Rapid Method for Determination of Chemical Oxygen Demand V. A. Stenger and C. E. Van Hall Analytical Science Laboratory, The Dow Chemical Co., Midland, Mich.
A method has been developed whereby the oxygen demand of an aqueous sample can be determined in about 2 minutes after homogenization or dilution. A microsample i s injected into a heated combustion tube through which carbon dioxide is flowing. Reducing materials react with the latter to form carbon monoxide, which is measured specifically while passing through an infrared stream analyzer. The procedure as described is best suited for oxygen demands in the range of 10 to 300 mg per liter.
duction product of which is also gaseous and capable of being measured rapidly, an unambiguous determination should be possible. Under the proper conditions, carbon dioxide appears to be an oxidant meeting these qualifications. The suitability of its stoichiometry can be demonstrated, assuming that Reaction 2 also proceeds to completion:
+
C,H~N,O~ m
coZ+ (m
+ a>co + b- H ~ O+
~2
(2)
2
WORKERS IN THE FIELD of water pollution control frequently need to know how much oxygen will be required to oxidize the impurities in domestic or industrial waste waters. The standard method for chemical oxygen demand (COD) ( I ) requires a relatively long digestion with chromic and sulfuric acids, followed by backtitration of oxidants. It would be advantageous to have a method capable of measuring oxygen demand by a much more rapid technique, such as that described for total carbon determination (2). An apparent solution to the problem is presented in this paper. Ideally a determination of chemical oxygen demand by the chromic acid method should oxidize the carbon of any organic compounds to carbon dioxide and the hydrogen to water. Amino nitrogen is considered to be converted to an ammonium salt. Similarly a theoretical determination of biochemical oxygen demand (ultimate BOD) should yield carbon dioxide and water, but nitrogen should be converted to nitrate. In practice, oxidation by either of the standard methods frequently fails to proceed completely. For a high temperature combustion process in the gas phase with excess oxygen, the reaction products would normally be carbon dioxide, water, and elemental nitrogen. Thus the result of a theoretical combustion process for COD should lie between the results for theoretical COD and BOD, because of the behavior of nitrogen. Differences should be minor in most cases, however, as the organic nitrogen content of waste water is usually small. In many instances a determination including partial oxidation of nitrogenous material might give a better measure of potential pollutional effects than the present COD method. A generalized equation for oxidation, by a combustion process, of the more abundant kinds of organic matter present in domestic sewage can be written as follows: L
L
L
In the rapid determination of total carbon, aCO2 is measured by the combustion-infrared method (2). The result cannot be correlated exactly with the oxygen demand (n/z O2 or n 0) because values of b and d are unknown. However, if in place of oxygen another oxidizing gas could be used, the re-
(1) Am. Public Health Assoc., “Standard Methods for the Examination of Water and Wastewater,” 12th ed., p. 510, New York, 1965. (2) C. E. Van Hall, J. Safranko, and V. A. Stenger, ANAL.CHEM., 35, 315 (1963). 206
ANALYTICAL CHEMISTRY
In order to balance both Equations 1 and 2 with respect to oxygen, Expressions 3 and 4 must hold: d + n = 2 u + ;b
d+2m = m + a
+ 2b
(3)
(4)
As the same material is to be oxidized in either case, 3 and 4 can be treated as simultaneous equations, whether C,HaN,Od represents a single compound or a mixture. Subtracting Equation 4 from 3 and transposing terms, one obtains: n=m+a
(5)
In other words, the number of moles of carbon monoxide produced in Equation 2 is the same as the number of oxygen atoms required in 1. At first glance it may seem that this conclusion would not hold for all cases, because carbon dioxide cannot oxidize any carbon that is already in an apparent oxidation state as high as carbon monoxide. Here one must recognize that such carbon upon being heated will itself yield carbon monoxide in an amount corresponding to its oxygen demand. Oxalic acid is an example:
For any specific compound containing only the elements in question, appropriate equations similar to Equations 1 and 2 can be written to verify the correspondence of oxygen atoms required in the former with carbon monoxide produced in the latter. As a trace of carbon monoxide in carbon dioxide can be determined rapidly in an appropriately sensitized nondispersive type infrared gas analyzer, it appeared to be a simple matter to develop the foregoing concept into a workable method. In practice several difficulties were encountered. The major problems were the presence of a trace of oxygen, about 20 to 30 ppm, in the carbon dioxide gas available, and the ease with which more oxygen is gained through connections. The effect of this impurity is to cause loss of sensitivity. Being a stronger oxidizing agent than carbon dioxide, the oxygen reacts preferentially with any reducing material in the sample or standards. Therefore the carbon monoxide produced is low by an amount equivalent to the oxygen present. Passage of the carbon dioxide through a bed of hot carbon was found to be a convenient way of overcoming
ADAPTER SLEEVE
llpl
SYRINGE ADAPTER
L5.0 cm.
