$ 20
lo
100
ATTRITION TIME (Seconds1 Figure 5. Attrition of nanideal granular solid
Some weight loss occurs on heating because of volatilization of water. I n this instance, the heat treatment (sintering) strengthened interparticle bonds as shown by a smaller attrition constant. Figure 3 shows these results. In a second experiment, a higher tem-
perature, 850' C., was used. In this case interparticle bonds were weakened as shown by an increase in attrition constant (Figure 4). Concurrent x-ray diffraction esaminations were conducted. These, combined with attrition, defined the changes which occurred. We have found that the ratio of attrition constant before a treatment to that after a treatment is useful. If this ratio is greater than one, intergranular bonds are strengthened. If it is less than one, they are weakened. While many granular solids closely obey the attrition law, dw/dt = -Kw, some do not. Thus granular solids can be classified as ideal granular solids obeying the attrition law, and nonideal granular solids, showing deviations. When macrolayer structure is present, nonideal behavior occum In one problem, onion-like layer structure was observed. In this case the experimental attrition curve oscillates about a straight line on semilogarithmic coordinates. The oscillations represent layer penetration. Figure 5 shows the attrition curve for an alumina base catalyst showing this behavior. When one of the spberical catalyst beads was cut in half and stained, the layer structure was
Figure 6. Layer structure of silicaalumina catalyst 11 2 X)
apparent. Figure 6 shows a photomicrograph of a treated hemisphere. LITERATURE CITED
(1) Forsythe, W. L., Jr., Hertwig, W. R., rnd. ~ n gc.h m . 41,1200 (1949). (2) Holowiecki, K., P i ~ z e m s l .Chem. 22, 369 (1938). (3) Sheinharts, I., McCullough, H. RI., Pmoder Met. Bulletin 7, No. 1, 22, (1954); C.A. 49, 6058h. RECEIYED far review October 22, 1962. Accepted January 4, 1963.
Rapid Combustion Method for the Determination of Organic Substances in Aqueous Solutions C.
E.
Van HALL, JOHN SAFRANKO,' and V. A. STENGER
Special Services Loboratary, The Daw Chemical Co., Midland, Mich.
b A method has been developed for the rapid determination of total carbon in aqueous solutions in concentrations down to 2 mg. per liter. The sample is injected inia a combustion tube where the organic matter is oxidized to carbon dioxide in a stream of oxygen. The gas stream is passed through a nondispersive infrared analyzer sensitized specifically for carbon dioxide, and the instrument signal is recorded on a strip chart. Height of the resulting peak is measured and the corresponding carbon content is read from a calibration curve. The time required for making a single determination (apart from preliminary dilulions) i s about two minutes. methods for the deof small quantities of organic matter in dilute aqueous solutions are generally based upon wet CCEPTED
oxidation. The amount of oxidant consumed may be found volumetrically (4) or photometrically (5),or the quantity of carbon dioxide evolved may be determined by manometric (fS),gravimetric (If), or alkalimetric (7) procedures. In addition, thermal conductivity (8) and mass spectrometry (93) have been utilized to determine the carbon dioxide. These methods, though some are widely used, suffer from the variable susceptibility of organic compounds to oxidation by common reagents such as chromic acid, the interference of chloride and other ions in some cases, and the length of time required for analysis. Recently a highly sensitive, specific, and accurate determination of organic carhon in water has been described (fO),in which a sample is evaporated to dryness and burned in oxygen, all of the vapors being passed through a combustion tube, collected, and circulated through an infrared analyzer. Even this method
requires from 1to 3 hours per determination. Increasing concern with the problems of water pollution and waste treatment has brought about a need for a rapid and precise determination of total carbonaceous matter in water, free from interference by salts. The present authors have been led to investigate the combustion of aqueous samples in a flowing oxygen atmosphere. The major problem involved here is the relatively large volume of water vapor produced when liquid water is placed in a hot combustion chamber. As calculated from ideal gas behavior, 1 ml. of liquid water yields 5.6 liters of steam a t 950' C. To work with a tube of reasonable dimensions, one is restricted to a small sample. For this reason a very
1 Present address, Aerojet-General Corp., Sacramento, Cahf. I
VOL 35, NO. 3, MARCH 1963
315
El I
2 3 4
5 6 7
8 9 10 II
12
13 14
I5
OXYGEN SUPPLY REGULATOR NEEDLE VALVE FLOW METER CHECK VALVE COMBUSTION TUBE TUBE FURNACE PYROMETER TRANSFORMER CONDENSER STOPCOCK FILTER ANALYZER AMPLIFIER RECORDER
Figure 1.
