dilutions were made and carried through the proposed procedure for cerebrospinal fluid. The data are presented in Table 11. The plot in the data is linear from 28 to 734 y. The per cent recovery is quantitative over this range. The coefficients of variation (2) for 28.3, 104.1, and 193.1 y of protein are given in Table 111. I n the usual working range, 200 9 of protein, the coefficient is less than 1%; it increases to 10.2% for the lower limit of sensitivity, 28.3 y. The procedure detailed is for protein nitrogen, but if a solution were cleared of protein and dried properly, this procedure could be adapted for the determination of nonprotein nitrogen.
ACKNOWLEDGMENT
The authors wish to thank Vivian Iob and Madeline McMath for carrying out independent micro-Kjeldahl nitrogen determinations on the ammonium sulfate and protein standards used in these experiments. LITERATURE CITED
Lowry, 0. H., ANAL.
York, 1956. (3) Byer, J. B., Dailey, M. E., FremontSmith, F., A.M.A. Arch. Neurol. Psychiat. 26, 1038 (1931). (4) Cipriani, A., Brophy, D., J . Lab. Clin. Med. 28, 1269 (1943).
(5) Jenden, D. L., Taylor, D. B., ANAL. CHEM.25,685 (1953). (6) Kendell, J., Davidson, A. W., J , Am. Chem. SOC.43, 979 (1921). (7) Kirk, R. L., Advances in Protein Chem. 3, 139 (1947). ( 8 ) Lowry, 0. H., Roberts, N. R., Leiner, K., Y., Wu, M. L., Farr, L., J . Biol. Chem. 207, 1 (1954). (9) Peters, J., Van Slyke, D., “Quantitative Clinical Chemistry,” Williams and Wilkins, Baltimore, 1943. (10) Samson, K., Ergeb. inn. Med. u. Kinderheilk. 41, 553 (1931). (11) Sperry, W. M., Methods of Biochem. Anal. 2, 83 (1955). RECEIVEDfor review August 22, 1957. Accepted Ma 2, 1958. Work supported Campbell Foundation for by Kenneth Neurological Research.
6.
Microdetermination of Sulfur and Halogens by Rapid Automatic Combustion E. J. AGAUI, E.
M. FREDERICKS, and F. R. BROOKS
Shell Development Co., Emeryville, Calif.
b A simple apparatus has been designed for the microdetermination of sulfur and/or halogen in organic materials by rapid and completely automatic combustion, in a horizontally mounted quartz tube, a portion of which is maintained a t high temperature. The sample is placed in the cool zone of the combustion tube cnd vaporized by a two-stage, electrical sample heater. Sample vapors are swept into the high temperature zone (900’C.) by a stream of nitrogen and there mixed with a large excess of oxygen. Combustion takes place and the oxidation products are swept out of the tube, absorbed, and measured by conventional methods. Eight minutes are required to complete the combustion and absorption cycle. The apparatus has been applied to a wide variety of organic materials containing sulfur, chlorine, bromine, and iodine. Data show good agreement with known values.
heating (3, 6,7 , 8), and those in which the rate of sample vaporization is controlled by the combustion characteristics of the sample itself (2,4,9). A very simple combustion apparatus ( I ) permitted a sample of 50 mg. or less to be burned in 1 to 2 minutes without any control devices, by rapid vaporization of the sample in a stream of nitrogen and injection of a large excess of oxygen in the hot zone of the combustion tube. Maintenance of high gas flow rates ensured complete combustion without explosion hazard. Although the apparatus was designed initially for the determination of chlorine in pesticide residues, it seemed adaptable to a convenient and general method for the determination of halogen and sulfur in a wide variety of materials. This report describes improvements in the apparatus and presents data which demonstrate its utility as a general tool in combustion analysis. APPARATUS
T
maintenance of slow and controlled sample vaporization has long been regarded as a key factor in the combustion analysis of organic compounds. Because this requires considerable skill and training of operators, much attention has been given to automatic control of sample vaporization. Apparatus of two general types utilizes automatic sample vaporization : those in which the sample is vaporized by a fixed or preselected pattern of HE
1566
ANALYTICAL CHEMISTRY
The apparatus is shown in Figures 1 and 2. Three service lines are connected to the rear of the unit: oxygen a t 5 p.s.i.g., nitrogen a t 5 p.s.i.g., and vacuum. Oxygen enters through a twc-way solenoid valve, and passes through a rotameter and a needle valve hy which the flow is adjusted to 400 ml. per minute, and then into the hot zone of the quartz combustion tube via a platinum10% iridium tube. Nitrogen is introduced into the unit in much the same
way, except that the flow is adjusted to 100 ml. per minute and it enters the combustion tube a t the entrance end fitting as shown in detail in Figure 2. Three-way solenoid valves are provided in each line, so that the oxygen and nitrogen entry ports may be reversed a t the start of the burnout period.of the combustion cycle. Vacuum is applied a t the end of the scrubber train via a needle valve after the nitrogen and oxygen flows have been adjusted. The valve to the vacuum line is adjusted t o maintain a slight reduced pressure in the unit a t all times: This is achieved by adjusting the vacuum valve until a slow trickle of air enters the pressure indicator in the oxygen line. Except for the absorber, the components are connected with each other by ‘/&ch copper tubing. Rubber tubing connects the absorber to the needle valve that regulates the vacuum. Sample Heater and Furnace. The sample heater is space-mound on a 3/4-inch mandrel and consists of 60 turns of No. 22 Kanthal A (Kanthal Corp., Stamford, Conn.) resistance wire wound at 18 turns per inch. This heater provides about 750 watts a t 115 volts, ahich is more than adequate to volatilize organic materials under test conditions, An air blast is located directly under the heater to allow rapid cooling of this portion of the combustion tube prior to introduction of the sample. The furnace is also wound with No. 22 Kanthal A resistance wire on a 15/lginch Alundum core, a t 16 turns per inch for 1 inch a t either end and a t 8 turns per inch for 6s/4 inches of center winding. Insulated with Sil-0-Cel brick, this furnace provides 83/4inches of combus-
I
i
'f
!'
