRAVIMETRIC METHOD
Figure 4. Comparison of Conductometric and Gravimetric Methods The conductometric method is sn-ifter than the conrentional gravimetric method because the conductivity of the sodium hydroxide solution can be measured more rapidly than an rlscarite bulb can be rreighed, and, if desired, a permanent record of the rate of combustion can be obtained. Another timesaving feature lies in the fact that t'he combustion can be discontinued as soon as the measurements show no additional evolution of carbon dioxide. The conductometric method has also been employed as a tool for finding the optimum conditions for the determination of burnable carbon by the grarirnetric method. It was found that the opt,imum sample size, temperature, time, and rate of comhustion vary with t,he type and history of the catalyst being investipatrd. LITERATURE CITED
(1)
Bennet, E. L., Harley, J. H., and Fowler, R. \I.. .\SL.
CHEM., 22,
445 (1950). (2)
Holdeheide, W., Huber, Br., and Stocker, O., Ber. d e u t . totan.
Ges., 54, 168 (1936). ( 3 ) Lescher, v. L., h . 4 L . CHEM., 21, 1246 (1949). (4) Schmitkons, G. E., paper presented a t Symposium on Rapid Methods of A4nalysis,Am. Petroleum Inst., Committee on
.4nalytical Research. April 1949.
TIME OF
Figure 3.
COMBUSTION, MINUTES
Burning Rate of Carbon on Catalysts
Temperature of combustion, l l O O o F.
two different cracking catalysts. Presumably, a recording conductivity bridge would furnish continuous burning rate curves. Burning rate curves of this type would be valuable in the study and comparison of various kinds of catal) sts. The oxygen rate was kept constant a t about 200 nil. per minute during all of these studies. COMPARISON WITH GRAVIJlETRIC METHOD
,4comparison of the results of carbon determinations on twelve catalysts by both conductometric and gravimetric methods is
(5) Touden, W. J., ANIL. CHEM.,19, 946 (1947). RECEIVED December 14, 1950. Presented before the Diviiion of Petroleum Chemistry a t t h e Sixth Southwest Regional Meeting of the AMERICAS CHEVICAL SOCIETY, San Antonio, Tex., December 1950.