CARBON MONOXIDE GENERATOR TUBE I
I
I
I FUSED S I L d
1.2 cm. 1.0
QUARTZ
CHARCOAL
"VYCOR"
TOTAL OXYGEN DEMAND COMBUSTION TUBE
Figure 1. Oxygen demand apparatus
1. Carbon dioxide supply 2. Regulator 3. Needle valve 4. Flowmeter 5. Carbon monoxide generator 6. Tube furnace 7. Temperature controller 8. Check valve 9. Sample injection port
10. 11. 12. 13. 14. 15. 16. 17. 18.
Combustion tube Tube furnace Temperature controller Condenser Drain stopcock Filter Infrared analyzer Amplifier Recorder
the problem. By adjus1:ment of the bed temperature a controlled concentration of carbon monoxide is generated. With a slight excess of monoxide present in the gas stream, all oxygen is subsequently reduced in the combustion zone and the instrument records a ,stablebase line. EXPERIMENTAL
The apparatus, shown schematically in Figure 1, consists basically of a carbon dioxide supply, a carbon monoxide generator, a combustion tube with sample inlet, an infrared analyzer sensitized to carbon monoxide, and a recorder. Carbon dioxide of the "bone dry" grade is obtained from a cylinder, 1, with the usual regulating valves. The flow is further controlled by a Watts regulator, 2, Type 26 Model M-1, and a Hoke needle valve 2PY 281, 3. The flow rate is measured with a Brooks flowmeter, 4, Type 2-1 110, with an R-2-15 AA tube and a stainless steel float. The flowmeter should be calibrated by the soap bubble technique (3). The tube in which the carbon monoxide is generated, 5 (Figure 2), is constructed of 12-mm i.d. fused silica with a 10/30 T joint at the entrance end and ball joint at the exit end. Both joints are se1:ured with spring clips, the T joint requiring a slotted washer to accommodate its clip. A small amount of Kel-F grease is used on the ball joint. The tube packing consists of 3 cm of charcoal held in place by two plugs of quartz wool 1 cm long. The tube heater, 6, is maintained at a temperature (about 540" C) high enough to produce a slight excess of carbon monoxide above that required to react with the (oxygen impurity that may be present in the carbon dioxide. The temperature is regulated with a controlling pyrometer Model 1602 L and control module 915 A, 7, obtained from Assembly Products, Inc. A small check valve, Kimble No. 38006, 8, is inserted in the carrier gas line before the combustion tube to improve reproducibility by restraining backflow following sample injection. The combustion tube, 10 (shown in Figure 2 also), is identical in size and shape with the carbon monoxide generator tube. Tube 10 contains a packing consisting of 5 inches (12 to Apparatus.
(3) G . Barr, J. Sci. Znsrr., 11, 321 (1934).
/ 9 "VYCOR"
I3 cm.
