164
ANALYTICAL CHEMISTRY
dilution of 1 to 20 was necessary. Table Is’ shows the recovery of added increments of detergent from the diluted sewage. I n this experiment] solutions containing 90% sewage mere used with definite amounts of detergent. The solutions \yere then diluted 1 to 20 and the detergent was determined. The recovery data are averages of replicate determinations and thus do not reflect the wider variation found n ith the methylene blue method. FROM RIVER\trTaTER. Recovery of detergent from Ohio River water was determined following the same procedure as for sewage, but it was not necessary to dilute the river water, as its detergent content was low. Table V s h o w the recoveries of added detergent (corrected for the initial detergent present), ranging in concentrations from 0.25 to 3.0 17.p.m. The methylene blue method gave a higher initial detergent content than it did with seFvage. The recovery of these added increments T\as comparable by both methods. FROMLAKEWATER. The effect of vegetable decomposition products on the accuracy of a detergent analysis was tested by use of a sample of water taken from a small lake (approximately 0.75 acre), which had abundant vegetable growth. The possibility that this lake contained detergent compounds is very remote. The results of the tests indicate that the methylene blue may be reacting with some compound present other than detergent. The data on recovery of added increments of de-
tergent show a much wider fluctuation in the case of the methylene blue method. REFERENCES
(1) Barr, T., Oliver, J., and Stubbings, W. V.,J. SOC.Chem. I n d . (London) 67, 45-8 (1948). (2) Biffen, F. M., and Snell, F. D., IND.EXG.CHEM.,ANAL. ED.7, 234 (1935). (3) Degens, P. N . , Jr., Zee, H. van der, and Kommer, J. D., Sewage and I n d . Wastes 25, 20 (1953). (4) Edwards, G. R., Ewers, W. E., and h,fansfield, W. W., Analyst 77, 205 (1952). (5) Epton, S. R., Trans. Faraday SOC. 44, 226 (1948). (6) Evans, H. C., J . SOC.Chem. I n d . 69, Suppl. 2, 576 (1950). (7) Faust, S. D., Water and Sewage Works 100, 242 (1953). (8) Gowdy, TFT. R., Sewage and Ind. Wastes 25, 15 (1953). (9) Harris, J. C., IND.ENG.CHEM., ASAL. ED. 15, 254 (1943). (10) Hart, R., Zbid., 5 , 413 (1933). (11) Jones, J. H., J . Assoc. Ofic. Agr. Chemists 28, 398 (1945). (12) Karush, F., and Sonenberg, hl., ANAL.CHEM.22, 175 (1950). (13) RIarrow, T. o., and Schifferli, J., IND.ENG.CHEM.,ANAL.ED. 18, 49 (1946). (14) Sewage and I n d . Wastes 24, 658 (1952). (15) Stackelberg, AI. V. van, and Schuts, €I., Kolloid 2. 105, 20 (1943). (16) Wallin, G. R., h . 4 ~ .CHEX 22, 616 (1950). RECEIVED for review June 20, 1955. Accepted December 8, 1955. Division of Water, Sewage, and Sanitation Chemistry, Symposium on Problems i n Stream Pollution, 127th Meeting, ACS, Cincinnati, Ohio, hlarch-.4pril 1955. Other papers in this symposium appeared in the February 1955 issue of I n d u s trial and Engineering Chemistry.
Determination of l o w Chemical Oxygen Demands of Surface Waters by Dichromate Oxidation W. ALLAN MOORE
and
W. W. WALKER
Robert A. Taft Sanitary Engineering Center, Public Health Service, U. S. Department of Health, Zducation, and Welfare, Cincinnati, O h i o
The desirability of a more sensitive evaluation of the chemical oxygen demand (C.O.D.) test in the range of 5 to 50 p.p.m. justified a thorough reinvestigation of the oxidizing efficiency of 0.025 and 0.05N potassium dichromate and comparison with reported values for 0.25N dichromate. Reproducible results were obtained with the weaker dichromate solutions in replicate series of determinations when the condenser outlets were kept plugged with glass wool, acidified distilled water was used for dilutions and washing, and the tapered-joint seals between flasks and condensers were cleaned by wiping with a damp cloth. Theoretical oxidation of chlorides and representative organic compounds compared favorably with the 0.25N dichromate reagent. When silver sulfate is used as a catalyst, only 40 p.p.m. correction should be subtracted from the C.O.D. found, where chlorides are from 40 to 300 p.p.m. Without silver sulfate, corrections are quantitative. The concentration of organic material should be regulated so that not more than 50% of the available dichromate is used. With varying stream flows the C.0.D.-B.O.D. ratio may vary by several hundred per cent; the farther from the source of pollution, the higher the ratio, which indicates a stream more biologically balanced. With stable stream conditions, where the ratios are relatively constant, B.O.D. ranges at representative stream points can be estimated from previous representative C.0.D.-B.O.D. data.
