ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
1085
Semi-Micro Tube Method for Chemical Oxygen Demand Ronald R. Himebaugh"' and Michael J. Smith Depaflment of Chemistry, School of Engineering and Science, Wright State University, Dayton, Ohio 4543 7
Through chemical and biological oxidation, organic material released t o receiving waters can greatly reduce or deplete dissolved oxygen. There have been several methods developed to measure this oxygen demand and two, the Biochemical Oxygen Demand (BOD) test and the Chemical Oxygen Demand (COD) test, have emerged as the most popular ( I , 2 ) . While the BOD determination closely approximates the actual oxygen demand present under sample conditions, the COD determination measures that portion of organic matter present that can be oxidized by a strong chemical oxidant. Although both tests are parameters used for indicating the general presence of organic pollution, the COD measurement has several advantages that have increased its popularity in recent years. First of all, the conventional COD test takes considerably less time (2 h) than the BOD test ( 5 days); this makes it much more useful as a control parameter in plant operations. Secondly, since the COD test does not rely on biological oxidation but on known quantities of reagents and sample, the COD results are more reproducible and the method is less susceptible to interference from toxic chemicals. Finally, an estimated BOD can many times be computed from the COD results if a relationship is established between the two methods a t a specific sample source. Basically, the method involves a 2-h sample oxidation by a known excess of potassium dichromate in hot 50% sulfuric acid (3). Under these conditions susceptible organic material is oxidized to carbon dioxide and water with the subsequent reduction of an equivalent amount of hexavalent dichromate t o trivalent chromium:
3(CH20)+ 16H'
-
+ 2Cr2072-
4Cr3+
+ 3C02 + llH20
After oxidation the remaining dichromate is titrated with ferrous ammonium sulfate and the equivalents of oxidant consumed are converted t o milligrams of oxygen consumed per liter of sample. This method has been further optimized by the addition of several modifications. Silver sulfate is added as a catalyst t o improve the oxidation of straight chain aliphatic compounds, and mercuric sulfate and sulfamic acid are used to alleviate interferences from chlorides and nitrites, respectively ( I , 4 ) . Although the COD method does not oxidize all organic compounds (such as aromatic hydrocarbons and pyridines) completely, it does oxidize most of the organic compounds commonly found in wastewater with little interference ( I ) . Recently, the standard method for COD has been modified for the Technicon AutoAnalyzer by reducing reagent quantities and oxidizing the sample in a small Teflon-capped culture tube. Unlike the standard method, the remaining dichromate following oxidation is determined spectrophotometrically rather than titrimetrically. This method has resulted in a n increase in precision, accuracy, and ease of analysis with a reduction in bench space, cost, time, and glassware ( 5 ) . Since the AutoAnalyzer tube method is not applicable to the water and wastewater laboratories not equipped with an AutoAnalyzer, the tube titration method described herein was developed (6). The tube titration method still reduces reagent and sample quantities, b u t not t o the level used in the AutoAnalyzer tube method. Also, like the standard method, the Present address: 3648 Eastern Drive, Dayton, Ohio 45432.
dichromate remaining after digestion is titrated with ferrous ammonium sulfate, b u t without sample transfer. Like the AutoAnalyzer method, this method reduces analytical time, glassware cleaning, and reagent cost, yet requires less space and equipment than either the standard method or the AutoAnalyzer method.