Figure 2. Quartz apparatus and fittings 13 cm) of platinum balls with quartz wool plugs 1 cm long on each end to hold the balls in place. The platinum balls, about 3 to 4 mm in diameter, are prepared by rolling small pieces of 80-mesh platinum gauze. The temperature of the combustion tube furnace, 11, is maintained at 875" C with another controlling pyrometer, 12, from Assembly Products, Inc. The sample injection port, 9, shown in Figure 2 consists of a glass tee on the inlet joint. Into the straight arm of this a No. 18 stainless steel syringe needle is sealed with epoxy cement. The needle acts as a guide and holder for a Hamilton No. 705 N-LT syringe. The carrier gas of carbon dioxide enters the other arm of the tee. A glass sleeve is mounted over the syringe holder. Exit gas from the analyzer enters through the side arm and provides an oxygen-free atmosphere over the injection port. Except when an injection is being made, the port is closed by a plug consisting of a discarded syringe with the needle cut off and sealed and the plunger removed. Surplus moisture is removed from the furnace exit gases by an air condenser 20 cm long, 13, 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, 14, through which the accumulated water may be drained periodically. A Hoke filter, Model S 541, 15, with a No. 2 filter element (5 to 9 microns) is used to trap fog or dusts that may be produced in the combustion of some samples. A Beckman infrared analyzer, 16, 17, Model I R 215, fitted with 13.3-cm cells and a detector sensitized for carbon monoxide, is employed to monitor the gas stream from the furnace. The output of the analyzer is recorded on a Sargent Model SR recorder, 18, using the 0- to 2.5-mv range and a chart speed of 0.3 inch per minute. The pen speed should not be slower than 1 second for full scale. A Waring Blendor, preferably with a Polytron attachment (Will Corp.), may be used to homogenize samples containing solids. Reagents. SODIUMACETATESTANDARD SOLUTION.Accurately weigh 2.127 grams of sodium acetate trihydrate, NaC2H8023Hs0, ACS reagent, into a I-liter volumetric flask, dilute to volume with distilled water, and mix thoroughly (1 ml = 1000 pg of oxygen demand). Make further dilutions of this solution as desired. Barium hydroxide, Ba(OH)2 8H20, ACS reagent. Charcoal, activated coconut, 12-30-mesh, obtained from the Barnebey Che,ney Co. VOL. 39, NO. 2, FEBRUARY 1967
207
250
200
150
Figure 3. Calibration data Peak height for indicated oxygen demand, mg per liter, calculated from sodium acetate content Distilled water, free from organic matter and oxidizing or reducing agents. Instrument Adjustment. Turn on the infrared analyzer and allow a sufficient warmup time for stable, drift-free operation. A minimum of 4 hours is necessary for intermittent operation and if daily use is anticipated the analyzer should be left on continuously. With the flow rate adjusted to 130 ml per minute, the combustion tube at 875" C, the amplifier gain at 50 divisions, and the recorder span at 0 to 2.5 mv full scale (range selector at 2.5), raise the temperature of the carbon monoxide generator tube by IO-degree increments (above about 500' C) until an excess of carbon monoxide is indicated by a shift in the base line. Usually it is not necessary to go above 540". Adjust the amplifier gain so that a 0-p1 sample of a standard having an oxygen demand equivalent to 250 mg per liter causes a recorder pen movement covering approximately two thirds of the full scale, At this gain setting the noise should be less than 0.5% of full scale. If a larger noise level is observed, adjust the analyzer according to the service manual. Preparation of Standard Curve. Prepare a series of standard solutions containing 50, 100, 150, 200, and 250 mg per liter of oxygen demand by pipetting 5-, lo-, 1 5 , 20-, and 25-1111 aliquots of the standard sodium acetate solution into 100-ml volumetric flasks. Dilute each to volume with