A
AIETHOD proposed for estimation of the organic content
of industrial wastes and sewage by a wet combustion method (1, 2 ) , using 0.25Y potassium dichromate as the oxidant in a 50 volume % sulfuric acid solution, did not possess the desired sensitivity xhen applied to stream samples. I n seeking to obtain greater sensitivity with samples containing a lower organic content ( 5 to 50 p.p.m,), use of 0.025 and 0.05A- potassium dichromate was investigated. This study included the oxidation of representative organic compounds, chlorides, and the organic material contained in river waters. PROCEDURE
I n working with the weaker dichromate solutions, two possibilities are presented; 25 ml. of 0.025-V potassium dichromate may be used or 2.5 ml. of 0 . 2 5 s reagent can be substituted. I n the latter case, the sample size may be increased from 50 to 70 ml. Early in the course of this study, it became apparent that much more rigid control was necessary in working Tvith 0.025N potassium dichromate than with the 0.25005 reagent. The procedure used was the same as that previously reported ( I ) , except that in the titration of the excess dichromate present it was necessary t o use two to three times as much indicator as for the stronger dichromate reagent, I n many of the preliminary runs wide discrepancies in replicate samples or blanke, ranging from 0.50 to 1.50 ml., were encountered. These discrepancies were found to be due mainly t o several factors. The distilled water used to wash down the condensers sometimes contained algal growths which contributed to wide fluctu-
V O L U M E 2 8 , N O . 2, F E B R U A R Y 1 9 5 6
165
ation in replicate samples. This difficulty was overcome by adding 1 ml. of concentrated sulfuric acid per liter to the rinse water and keeping the wash bottle scrupulously clean. The condenser outlets were kept plugged with glass wool, which was removed only to wash the condensers and promptly replaced. This prevented the entrance of organic material from the air during the reflux period.
Table I. h-0. of Replicates 3
results were obtained with glass wool plugs in the condenser openings, and using the acidified distilled water for dilutions and washings. Parallel series of replicates were analyzed simultaneously both with and without silver sulfate. Replicate series of blanks usually agreed within a few hundredths of a milliliter. Only two cases occurred in the seven replicate blank control series (33 total determinations) in Table I, where an individual result gave approximately 1% difference between the maximum and minimum of the control. When silver sulfate was used a t the rate of 1 gram to each flask of the regular reflux mixture, colloidal silver chloride was precipitated. Coagulation of the suspended silver chloride proceeded rapidly, as in all cases the flasks became clear with a few minutes' boiling, and granular silver chloride remained when 2 ml. or more of 0.025~11sodium chloride (1.77 mg. of chloride) had been added. Addition of from 5 to 15 ml. of 0.025-V sodium chloride (4.43to 13.29 mg. of chloride) resulted in the average oxidation of slightly more than the equivalent of 2 ml. of 0.025-V sodium chloride, or approximately 1.9 mg. of chloride ion. With a 5-ml. stream sample, this would correspond to 38 p.p.m. Based on the data of Table I, it is apparent that in regular determinations of oxygen consumed, quantitative chloride corrections are reliable in all ranges up to the limit of dichromate available, with the maximum expected deviations in the samples of lowest chloride content. When silver sulfate is added to the regular reflux mixture, quantitative chloride corrections to samples containing less than 1.9 mg. of chloride ion may be made. With chloride concentrations up to 300 p.p.m. of chloride approximately 40 p.p.m. of the total chloride should be oxidized, and results corrected for this amount. Oxidation of Organic Compounds. In Table I1 are shown the results obtained in the oxidation of organic compounds by 0.05
Oxidation of Chlorides with 0.025N Dichromate c1ildded, Mg. 0.443
Max. Min. hv.
6
0.886
6
1.77
G
4.43
Max. hlin. Av. Max. &fin. Av.
Max. hlin. Av.
6
8.86
3
13.29
3
17.72
Max. hlin. AT. hlas. hlin. Av.
Max. hlin.
xr.
Chlorides Oxidized Without With With .kg?SO4, iMg. AgzSO4, 3' % .k%?SOa, % ... 108.0 132.0 ... 104.0 112.0 ... 106.0 119.3 ... 102.4 104.4 ... 99.5 93.6 ... 100.8 100.8 1.68 102.5 95.0 1.64 92.5 92.5 1,67 97.3 94.0 2.02 101.0 ... 1.82 94.0 ... 1.92 98.1 ... 2.14 98.4 ... 1 70 95.8 1.82 96.9 ... 1.99 97.3 ... 1.88 95.0 ... l,95 96.5 ... 2.38 97.9 ... 2.22 96.9 ... 2.30 97.6 ...