2
3
v Schematic diagram of combusiion apparatus
sensitive and specific detector for carbon dioxide becomes necessary. ,4 further consequence of the great expansion of a sample on vaporization is that part of the gas produced blows through the combustion tube with high velocity. To obtain complete oxidation of organic matter under these conditions it is important to have a packing material present. This must both serve a9 a heat reservoir and provide a surface capable of holding sufficient oxygen for reaction, even though the bulk oxygen atmosphere has been replaced by steam. When the quantity of organic matter is small, not much oxygen is required; nevertheless good opportunity for contact is essential because the oxidation must take place so rapidly. The situation is different from the ordinary microcombustion, Fhere even an open tube allows complete oxidation in a fast stream of oxygen (3). Using a combustion tube of about 50-ml. internal volume, it is feasible to inject 0.02 ml. of water. Under these conditions an instantaneous pressure increase of slightly over 2 atm. would be expected. In practice the pressure is relieved rapidly as the gases pass out of the tube and moisture condenses. With 2 to 100 mg. of carbon per liter of water, the carbon in the sample amounts to only 0.04 to 2 pg. The volumes of carbon dioxide equivalent t o such quantities are negligibly small but are present, after combustion and cooling, in roughly 50 ml. of oxygen saturated with water vapor. Fortunately, even in this low concentration range carbon dioxide can be determined readily by its infrared absorbance. A commercially available nondispersive instrument serves as the detector. This type of detector has been used for the determination of carbon dioxide in the combustion products from hydrocarbons in air (6, I d ) , in the microanalysis of organic compounds for carbon (9), and in the analysis for carbon in larger
316
ANALYTICAL CHEMISTRY
samples of fresh or ocean water (10, 14). In the proposed method, the sample is injected with a microsyringe and the output of the infrared analyzer is recorded on a strip chart. The curve produced shows a peak similar to a gas chromatographic peak for a single component, n hich can be evaluated through peak height or area by comparison with data bmed upon standard solutions. Results so obtained show that the method is applicable for most water-soluble organic compounds, including those that contain sulfur, nitrogen, and halogens. Nonvolatile organic substances and those such as carbon dioxide or light hydrocarbons, which are readily volatile, may be differentiated by determination of carbon before and after blowing the sample solution with an inert gas. The method promises to have broad applicability in mater analysis, in the determination of carbonates and organic impurities in inorganic materials, and in the study of organic solubilities in aqueous systems. EXPERIMENTAL
Apparatus. The complete apparatus is shown in Figure 1. A controlled oxygen supply from a cylinder is further adjusted t o a flow rate of approsimately 50 ml. per minute by means of a Watts regulator, Type 26, Model h'l 1, and a Hoke needle valve, No. 2 PY 281. The flow rate is measured with a Brooks
flowmeter, Type 2-1110 with an R-215-AA tube and stainless steel float, calibrated by the soap-bubble technique ( 1 ) . A small check valve, Kimble KO. 38005, inserted in the oxygen line between the flowmeter and the combustion tube, restrains backflow following sample injection and thus improves reproducibility. A small wad of quartz wool is inserted in the oaygen line to prevent dust from reaching the flowmeter tube. The combustion tube is heated to 950" C. by a 700-watt tube furnace controlled Ly a variable voltage transformer. A Sym-Ply-Trol pyrometer, Model 4526, is used to indicate temperature. Surplus moisture is removed from the furnace exit gases by an air condenser 8 inches long, minimizing the possibility of condensation in the analyzer. An integral part of the condenser is a small U-tube with a stopcock on the bottom, through which the accumulated water may be drained periodically. A Hoke filter, Model S 541, with a 10to 13-micron filtering elenlent, is used to trap fog or dusts which may be produced in the combustion of some acid or salt solutions. A Beckman L/13 infrared analyzer, Model 15 A, fitted with 5.25-inch (13.3-cm.) cells and a detector unit sensitized for carbon dioside, is employed to monitor the gas stream from the furnace. The cell compartment is thermostatically controlled a t 55" C., thus further reducing the possibility of moisture condensation in the cell. The output of the analyzer is recorded on a Sargent Model MR or SR recorder, using the 0- to 5 m v . range. A filter is placed at the input to the recorder to reduce the noise from the chopper motor in the analyzer. The filter consists of a series-connected 500()-ohm carbon resistor on one lead and a 40-pf. dry electrolytic capacitor across the input. The combustion tube and sample injection port are shown in Figure 2. The tube is constructed of 12.5-mm. i.d. fused silica and is 40 cm. long. A T joint a t the entrance provides a rigid connection with the sample port, whereas a ball joint a t the exit provides a flesible connection with the condenser. A small amount of Kel-F grease is used on the ball joint. Both joints are secured with spring clips, the T joint requiring LL slotted washer to accommodate its clip. The sample injection port consists of a glass tee on the inlet joint. Into the straight arm of this is sealed (with epoxy cement) a No. 18 stainless steel syringe needle, which acts as a guide and holder for a Hamilton No. 705 N syringe.