Table I. Precision of Calibration Data Calcd. oxygen Std deviation demand, COD, mg/l Peak height, chart divisions Av Div mgfl 50 100 150
200
250 300
208
*
10.0, 10.8, 10.0, 10.8, 10.4 21.5,21.4,22.4,21.7,22.0 35.0, 35.8, 35.8, 35.5, 35.8 50.0,50.2,48.4,48.6,48.6 61.9,62.0,63.0,61.8,62.4
17.1, 76.4, 75.6,75.7, 76.4
ANALYTICAL CHEMISTRY
10.4 21.8 35.6 49.2 62.2 16.2
0.40 0.34 0.35 0.87 0.49 0.61
1.9 1.6 1.5
3.6 2.0
2.4
distilled water and mix. Fill an additional flask with the same distilled water for use as a blank. Before making readings on the instrument, inject several 2 0 4 portions of a standard solution into the combustion tube to condition the catalyst and the detection system. Such conditioning causes a rise in base line which will hold for about 15 minutes. Whenever the base line falls, repeat the conditioning. Much of this effect appears to be caused by adsorption and evaporation of moisture on the cell windows, as it can be prevented by insertion of a drying tube after the filter. Introduce 20 p1 of each of the standards and the blank into the instrument successively at about 3-minute intervals and determine the peak height in scale divisions for each injection. The technique is as follows: Rinse the syringe several times with the solution to be analyzed, fill, and adjust to 20 pl. Make sure that no air bubbles are present. Wipe off the excess liquid with soft paper tissue, taking care that no lint adheres to the needle. Remove the plug from the syringe holder, insert the syringe needle as far as it will go, 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 gas flow returns to normal, then replace it with the plug. Run the solutions in duplicate, or until checks are obtained. Correct the results for the blank (using the positive peaks only) and prepare a standard curve of peak height us. the oxygen demand expressed in milligrams of oxygen per liter, on rectangular coordinate graph paper. If the peak heights for the standards are low, as indicated by the calibration curve passing not through the origin but through some negative value, increase the amount of carbon monoxide in the carrier gas by raising the temperature of the carbon monoxide generator tube heater and repeat the calibration. Procedure. If the material to be analyzed contains suspended solids, blend it for 5 to 10 minutes, so that a uniform sample can be obtained with a syringe. Inject 20 pl into the combustion tube and measure the height of the resulting positive carbon monoxide peak. Subtract the appropriate blank. From the corrected peak height and the standard curve determine the oxygen demand of the sample. RESULTS AND DISCUSSION
Precision and Accuracy. Peaks obtained during standardization against sodium acetate solutions under the conditions outlined above are illustrated in Figure 3. Table I indicates the precision of a set of repeated calibration readings. Above an oxygen demand of 100 mg per liter, the instrument response is practically a straight line. Because contamination from samples may affect the activity of the combustion tube packing, however, a calibration graph should be rechecked frequently. Occasional washing of the tube and pa5king is advisable, followed by drying and conditioning in the carbon dioxide-monoxide atmosphere to remove all oxygen. The first two or three readings after the instrument has not been used for an hour or more are likely to be erroneous and should be repeated until constant. Known solutions of various substances have been analyzed by the proposed method, with results as shown in Table 11. Theoretical oxygen demand was calculated from the formula of each compound and its concentration, assuming complete oxidation in accordance with Reaction 1. For several of the compounds the results are low; possible reasons for this are discussed later. The usual method for oxygen demand frequently yields low results also. Comparable COD values by the chromic acid method have been re-
6ol r !-I
r
50
I /
70
'
30
20
'
10
.
40
i I
- 0 0
" c
50
I
100 150 FLOW RATE, ml./MIN.