...
The tops of the condensers and the area around the taperedjoint seals were wiped with a clean damp towel before the flasks were removed to introduce or remove samples. RESULTS
Oxidation of Chlorides. Preliminary runs were made using 0.0025 and 0.025N standard sodium chloride solutions. While considerable variations among replicates of blanks and samples were encountered, chlorides were completely oxidized by the 0.0255 potassium dichromate, when either 40 or 50 volume 70 sulfuric acid was used in refluxing. I n some cases average depletions within a series varied from 0.5 to 2.5 ml., but in all series approximately 100'?40 of the chloride was oxidized. Chlorides in the lowest ranges gave the most erratic results, as variations of a few hundredths of a milliliter in titration figures became significant when multiplied by the necessary factors. A chloride content of 1 ml. of 0.0025.V sodium chloride (0.089 mg. of chloride ion) would be less than 2 p.p.m. in a regular 50-ml. stream sample. With both 0.025 and 0.05*V potassium dichromate the deviations from the theoretical depletions became progressively less and results more uniform as the chloride content increased from 0.1 to 0.5 ml. of 0.025N sodium chloride (0.089 t o 0.443 mg. of chloride). Characteristics of chloride oxidation are well illustrated in Table I. These
Table 11. Oxidation of Organic Compounds No. of ~ ~ ~ l i cates Normality 12 10 6 3 3
0.050 0.050 0.050 0.050 0.050
$?iesdq(
t',-,$f,tf
Gram
for, Mg.
0 1.0 0
0 0 4.43 1.77 1.77
0 1.0
Sample Used. M g . 1.0 3.0 5.0 1.0 3.0 5.0C.O.D. found/ 1000 mg. of compound % of theory
Glucose 1030 950 990 980 890
1020 940 980 980 880
1010 950 980 980 890
96.4 89.1 92.6 92.3 83.0
95.3 88.6 92.1 92.3 82.6
94.8 89.3 91.5 92.2 83.1
(0.5) 1790 1760
(1.0) 1780 1750
(2.0) 1780 1730
(0.5) 94.7 93.2
(1.0) 94.3 92.3
(2.0) 94.2 91.5
Acetic Acid (1.0) 0 60 0 1030 (0.5) 0.89 970 0 1020 0.89 960
(3.0) 48 950 (1.0) 930 990 950
(5.0) 22 960 (2.0) 930 1000 950
(1.0) 5.6 96.6 (0.5) 90.9 95.5 89.6
89.2 (1.0) 87.5 93.0 89.1
(5.0) 2.1 89.7 (2.0) 87.3 93.9 88.6
1280 1660
72.1 93.8
71.1 02.6
70.1 91.3
(2.0) 186 235 660 740
(5.0) 176 219 560 640
13.9 18.6 57.3 63.2
(1.0)
(2.0) 14.8 18.7 52.4 58.7
(5.0) 14.0 17.4 45.0 51.2
Glutamic Acid (1.0) (2.0) 0 402 382 0 922 895
(6.0) 345 782
(1.0) 41.0 94.1
(2.0) 39.0 91.3
(5.0) 35.2 79.8
R esor cin o1 6
3
0,025
0.050
0 1.0
0 1.0 1 .o
3
0,025
5
0,025
1.5 1.5
3
0.025
0 1.0
3 3a 3 3"
0.025 0.025 0.025 0.025
0 0 1.0 1.0
3
0.025
' Refluxed 3 hours.
0 1.0
0 0
0 0
0 0
0 0
Butyric Acid 1310 1290 1710 1690 Alanine (1.0) 175 234 720 800
All others regular 2-hour reflux time.