"2
Figure 2. Diagram of gas flow apparatus
Figure 1. Quartz tube combustion microopparotus tion zone maintained at 850" to 900' C. No control or pyrometer is provided for the furnace. Combustion T u b e and Absorbers. T h e quartz combustion tube and the glass absorber assembly are described by Figure 3. These components are similar to those used with the equivalent macrocombustion apparatus (67, except for size. The porosities and diameters of the fritted disks used in the absorbers are fairly critical, to give good scrubbing action without seriously restricting the gas flow. Combustion Cycle. T h e circuit diagram of the apparatus is shown in Figure 4. The main power switch, the furnace switch, and the sample heater switch are turned on and the furnace is allowed to reach operating temperature. If necessary, the sample vaporization zone of the combustion tube is cooled by turning on the air blast or, in the case of very volatile samples, by placing solid carbon dioxide around the outside of the tube. The sample is introduced into the front of the combustion tube, the entrance closure is capped, and the manual gas switch is turned on to sweep the sample vaporization zone free of air. The "start" button is then depressed and held for approximately 5 seconds until cam 1 closes its microswitch. As the switch closes, pilot light 2 goes out, the start button is then released, and the operator shuts off the manual gas switch and is free to turn to other work. The oxygen and n i t r u gen conthue to flow and current is supplied to the sample heater through a variable autotransformer preset to deliver about 60 volts. The voltage across the sample heater is maintained a t this level for 2 minutes. At the end of this time, cam 3, through its microswitch, connects the sample heater to full line
L a e r *b.orbar
" D P I *b.Olbll
n
re 3.
Q u o h and borosilicate glassware
voltage (115 volts). The sample is either vaporized completely or reduced to a carbonaceous residue during the next 2 minutes. At the end of this time (4 minutes total), cam 2 closes its microswitch, which causes the threeway solenoid valves to interchange the oxygen and nitrogen flows. The par, sage of oxygen over the heated sample boat causes complete combustion of any carbonaceous residue. At the end of 4 additional minutes cam 1 opens its microswitch; this stops the motor, turns on pilot light 2, and de-energizes the sample heater. Cam 2 causes the gas flows to interchange again, and cam 3 causes the sample heater microswitch to return to its starting position. This marks the completion of the combustion cycle. DETERMINATION OF SULFUR AND HALOGENS
Combustion of Samole. Weieh sufficient sample to glrve from 0.62 to 2 mg. of sulfur or 0.0005 to 0.5 meq. of halide, if possible, hut do not use more than 30 mg. of sample. Charge the absorber with 6 ml. of absorbent, placing 4 ml. in the lower
absorber and 2 ml. in the upper absorber. Use 6% hydrogen peroxide solution for sulfur and 0.5M sodium arsenite solution for halogens. Turn on the manual gas switch and adjust the oxygen flow rate a t 400 ml. per minute and nitrogen a t 100 ml. per minute. Turn on the air blast to cool the combustion tube or, in the case of very volatile samples, place solid carbon dioxide around the sample vaporization zone of the tube. Open the combustion tube and intrcduce the sample. If a capillary sample tube has been used, scratch the capillary about 3 em. from the bulb with a Carborundum chip, break the capillary a t the scratch, and lay both pieces in a platinum trough. Push the sample to within 4 cm. of the furnace and close the combustion tube entrance. Turn on the sample heater switch, push the start button and hold it in contact until pilot light 2 goes out, turn off the manual gas switch, and adjust the vacuum valve until there is a slight reduced pressure in the tube as indicated by a small flow of bubbles in the pressure indicator. A t the end of 8 minutes, the pilot light comes on again and the combustion is finished. Disconnect the VOL. 30, NO. 9, SEPTEMBER 1958
1567
Table I. Application of Microcombustion Method to Various Materials
Sample Weight, Present, >\laterial White oil High sulfur gas oil Diesel fuel Benzyl SUIfide Dimethylsulfolane Diphenylsulfone s-Trithiane Sulfur, floFers
Found, % Sulfur
RIg.