I 200
i 250
Figure 4. Effect of flow rate on peak height Sample. 20 pl of solution containing 0.5316 gram of NaCnHsOz. 3H20per liter. Oxygeii demand = 250 mg per liter. Furnace temperature 875' C ported for various materials (4, 5, 6). A principal difference between results by the two methods lies in the behavior of the nitrogen compounds. Ammonia and urea reduce carbon dioxide at high temperature but do not normally reduce chromic acid in boiling aqueous solution. Other differences may be exhibited by individual compounds, depending upon their ease of oxidation by chromic acid and their solubility or volatility. If a specijic substance of interest is found to yield low results by the combustion method, the error may be eliminated by calibration against that substance. Effect of Varying Conditions. The recommended conditions were chosen after preliminary studies had been made on solutions of acetic acid, oxalic acid, sucrose, and sodium acetate. Usually the last gave the highest instrument reading relative to calculated oxygen demand, so it was used in a study of the effect of variables. With all other conditions as specified, the effect of varying the rate of flow of carbon dioxide was found to be as shown in Figure 4. U p to a rate fo 50 ml per minute the jpeak height increases with flow velocity, but between 50 and 103 it remains almost constant. Beyond 100 ml per minute the peak height drops slowly. In practice a rate of 130 ml per minute is recommended as this rate provides a low time lag between injection and instrument response. Subsequent tests were made at the recommended rate. The effect of combustion tube temperature variation is shown in Figure 5. No plateau is reached where the peak height is independent (of temperature, but above 750" C the incremental change is relatively small. Data in Remy (7) indicate that in the reaction between carbon and carbon dioxide:
c:o,f c * 2co
the composition of the gas in equilibrium with solid carbon at one atmosphere pressure varies in the following way with temperature: (4) R. A. Dobbs and R. 'r. Williams, ANAL.CHEM., 35,1064(1963). (5) W. A. Moore, R. C. Kroner, and C. C. Ruchhoft, Ibid., 21,953 (1949). (6) R. B. Schaffer, C. E. Van Hall, G. N. McDermott, D. Barth, V. A. Stenger, S. J. Sebesta, and S. H. Griggs, J. Water Pollution Control Federation, 31, 1545 (1965). (7) H.Remy, "Treatise on Inorganic Chemistry," Vol. I, p. 441, Elsevier, New York, 1!956.
4
"t 10
0
300
400
500 600 700 800 FURNACE TEMPERATURE, *C.
900
1000
Figure 5. Effect of temperature on peak height Sample. 20 ~1 of solution containing 0.5316 gram of NaCzHsOz.3H~0per liter. Oxygen demand = 250 mg per liter Flow rate. 130 ml per min
c % coz % co t,
o
800 6.0 94.0
900 2.8 97.2
lo00 0.7 99.3
In view of the facts that under the conditions of analysis one has a vanishingly small amount of carbon and a great excess of carbon dioxide, it is evident that the formation of carbon monoxide should be substantially complete at any of these temperatures. Although the higher temperatures might be expected to be more favorable, there is a disadvantage in going above 900" C because of an effect on the water blank (see below). An operating temperature of 875" C is recommended. Platinum was found to be a useful catalyst in the early experiments with this combustion system and little effort has been made to find a less expensive substitute. Asbestos and thorium oxide-impregnated asbestos were unsuitable, possibly because of iron impurities. Quartz chips coated with a thin film of platinum were tried also, but this packing did not produce the sensitivity obtained with pure platinum, Obviously the materials used in the combustion tube or packing must not react with oxygen, carbon monoxide, or carbon