(;)
166
ANALYTICAL CHEMISTRY
and 0.025N potassium dichromate. The solid organic compounds Glutamic acid also showed a drop in per cent oxidation in the used were dried overnight in the oven and cooled, and 1 gram 1-, 2-, and 5-mg. series, both with and n-ithout silver sulfate. Fvas dissolved in 1 liter of distilled water. Theoretical chemical With the regular procedure, the total difference was 6% (41 to oxygen demand (C.O.D.) values shown in the tables were cal35%). Where silver was used, a total difference of 14% (94 culated for these 1000 mg. per liter or 1000 p.p.m. solutions. 4 s to 80%) was obtained using the same glutamic acid concentra1 ml. of these solutions contained 1 mg. of the organic compound, tions. convenient amounts for each sample could be accurately measRiver Samples. Samples were collected from the Ohio and ured by pipet, and by multiplying the results by the appropriate Little Miami rivers. Replicate 5-day biochemical oxygen factor or 1000 divided by milligrams of sample used, the C.O.D. demands (B.O.D.) mere run on all samples to obtain the C.0.D was obtained for comparison. Glucose solutions of various B.O.D. ratios. Replicate series of C.O.D. determinations u ere glucose contents were 95 t o 97% oxidized with 0.05AVdichromate made on each sample, with 0.025 and 0.05S potassium dichroby the regular reflux procedure. When 1 gram of silver sulfate mate, both with and without silver sulfate. Chloride corrections was used as a catalyst, only 89% of the glucose was oxidized. were made on all river samples. The agreement between series Addition of 1.77 or 4.43 mg. of chloride, with the necessary corof samples using 0.028 and 0.05-V dichromate was slightly better 1ections, resulted in approximately 92% oxidation in all glucose in almost all cases when silver sulfate was used. concentrations by the regular procedure. When silver sulfate Results obtained on stream samples are shown in Table 111. was used, with chlorides added and corrected for, oxidation reThe first tTvo samples of Rlay 28 and June 2 xere collected from sults were only 83% of theory in all cases. The silver ion lonered the Ohio River a t point 1 above Cincinnati. On both days, the oxidation efficiency approximately twice as much as the the Ohio River had been in the pool stage for several Reeks. corrected chloride ion, and when both mere present, the per cent The C.0.D.-B.O.D. ratios aere in a comparable range hoth nith glucose oxidized tTas 12 to 14% lower. and without silver sulfate on May 28, lvhile those on June 2 were Resorcinol oxidation was approximately 2% lower when silver slightly higher Lvith silver sulfate. Samples taken June 18 during sulfate was used. A slight decrease in oxidation efficiency ochigh water a t this point were turbid and gave slightly higher curred as the resorcinol content was increased with silver sulfate C.0.D.-B.O.D. ratios. On July 1, the ratios dropped to appresent. proximately one half the June 18 value. Samples of July 7 gave With both glucose and resorcinol, increasing the amount of the lowest C.0.D.-B.O.D. ratios of the series. I n this series, material available for oxidation up to four or five times resulted samples of 25, 50, and 70 nil. were analyzed simultaneously, in comparable oxidation values in practically all cases. with and without silver sulfate. This series gave the highest Acetic acid was very resistant to oxidation by the regular B.O.D. and next to the lowest C.O.D. values found at point 1. reflux conditions with 0.05X ootassium dichromate. With Point 2 on the Ohio River was located at the foot of Broadway 1 mg. of acetic acid present, 5.6% oxidation occurred. Three milligrams of acetic acid resulted in 4.5% oxidation, while when 5 mg. was present, only 2.1% \vas oxidized. When Table 111. Oxidation of Ohio River Water silver sulfate was used, corresponding oxidaAgzSOi tion values were 96 to 90% of theory. With Date ~ d d ~ d , K ~ c ~ C.O.D. ~ ~ Found. , P.P.11. "'.?;D' pRig:1954 Gram Normality Max. blin. .IT-. P.P.M. H.O.D. 0.025N potassium dichromate and 1.5 grams of silver sulfate, average oxidation values were Sample Point 1 96 to 93%. Addition of 1 ml. of 0.025.V soM a y 28 11 9 13.9 1.08 12.9-1 0 0,025 15 6 13 1 1 3 . 