/c
24 22 23 15 21 15
... 1.58 1 02
10 11
14 96
12 12 18 14 5 6 5 5
21.63 14 66 69.57
100
0.04 0.02 154 1.57 0.91 0.94 14 78 14 92 21.03 21.21 14 71 14 65 69.40 69.44 99.6 99.2
Pklol Light 3
SPDT M i c r o Sw
POWerStal
Chlorine Chlorinated wax white oil Chlorinated wax white oil p-Chloroacetanilide
+
41 48
2.01
2.04 2.00
+
14 20
0.13
0.15 0.13
11 12
20.91
20.92 20.73
Bromine Tetrabromoethane Bromodecane
92.47 32.33
Bromoethylbenzene
43.18
92.4 92.3 32.10 32.27 43.03 42, 70
Iodine Iodonaphthalene Iodoanisole Iododecane
5 6 11 8 8 4
49.95 54 23 47.32
49.36 49.32 53 96 54 07 46.46 46.93
a Composition values of compounds calculated from formulas and those for etroleum fractions and mixtures obtained !y macroanalysis (quartz tube combustion).
absorber from the combustion tube and turn off the vacuum. When samples are burned in a capillary bulb, manually turn the sample heater on and off during the last 4 minutes of the combustion cycle to allow oxygen to enter the bulb and burn out any remaining residue.
Analysis of Absorber Solutions. SULFUR. If the sample is free of
1568
ANALYTICAL CHEMISTRY
Figure 4.
Circuit diagram
halogens, phosphorus, and nitrogen, the sulfuric acid formed may be determined by titration with standard sodium hydroxide solution. (Sote. Although combustion of nitrogencontaining materials results in the formation of some interfering oxides of nitrogen or ammonia, the error is generally negligible when the saniple contains less than 3% nitrogen.) Transfer the absorber liquid to a 25ml. Erlenmeyer flask, using approximately 10 ml. of water to effect the transfer. Add 2 to 3 drops of methyl red indicator solution and titrate with standard 0.Ol.V sodium hydroxide solution, Make a blank determination, using 6 ml. of 6% hydrogen peroxide solution and 10 nil. of water. If the sample contains phosphorus, halides, or nitrogen, determine the sulfur microgravimetrically by precipitation as barium sulfate. HALOGENS. Determine chloride or bromide by amperometric titration with silver ion, using a rotating platinum electrode (1). Iodine may also be titrated amperometrically; however, oxidation t o iodate and titration with thiosulfate to the starch end point are preferred because of the favorable stoichiometry and ease of application. DISCUSSION
OF RESULTS
The apparatus, described ( 1 ) in connection with' the determination of chlorine in pesticide residues, differed from the unit described above primarily in that sample vaporization proceeded
more rapidly because no provision was made for starting the sample heater a t a reduced voltage. Preliminary tests were made with the earlier apparatus to determine the general applicability of the technique to materials containing sulfur as well as halogen. With few exceptions, good results were obtained. The occasionally poor results were due to too rapid sample vaporization and consequent incomplete combustion. T o eliminate this difficulty, a simple circuit \vas devised which permitted the sample vaporization to be effected in two steps. At this point the present apparatus was constructed and the entire combustion cycle, including burning out of carbonaceous residues, was made completely automatic, by incorporating a simple program timer and solenoid valves t o control the flow of gases through the combustion tube. To illustrate the validity of this combustion technique, the method was applied to a wide variety of sulfur and halogen compounds. It performed satisfactorily on all types of samples encountered. Analytical data for a variety of materials (Table I) agree well with theoretical values and with those obtained by macrocombustion analysis (6). The data show a slight bias toward low values. This appears to be primarily related to the purity of the materials used for testing, as the good results obtained for materials high in sulfur or halogen indicate that the absorbing system is efficient. KO evidence of incomplete combustion was observed. LITERATURE CITED
(1) Agazzi, E. J., Peters, E. D., Brooks, F. R., B S A L . CHEM. 25, 237 (1953). (2) Clark, R. O., Stillson, G. H., ISD. ENG.CHEM.,ANAL.ED. 19, 423 (1947). (3) Deiglmayr, I., Chem. Ztg. 26, 520 (1902). (4) Fischer. F. 0.. ANAL.CHEM.21, 827 ' (1949). ' (5) Hallett, L. T., IXD.ENG. CHEM., ANAL.ED.10, 101 (1938). (6) Peters, E. D., Rounds, G. C., Agazzi, E. J., ASAL.CHEU.24,710 (1952). ( 7 ) Riehlen, H., Weinbrenner, E., Mikrochenzie 23,285 (1937/38). (8) Royer, G. L., Sorton, A. R., Sundbern. 0. E.. IND.ESG. CHEY.,ANAL. E D ~ Z688'(1940). , (9) White, T. T., Penther, C. J., Tait, P. C., Brooks, F. R., ASAL. CHEM. 25, 1664 (1953).
RECEIVED for review February 2, 1958. Accepted June 9, 1958. Division of Analytical Chemistry, 133rd Meeting, ilCS, San Francigco, Calif., April 1958.