EXPERIMENTAL Apparatus. Samples were digested in Corning No. 9826 20 X 150 mm screw cap (cap No. 9998) culture tubes. An aluminum block (6-hole, 25.75 mm diameter X 48 mm depth) placed on a laboratory hot plate was used to heat samples. Both a 0-5 mL and a 5-10 mL Oxford adjustable pipet were used in addition to a magnetic stirrer and Teflon stir bars, size 3 / 4 X 5 / 6 inch. Reagents. All chemicals were ACS reagent grade and all water was distilled and deionized. Digestion solution was prepared by adding 4.9035 g of K2Cr207, 83.5 mL of concentrated H2S04and 16.7 g of HgS04to 200 mL of water and diluting the cooled solution to 500 mL. Sulfuric acid-silver sulfate solution was prepared by dissolving 22 g of Ag2S04in a 9-lb bottle of concentrated H2S04. Titrant was prepared by adding 9.75 g of Fe(NH,)2(S04)2.6H20 and 5 mL of concentrated H2S04to 200 mL of water and diluting the cooled solution to 500 mL. Ferroin indicator was prepared by dissolving 1.49 g of 1 , l O phenanthroline monohydrate and 0.70 g of FeS04.7H20in 100 mL of water. Procedure. All culture tubes and screw caps were cleaned with soap and water and chromic acid to prevent contamination. Alternately, tubes may be heated in a muffle furnace at 450 "C for 1 h instead of washing. Samples containing high suspended solids content were homogenized in a blender prior to digestion, and 10 mg of sulfamic acid per mg nitrite were added to samples suspected of containing substantial nitrite nitrogen. Digestion was conducted by pipetting 5 mL of sample and 3 mL of digestion solution into a culture tube. Seven ( 7 ) mL of the sulfuric acid-silver sulfate solution were then added carefully down the side of the tube so that an acid layer formed beneath the digestion solution-sample mixture. The tube was capped tightly and inverted several times to establish complete mixing. Three blanks were prepared in the same way as samples and analyzed with each set of samples. All samples and blanks were placed in either an oven or a block heater at 150 "C (the reflux temperature of 50% sulfuric acid). After 2 h the tubes were removed and cooled at room temperature. The potassium dichromate remaining was titrated conveniently in the tube by placing a stir-bar and a drop of indicator in the tube and stirring the sample on a magnetic stirrer with the use of a test tube rack or finger clamp. Several tubes were stirred in the rack simultaneously. The volume of titrant consumed in the titration of samples and blanks was recorded and the mg/L COD determined using the standard COD formula.
RESULTS AND DISCUSSION T h e precision of the new method was determined through the analysis of 11 replicates of two samples by both the standard method and the tube method. One sample was a wastewater effluent sample in the lower range of 60 t o 70 mg/L and the other was a raw wastewater sample a t a higher COD level of 175 t o 185 mg/L. In addition, precision was determined for the tube method on two reference samples obtained from the Environmental Monitoring and Support Laboratory, U S . EPA. For a low level tube COD determination, a titrant normality of 0.025 N was used on nine replicates of an effluent sample a t a COD concentration of
0003-2700/79/0351-1085$01.00/0 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 7 , JUNE 1979
Table I. Estimated Precision for the Tube and Standard COD Methodsa
Table 111. COD Sample Recovery by the Tube Methoda
no. of
method
sample
standard tube standard tube tube tube tube (low level) tube tube (low level)
upper range upper range middle range middle range U.S. EPA 31 U.S. EPA -12 lower range
COD, mg/L
replicates mean
9
186 175 70 65 14.6 229 33.6
9 9
7.6 3.8
11 11 11 11
9 10
blank blank
source
std range dev. 10
13 19 14 3.9 9 5.8
3.3 4.7 5.5 4.0 2.9 3.0 1.8
14.3 4.4 2.0 0.9
All samples were collected and preserved as stated in reference 6.
added KHP, mg/Las COD
sample COD, mg/L
Ind-1 Ind-2 Inc. Dairy
324 454 46 363 78
TF AS
117
cs
100 100
100 100
100 100
107
ww
100 100
0
source
cs
Results should be 9 5 to 100% of the actual value. 14th Edition. oratories.