Table 11. Analyses of Known Solutions Oxygen demand, mdl Oxidation Compound Calcd Found efficiency, Acetic acid 246 239 97.2 Benzoic acid 250 248 99.2 Oxalic acid 250 244 97.6 Glycine 250 248 99.2 Urea 250 250 100.0 pNitroaniline 250 244 97.6 Phenol 245 216 88.2 Sucrose 248 215 86.7 Acetone 173 145 83.8 Ethanol 235 200 85.1 Methanol 238 205 86.1 Ammonium hydroxide 250 204 80.6 Ammonium chloride 109.6 250 274
VOL 39, NO. 2, FEBRUARY 1967
209
dioxide. This excludes the use of most metals and their oxides. The response of the instrument is sensitive to the length of the packing as well as to the nature of the material being oxidized. Figure 6 illustrates the effect of packing length on the per cent of oxidation for solutions of several compounds. Essentially complete oxidation appears to be obtained with such solid materials as oxalic acid, benzoic acid, p-nitroaniline, glycine, and urea, whereas phenol and volatile liquids such as methanol, ethanol, and acetone gave low results even at the optimum packing length of 5 inches. The curves of Figure 6 can be interpreted on the basis of two factors, The first involves the assumption made originally, that Reaction 2 goes to completion. With at least 4 inches of packing this appears to hold true for sodium acetate and urea, but the more volatile compounds such as acetone and phenol apparently may not be completely oxidized even with 5 inches or more. The second factor may be a result of the steam pressure developed when an aqueous sample reaches the combustion zone. With a longer packing, proportionately more of the vapor is forced back into a cooler part of the tube. The consequent delay in reaction causes a lowering of peak height. Interferences. Ammonium chloride is the only compound yet tested that yields results higher than calculated according to the theoretical equation. That the effect is not a consequence of nitrogen oxide formation is shown by good results for other nitrogen compounds such as urea. That it is not due to absorbance of infrared radiation by hydrogen chloride or a hydrochloric acid fog in the gaseous products was demonstrated by insertion of an alkaline scrubber between the combustion tube and the detector; the peak height was not decreased thereby. The reason for the high value may be a secondary reaction of hydrochloric acid with carbon dioxide and platinum: 2 NHICl
+ 3 COz
2 HCI
-P
3 CO
+ COz + Pt
-P
+ Nz + 2 HCI + 3 Hz0 CO + HzO + PtClz
Very likely the platinum chloride is a transient surface compound, which dissociates quickly to platinum and chlorine under the furnace conditions. The extra carbon monoxide, however, would account for the high reading obtained. Samples of hydrochloric acid or sodium chloride containing the same chloride concentration as the ammonium chloride standard (355 mg per liter) gave peaks corresponding to approximately 30 to 40 mg of oxygen demand per liter. This is about the amount by which the ammonium chloride standard is high (10z)and tends to substantiate the proposed reaction of chloride with carbon dioxide to produce carbon monoxide. However, standards containing both sodium acetate and sodium chloride do not show the same magnitude of interference as when chloride alone is present. With a standard containing sodium acetate corresponding to 250 mg of oxygen demand per liter and sodium chloride amounting to 300 mg of chloride ion per liter a positive interference of only 1 was found. As chloride is a common constituent
Table 111. Interference of Inorganic Salts Peak height, div Solution Total Error Error, Acetic acid only 65.8 Acetic acid Na2SOb 57.3 -8.5 -12.9 12.4 -53.4 -81.2 Acetic acid NaNOs 63.5 -2 . 3 - 3 . 5 Acetic acid NaHZPOd. HzO
+ +
+
210 *
ANALYTICAL CHEMISTRY
100
t
gOl 80
0 I-
60
z
w
aw
50 40
I
c
(OI
30
IO
01
0
i
IO/
i
13'
I 1
I
1
2 LENGTH
3
I I I 4 5 6 PLATINUM PACKING, IN.