NEEDLE
FUSE0 SILICA 16 MM 0 0 125MM I D
Figure 2.
Combustion tube and syringe adapter
53
I50
IW
zoo
FLOW R I T E , ML 2EQ Mih
Figure 3.
Effect of flow rate on peak height and peak area
Sample. 20 pl, of 250 p.p.rn. acetic acid in water (100 p.p.rn. carbon) Furnace temp. 950' C.
Between sample runs, the injection port is closed with an old No. 705 N syringe, the needle of which has been sealed. Osygen enters through the other arm of the tee. A wad of ignited asbestos fiber, enough to make a packing about 4 CUI. long, is inserted into the combustion tube from the entrance end and pressed against several restraining indentations located two thirds of the way along the tube. The asbestos is tamped into place gently with a quartz rod. A rolled piece of 45-mesli platinum gauze 8 em. square is inserted into the tube behind the plug and pushed tightly against the indentations. The combustion tube is placed in the tube furnace so that the end of the needle guide is even with the end of the furnace. Asbestos tape is then used to plug the opening around the tube. Reagents. A standard acetic acid solution is prepared by accurately weighing 1.000 gram of ACS grade glacial acetic acid into n 1-liter volunietric flask, diluting to volume with carbon dioxide-free distilled water, and mixing thoroughly ( I ml. = 1.00 m g . of acetic acid or 0.400 mg. of cubon). Solutions of other compounds used in this study were prepared from the purest grades of chemicals available. The asbestos fiber packing used in the combustion tube is conditioned by ignition in the full heat of a Fisher burner for 2 hours. Instrument Adjustment. Turn on the infrared analyzer, recorder, and tube furnace and allow a sufficient warm-up time for stable, drift-free operation. A time of 2 hours is usually necessary; if daily use is anticipated, the analyzer can be left on continuously. TT'ith the oxygen flow rate adjusted to 50 ml. per minute, the combustion tube a t 950' C. and the recorder set to the 5-mv. range, adjust the amplifier gain so that a 20-pl. sample of the 100-p.p.m. standard (see below) gives a peak height of approximately half-scale deflection on the recorder. At this level of gain the noise level should be less than 0.5% of full scale. If a higher noise level is
observed, adjust the analyzer according to the service manual. Preparation of Standard Curve. Prepare a series of diluted standard solutions containing 20, 40, 60, 80, and 100 mg. of carbon per liter by pipetting 5-, IO-, 15-, 20-, and 25-ml. aliquots of the above standard acetic acid solution into 100-ml. volumetric flasks. Dilute to volume with carbon dioxide-free distilled water and mix. Successively introduce 20 pl. of each standard into the instrument and read the height of the corresponding peak. Between injections allow the recorder pen to return to its base line. The actual injection technique is as follows: Rinse the syringe several times with the solution to be analyzed, fill, and adjust to 20 ~ 1 . Wipe off the excess with soft paper tissue, taking care that no lint adheres to the needle. Remove the plug from the syringe holder, insert the sample syringe, and inject the sample into the combustion tube with a single, rapid movement of the index finger. Leave the syringe in the holder until the flow rate returns to normal, then replace it with the plug. Run duplicate determinations on each solution. Prepare a standard curve by plotting milligrams per liter (or parts per million) of carbon taken os. peak height, on rectangular coordinate graph paper. Procedure. After mixing the sample thoroughly and making any appropriate dilutions to bring the carbon content into the proper range, take a 2O-pl. sample with the precautions noted above and inject it into the combustion tube in the same way. hleasure the height of the resulting carbon dioxide peak. From the peak height and the standard curve determine the carbon concentration of the solution. To distinguish between nonvolatile organic materials and carbonates or other volatiles, acidify a portion of the sample to a pH of 2 or less. Generally a few drops of concentrated hydrochloric acid are sufficient for 100 ml. of sample. Test pH either with a meter or by spotting on indicator paper. Bubble nitrogen or other inert gas, free from
FURNACE T E H P E R I T U R E . ' C
Figure 4. Effect of temperature on peak height and peak area Sample. 20 pl. of 250 p . p m aceric acid in water ( 1 00 p.p.rn. carbon) Flow rate. 56 ml. per minute
carbon dioxide and organic impurities, through the acidified solution for at least 5 minutes. Determine the remaining carbon in the usual manner. RESULTS A N D DISCUSSION