TF AS
Determined by 74 collaborative lab-
34 mg/L. Similarly, nine replicates of a blank were analyzed for detection limit data. Table I indicates that the precision of both methods is well within the limits suggested in the 14th edition of “Standard Methods” (Table 11). Both methods produced similar results, with the tube method precision slightly poorer a t the upper range (4.7 mg/L) but better than the standard method in the lower concentration range (4.0 mg/L). Similarly, the U S . EPA reference samples and the low level COD sample were analyzed with precision well within the limits defined by standard methods (Table I). Defining the detection limit a s the mean value of blank analyses plus two standard deviations, the tube method is applicable in a range from 16.5 to 960 mg/L COD; however, by using a reduced ferrous ammonium sulfate concentration (0.025 N) a range of 5.6 to 480 mg/L is achievable. T h e accuracy of the new method was determined by spike recovery and comparison with the standard method. Nine different real sources were sampled, spiked with a known standard, and the percent recovery was calculated. T h e sample types consisted of three wastewater effluent samples (contact-stabilization plant (CS), trickling filter plant (TF), and activated sludge plant (AS), one solid waste incinerator effluent sample (Inc.), one dairy effluent sample (Dairy), one well water sample (WW), and two auto industry samples (Ind-1 and Ind-2). In addition, the recovery data of the tube and standard method were compared and recovery by the tube method on the U S . EPA reference samples was determined. Both methods produced average recoveries of K H P spikes in real samples greater than 99% with precision comparable to that encountered in the replicate study (standard and tube method standard deviations of 5.4 and 4.9 mg/L, respectively); see Tables I11 and IV. Recovery of the U.S. EPA reference samples by the tube method was an acceptable 94.8% a t the low level and 99.1% at the upper level (Table V). Also, comparison of the tube method and the standard method
added KHP, mg/Las COD
sample COD, mg/L
Ind-1 Ind-2 Inc. Dairy
1 3 mg/L mg/L
I 14
Accuracyb a
98 108 97 95 100
97 95 106
Table IV. COD Sample Recovery by the Standard Method
std. dev. t
ery of added KHP
99.5 4.9 All samples were collected and preserved as stated in reference 6.
Precisionb 200 mg/L COD 1 6 0 mg/L COD
% recov-
mean standard deviation
Table 11. Standard Methods Limits for COD Analysesa sample concentration
sample KHP, COD, mgiL 422 562 143 458 178 214 202 106
T
ww
301 412 42 395 79 112
100 100 100 100 100 100 100 100
107 0
sample KHP, COD, mg/ L 407 518 134 487 179 210
+
206
101
mean standard deviation
recovery of added KHP
%
106 106 92 92 100
98 99 101 99.3 5.4
Table V. Recovery of COD from U.S. EPA Reference Samples by the Tube hlethod no.
mg/L COD knowna avCOD liCOD recov- % r e cates range (mean) ered covery of
rep-
source U.S. EPA x l U.S. EPA =2
9 10
3.9 10
15.4 231
mean
14.6 229
94.8 99.1 97.0
The U.S. EPA reference samples were prepared as concentrates by dissolving known amounts of analytical reagent grade chemicals in distilled water for exact and preplanned concentrations. Each sample has been analyzed repeatedly over a period of months to ensure stability. These samples are specifically designed to check quality control. a
-
Table VI. Comparison of COD Sample Recovery by the Tube and Standard Method
source Ind-1 Ind-2 Inc. Dairy
cs
TF
AS
ww
standard method COD, mg/L
301 412 42 39 5 79 112 107 0
mean standard deviation
tube method COD, mg/L 324 454 46 363 78 117 107 0
recovery as percent of the standard method 107.6 110.2 109.5 91.9 98.7 104.5 100.0 100.0
102.8 6.3
ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
Table VII. Comparison of the Semi-Micro Tube Method with the Semi-Automated Tube Method' no.
a
method
sample
semiautomated semimicro
wastewater wastewater
of COD, mg/L replistd. cates mean range dev. 11 10 9 11
26
4
1.3
270 34 175
12 6 13
4.6 1.8
4.7
Semi-automated tube method by Jirka and Carter ( 5 ) .