I
I
7
6
OF Figure 6. Effect of packing length on oxidation of various substances Sample. 20 PI solutions as in Table II Furnace temperature. 875 C Flow rate. 130 ml per min 1. Ammonium chloride 8. Sucrose 2. Sodium acetate 9. Acetic acid 3. Glycine 10. Phenol 4. Urea 11. Methanol 5. Oxalic acid 12. Ethanol 6. p-Nitroaniline 13. Acetone 7. Benzoic acid O
of waste waters, it may be necessary when the oxygen demand is low to determine the magnitude of the interference by means of a known solution containing the amount of chloride present in the sample. Alternatively, the standards may be made up with the appropriate concentration of chloride. Several other inorganic anions, including sulfate, nitrate, and phosphate, cause negative interference with this method. Table I11 contains the analyses of several standard solutions each containing 250 mg per liter of total oxygen demand and 500 mg per liter of one of the sodium salts of these anions. All three of the salts tested interfere by decomposing to yield available oxygen, with nitrates showing the greatest effect. Nitrates would yield available oxygen in BOD tests also and probably should not be regarded as interfering if actually a valid part of the sample. Sulfate does not interfere in the usual COD or BOD determinations, but under the combustion conditions of the method it is probably converted to sulfite. If the organic matter present in a sample is entirely soluble in water, the sulfate interference can be avoided by preliminary precipitation with barium hydroxide in slight excess. Other Causes of Variation. In the early stages of this investigation variations in base line were observed that coincided with the removal of the syringe from the adapter. This be-
havior may be attributed to one or more of several possible factors. One would be a change in flow rate through the system, resulting in a change of the carbon monoxide concentration in the detector cell. Another would be the introduction of a small amount of oxygen into the combustion tube through the open syringe adapter. Any oxygen entering the tube would decrease the carbon monoxide level reaching the detector. The latter explanation seemed the most plausible. To overcome the effect, provision was made to protect the injection port with an inert atnosphere, by means of a glass sleeve enveloping the injection port, through which was passed the exhaust gas from the infrared analyzer (substantially pure carbon dioxide). Much of the variation was eliminated by this means. Another source of variation in results is the water blank. Pure water injected intl3 the combustion furnace under the conditions specified in the method produces a curve with two peaks, one negative and the other positive. This is illustrated in the blank run of Figure 3. Ordinarily the negative peak is ignored and the blank is read as the height of the positive peak above the base line. The blank is deducted from readings on standards before preparing the calibration curve and also from readings on samples. Variations can be minimized by controlling the combustion tube temperature closely at 875" C and operating with a rather low but constant level of carbon monoxide in the feed gas stream. Application to Waste 'Water. The oxidation products listed in Equations 1 and 2 differ, with respect to the behavior of nitrogen, from those expected in measuring COD or BOD. In the case of COD determination, nitrogen bonded to hydrogen is normally converted to ammonium sulfate (though some gaseous nitrogen may be liberated if chloride is high and is not complexed with mercuric ions). In ultimate BOD, nitrogen may be oxidized to nitrate. Because of these differences, one cannot expect the proposed method to yield results for nitrogenous wastes agreeing exactly with those of either COD determinations or ultimate BOD tests. The results will also differ in respect to compounds that are resistant to attack by chromic-sulfuric acid or organisms. Therefore the authors suggest that the quantity determined by the proposed method be given the slightly different designation of C02D, signifying an oxygen demand obtained by the carbon dioxide combustion method. Under well-controlled conditions the C 0 2 D method is capable of yielding consistent results on solutions of particular compounds. It also appears to be applicable to the analysis
Table IV. Results on Domestic Sewage
Sample No. 1 2 3 4 5 6 7 8 9 10 11
Oxygen demand, mg/l Proposed Standard method method (COD) (COD) 213 220 235 195 240 213 158
190 370 210 190
205 205 229 137 252 198 157 189 350 196 166
AV
Ratio C02D :COD 1.04 1.07 1.03 1.04 0.95 1.08 1.01 1 .oo 1.06 1.07 1.14 1.045
of domestic sewages after settling or homogenization. Table IV presents data obtained on samples of effluent from the primary settling step in the municipal sewage treatment plant at Midland, Mich. Before COzD determination, the samples were homogenized in a Waring Blendor. The C 0 2 D results are averages of two or three determinations by the proposed procedure, while those for COD are averages of duplicates by the standard method ( I ) . Generally the former are somewhat the higher, as would be expected considering the presence of nitrogenous components. However, the average difference is probably within the normal experimental variation of both methods. Further applications to domestic sewage and industrial wastes have been attempted with reasonable success (8). RECEIVED for review September 12, 1966. Accepted December 9,1966. Division of Analytical Chemistry, 152nd Meeting, ACS, New York, N. Y., September 1966. A patent application has been filed on certain features of the apparatus and procedure described herein. An instrument incorporating these features will be made commercially available in due time. (8) V. A. Stenger and C. E. Van Hall, Preprint 5.3-4-66, Instrument Society of America, 21st Annual Conference, New York, Oct. 24-27, 1966.
VOL. 39, NO. 2, FEBRUARY 1967
21 1