Numerous experiments were performed t o find the operating conditions necessary for optimum results. The peak height for a given concentration of carbon is independent of oxygen flow rate a t rates between 40 and 200 ml. per minute. This is a major advantage, in that small variations in flow rate, which are difficult t o eliminate, do not affect peak height. Data on peak height and peak area os. flow rate, for 20-p1. samples of dilute aqueous acetic acid equivalent to 100 p.p.m. of carbon, are shown in Figure 3. The greater dependence of area upon flow rate makes it less practical to use than peak height. The general shape of the curve for peak area us. flow rate is what one would expect, considering the fact that the area is measured on a time-base curve rather than on the basis of gas volume. However, correction for this through multiplication of the area by the flow rate, and plotting the product against flow rate, yielded a very erratic curve. Figure 4 illustrates the effect of temperature on peak height and peak area. The peak height is practically temperature-independent a t furnace temperatures above 900' C. Typical calibration data are shown in Figure 5. Calibration curves constructed from such data are shown in Figure 6. The peak-height curve follows closely the response curve of the infrared analyzer as furnished by the manufacturer. Within experimental error, the peak-area curve is essentially a straight line. VOL 35, NO. 3, MARCH 1963
317
d Figure 5.
Calibration data
Standards. Acetic acid in water Flow rate. 56 ml. per minute Furnace temp. 9500 c.
A critical part of the apparatus is the packing in the combustion tube. Without it, combustion is incomplete and erratic. In addition to the functions described above, the packing serves to catch the thin, rapidly moving, stream of sample ejected from the syringe. The packing should be in the latter half of the tube, well m-ithin the heated zone. If i t is too near the entrance port, part of the sample may be blown back and lost in the inlet tube and the syringe adapter. The open half of the tube allows some room for gas expansion and provides a cushion of oxygen which restrains the sample vapors from blowing back. Several types of packing were investigated. A baffle plate of silica permanently mounted in the middle of the tube was fairly effective, but results lacked the desired precision. A packing of quartz wool was found t o be suitable for use with solutions of low salt content, but brines caused a rapid disintegration with subsequent blowthrough of sample. Asbestos fiber is more stable and is recommended, although it causes larger back pressures 318
ANALYTICAL CHEMISTRY
Table 1.
Analyses of Standard Solutions
Compound
Calcd.
Benzoic acid Phenol Sucrose Glycine Pyridine Urea Sodium cyanide Acetanilide p-Ni tr oaniline 4Aminoantipyrine Sulfanilic acid Diphenylaminesulf onate, Ba salt &Methionine 2,4,6-Trichlorophenol Sodium carbonate Acetic acid in 207, NaC1 Acetic acid in 2070 CaClz
68.8 76.6 104.8 100.7 105.6
a
than were experienced with either quartz wool or baffles. A further advantage of asbestos fiber is that it can be replaced easily. A platinum gauze mas necessary for the complete oxidation of some nitrogen-containing compounds, perticularly cyanides. Quartz chips n-ere also tested but were less effective than platinum and harder to handle. B packing of platinized asbestos, used without a roll of platinum gauze, was effective for a short time but subject to poisoning or some other loss of activity. After several days of continued use this packing yielded results similar to those from plain asbestos alone. A sample volume of 20 fil. appears to be most suitable for general use. Larger samples increase the ri5k of excessive internal pressures and blowthrough of the vapors. Smaller samples may be used, but the lower reproducibility of the smaller syringe volumes decreases the precision of the method. Dilution of stronger samples with pure water is to be preferred over cutting the sample volume. Alternative posaibilities, of changing the amplifier gain or the recorder voltage range, are acceptable (within the useful limits of the infrared analyzer capabilities) but would require recalibration under the new conditions. The sample must be injected from the syringe into the combustion tube rapidly with a single movement of the plunger. -4 very slow or erratic injection will result in variations in peak shape and noticeable deviations in reproducibility.
100.0
122.5 75.4 106.2 111.5
89.3 87.8 103.0 75.4 99.5 100.0 100.0
Carbon. 13.D.m. Found Max. A h . 69.0 77.2 105.1 101.2 104.4 100.9 122.1 76.0 105.8 110.6 90.5
67.4 76.5 104.3 99.5 103.6 99.1 119.5 i5,O 104.9 108.9 88.6
68.2 76.9 104.5 100.3 104.2 99.8 120.5 75.4 105.4 110.2 89.3
87.6 102.7 76.0
86.8 101.8 74.0 99.2 99.0 98.1
87.4 102.5 75.0 99.4
100.0 101.0 100.0
Std.