showed that the tube method recovery averaged 102.8% of the standard method on a variety of sample types (Table VI). On the average, the accuracy data collected fell within the limits (Table 11) of 95-100% recovery, with only the low level reference sample at a border 94.8%. Good recovery a t low levels, however, could be obtained by using reduced normalities of ferrous ammonium sulfate. Detection limits and precision on wastewater using a reduced titrant normality agreed favorably with data achieved by Jirka and Carter (Table VII). T h e acceptance of a new analytical method in water and wastewater is usually determined by one or more of at least five factors: precision, accuracy, cost, method simplicity, and safety. Without exception, the tube COD method, described herein, excels in these factors. Precision and accuracy of the tube method falls within the required limits and, in many respects, is a n improvement to the present method. By reducing the reagent volumes and using culture tubes in place of reflux apparatus, several advantages have been achieved. First, the elimination of boiling chips and the reduction of glasssample contact surface in the tube method minimizes the possibility of contamination. Secondly, a source of error introduced by heat generation during reagent mixing and the subsequent loss of volatile sample components is eliminated by adding the digestion solution down the side of the tube, capping, and mixing so that heat generation occurs only in closed tubes. Such losses are minimized in the standard method by submerging the flask in a n ice bath or adding reagents through a condenser. One further improvement effected by the tube method is the addition of mercuric sulfate in liquid form, as part of the digestion solution. In this way, the addition of this important reagent is more reproducible than when added as a powder as is done in the standard method. Probably the greatest asset of the tube method is cost reduction. Because of the high cost of mercuric sulfate and silver sulfate, the 75% reduction in reagent use makes the tube method very attractive t o t h e budget-minded laboratory. Furthermore, the initial equipment cost for the tube method is much less than the standard method. The estimated initial cost of glassware, for example, of one analysis by the standard method is $17.00 ($15.00, condenser; $2.00, flask), while the cost of one culture tube with cap, used in place of the condenser and flask, is $0.65. Other expenses such as condenser
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tubing and lattice framework, commonly used in the standard method, are eliminated in the tube method completely. The tube method is inherently simpler and less timeconsuming than other methods. The omission of much of the standard glassware eliminates condenser rinsing and reagent cooling plus glassware cleaning. Time is further saved because of smaller reagent additions, and the elimination of boiling chips and temperature control during reagent additions. One of the more attractive features of the tube method, however, is the conservation of bench space. In the standard method, the number of samples analyzed is usually determined by work space, condenser-hot plate accommodation, a n d water availability for condenser cooling. These limitations are all eliminated in the tube method, and existing laboratory equipment can be utilized with the purchase of only culture tubes. As many digestions as necessary can be done in an oven with essentially little space limitation. Block heaters may also be used to complete the digestion and are preferred. Although both the oven method and the block heater method work, the block heater method appears to be superior in several practical ways. After oven heating, it has been found that tube caps tend to stick and may require the use of pliers for removal. This does not occur when digestion is completed in a block heater since only the lower portion of the tube is heated. The solution level in the tube comes just to the top of the block and is completely surrounded by heat, producing even heating and excellent reflux. Lastly, cast aluminum heating blocks can be purchased a t low cost ($20.00) and heated on an existing laboratory hot plate. Finally, safety is a positive factor in the tube method. Mercury compounds are hazardous materials in the laboratory and mercuric sulfate is no exception. The standard method requires the addition of mercuric sulfate as a powder, usually using a measuring spoon. The prolonged use of this material and dust generation could make this procedure less than healthy. The tube method minimizes the manipulation of dry mercury salts by making the mercuric sulfate addition as part of the liquid digestion solution. The safety problem associated with the disposal of mercury compounds has generated recovery methods for mercury in the COD method which are not always followed (7).The smaller volumes used in the tube method reduce the amount of mercury released when mercury recovery is not practiced.
LITERATURE CITED "Chemical Analysis for Water Quality", Training Manual, Environmental Protection Agency, Cincinnati, Ohio, 1973. W. A. Moore, E. J. Ludzack. and C. C. Ruchhoft, Anal. Clem., 23, 1297 (1951). "Standard Methods for the Examination of Water and Wastewater", 14th ed., American Public Health Association, New York, 1975. R. A. Dobbs and R. T. Williams, Anal. Chem., 35, 1064 (1963). A. M. Jirka and M. J. Carter, Anal. Chem., 47, 1397 (1975). R. R. Himebaugh, M.S. Thesis, Wright State University, Dayton. M i , 1977. R. B. Dean, R. T. Williams, and R. N. Wise, Environ. Sci. Techno/.,5 , 1044 (1971). "Methods for Chemical Analysis of Water and Wastes", Environmental Protection Agency, Cincinnati, Ohio, 1974.
RECEIVED for review August 2 2 , 1978. Accepted cJanuary22, 1979.
Oxygen Plasma Asher J. E. Patterson Chemistry Division, DSIR, Lower Hutt, New Zealand
Electrodeless discharge lamps are used in many laboratories as light sources for the determination, by atomic absorption 0003-2700/79/0351-1087$01.OO/O
spectrometry (AA), of volatile metals or metals with volatile halides such as As, Cd, Pb, Sb, Se, Sn, T1, etc. Lamps powered 6 1979
American Chemical Society