.4v. %
0.66 0.30 0.40 0.69 0.40 0.86
99.1 100.4 99.7 99.6 98.7 99.8 98.4 100.0 99.2 98.8
Av. dev. ( & ) " recovery
1.11
0.48 0.52 0 85 0.90
100.0
99.5 99.5 99.5 99.9
100.0
0.40 0.45 0.84 0.40 0.82
100.0
99.1
0.78
99.1
All results based on 4 determinations. Calibrations made with standard solutions
of acetic acid in n-ater.
samples. One can generally determine the magnitude of the interference by running a blank consisting of a solution similar to that being tested, but free from organic matter. The method is not necessarily limited t o the analysis of true solutions. I n some cases it may be used to analyze dispersions of insoluble carbon-containing materials. I n such applications the inside diameter of the syringe needle becomes a n important factor. The needle specified has an opening 170 microns in diameter. It can therefore act as a filter, excluding larger particles and thus causing low results. If the particle size can be made small enough by grinding, homogenizing, or blending, and if the suspension is uniform, slow to settle, and not too viscous, the method is applicable. LITERATURE CITED
(1) Barr, G., J. Sci. Znstr. 11,321 (1934).
Figure 6.
Calibration curves
Standards. Acetic acid in water Flow rate. 56 ml. per minute Furnace temp. 950’ C.
Table I illustrates the results obtained with a beries of standard solutions of different compounds. Generally the data are reproducible to f l p.p.m. with a relative standard deviation of *1.001, at the 100-p.p.m. level. The recovery of acetic acid standards is essentially 1 0 0 ~ o&-henthe system is
Tab’e II.
Anion (1% solution) NOs-
Effect of Foreign Ions
Carbon, mg. per liter Calcd. Found@ 100.0 100.9 c1100.0 100.1 504-2 100.0 100.0 po4-3 100.0 99.0 Average of four determinations.
calibrated against a sodium carbonate standard. This establishes that the conversion of acetic acid to carbon dioxide is complete. I n tests of solutions of several common anions, no interference was encountered with concentrations at least up t o 1%. Table I1 shows the results obtained for various sodium salt solutions containing known quantities of acetic acid. Strong brines and certain acid solutions interfere slightly with the method by producing fogs which may be counted as carbon dioxide. Presence of the fine filter in the line t o the analyzer minimizes this interference by removing most of the particulate matter. It is advisable, however, t o test for this effect when analyzing these types of
(2) Beattie, J., Bricker, C., Garvin, D., ANAL.CHEY.33, 1890 (1961). ( 3 ) Belcher, R., Ingram, G., Anal. Chim. Acta 4 , 118 (1950). (4) Bertram, F. W., Carlisle, 0. T., Murray, J. E., Warren, G. W., Connell, C. H., ASAL. CHEM.30, 1482 (1958). (5) Englis, D. T., Wollerman, L. A., Ibid., 24, 1983 (1952). (6) Heaton, W. B., Rentworth, J. T., Ibid., 31, 349 (1959). 17) , , Kav. H.. Kiel. Xeeresforsch. 10. 26 (1954j; c.’A. 48, 9869a. (8) Kieselbach, R., ANAL. CHEX 26, 1312 (1954). (9) Kuck, J. A,, Berry, J. W., Andreatch, A. J., Lentz, P. A., Ibid., 34,403 (1962). (10) Montgomerv. H. A. C..’ Thom. N. S.. ’ A n a l y s t 8u7, 68s 11962). (11) Pickhardt, W. P., Oemler, 9. N., Mitchell, J., Jr., ANAL. CHEW 27, 1784 (1955). (12).Rosenbaum, E. J., Adams, R. W., King; H. H., Jr., Ibid., 31, 1006 (1959). (13) T’an Slyke, D. D., Folch, J., Plazin, J., J..Biol. Chem. 136, 509 (1940). (14) Wilson, R. F., Limnol. and Oceanog. 6 , 259 (1961). RECEIVED for review October 29, 1962. Accepted January 7, 1963. Tenth Detroit Anachem Conference, October 24, 1962. Patent application has been filed on certain features of the apparatus and procedure which are thought to be novel. It is intended that an instrument incorporating these features will be made commercially available in the near future. Inquiries should be directed t o the Technical Service and Development Department. The Dow Chemical Co.
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