8 1.08 12.8-1 1 0.02.5 14.4 dium chloride (0.89 mg. of chloride) and quan12.9 11.6 12.4 1.08 11,s-1 0 0.05 1.08 11.2-1 1 0.05 12.9 11 7 12.1 titative correction resulted in approximately 12.6 13.1 1.03 12.7-1 0 0,025 13.7 June 2 the same per cent oxidation (90%) with either 13.3 13.8 1.03 13.4-1 1 0,025 14.7 1.0 or 1.5 grams of silver sulfate present. 13.1 13.5 103 13.1-1 0 0.05 14.1 14.5 1.03 11.1-1 1 0.05 14.8 13.3 Butyric acid was 70 to 72% oxidized by the 0 0.025 23.3 21.2 J u n e 18" 22.5 1.50 15 0-1 regular procedure Kith 0.025-V potassium di15.t-1 1 0,025 26.1 51.7 23.3 1.50 19.9 20.3 1.50 13.5-1 0 0.05 20 5 chromate. When silver sulfate was used, oxi20.5 23.7 1.50 15 8-1 1 0.05 26 8 dation was increased to 91 to 94%. I n both 7.8 1.20 6 5-1 0 0.025 8.0 7.6 July 1 cases, the highest oxidation values were obtained 8,4 1.20 7.0-1 1 0.025 8.5 8.3 6.3-1 7.6 1 20 0 0.05 7.8 7.4 with the series of lowest butyric acid content. 9 5 1.20 i .9-1 9.7 9.3 1 0.05 July 7 With the butyric acid doubled and quadrupled, 4.8-1 10.1 2.12 25-ml. sample 0 0.025 11.5 8.9 a slight decrease in oxidation values resulted. 5.2-1 2.12 0.025 11.9 10.5 11.0 1 The amino acids exhibited the most erratic 6 9 10.0 2.12 4 7-1 0.025 11.3 .50-ml. sample 0 9.1 9 3 2.12 4 4-1 0.026 10.0 1 behavior of any class of organic compounds 9.5 2.12 4 5-1 9.6 9.2 70-inl. sample 0 0.025 used. Both alanine and glutamic acid were 2.12 4.4-1 9.0 9.4 1 0.025 9.7 only partially oxidized with the regular Moore Sample Point 2 ( 1 ) procedure, alanine being the most resist7.98 '> (3-1 19 8 20.5 0 0.025 21.1 June 8 ant to oxidation. With the regular 2-hour 1 0 025 22.2 21.4 21.9 7.98 1 7-1 2.4-1 18 8 7.98 1 7 . 7 0 0 . 0 5 2 0 . 1 reflux time, only 14 to 15% of the alanine 21.5 21.8 7.98 2 7-1 1 0.05 22.1 mas oxidized. Extending the reflux time to 3 3.27 fi 8-1 21.2 22.3 0 0.025 22.9 June 21 a 3.27 l i 9-1 22.3 22.7 1 0.025 23.3 hours resulted in oxidation of 17 to 19%. I n 3.27 5 7-1 0 0.05 1 9 . 5 1 8 . 1 18 6 7.0-1 22.9 3.27 22.4 1 0.05 24.1 both cases 1, 2, and 5 mg. of alanine gave 15 7 16.0 8,l4 2 0-1 0 0.025 16.4 J u n e 23 practically the same amount of oxidation. 2 3-1 18.7 8.14 18 2 1 0.025 19.4 2 1-1 16.8 8.14 Replicate series x i t h 1 gram of silver sulfate 17.3 16.2 0 0.05 18.9 8.14 2 3-1 19 5 16.4 1 0 05 added gave oxidations of 57, 52, and 45% in a Stream high and turbid. 2 hours and 63, 59, and 51% in 3 hours, a Three replicate analyses on all B.O.D. and C.O.D. determinations. -4verages in tahle total decrease of 12% in each case in a pracinclude all three results. Chlorides were determined and quantitative correction made on all samples. tically uniform pattern for the 1-, 2-, and 5-mg. series.
V O L U M E 28, NO. 2, F E B R U A R Y 1 9 5 6 Street in Cincinnati. Several se\! ers discharge above the point sampled, eo that when the river was in pool stage, this area was heavily polluted. The low average C.0.D.-B.O.D. ratios of 2.6 to 1 on July 8 and 2.2 t o 1 on June 23 clearly indicate the presence of unstable organic material which was oxidized biochemically. The sample taken June 21 during high nater, when the discharged sewage was diluted b j the increased flow, resulted in an average ratio of 6.6 to 1, approximately three times a. large as during the quiet pool stage. Sampling of the Little Miami River as interrupted by high water on June 14 and 25, T\ hich flushed the river thoroughly and increased the C.0.D.-B.O.D. ratio from an average of 5.7 to 1 on June 3, to 10.2 to 1 on June 14, and 11.3 to 1 on June 25. The saniple of June 3 was taken during low normal flow. On all of the Miami River samples, use of silver sulfate gave approxiniately 10% higher C.O.D. values than the regular Moore procedure. hIill Creek, which was sampled on June 16, carried a large amount of raw sewage. This sample had a B.O.D. of 115 p.p.m. and an average C.O.D. of 256 p.p,m., giving approximately a 2 t o 1 ratio of C.0.D.-B.O.D. Chlorides were 96 p.p.ni.; correction for 40 p.p.m. of this amount was macle on samples that contained silver qiilfate. DISCUSSION
In this study, chemical determinations of oxygen consumed mith 0.023 and 0.05.V potassium dichromate solutions gave reproducible, comparable results in replicate series of experiments. Oxidation of chlorides and most of the representative organic compounds used compared favorabl! with iesults reported for the 0 . 2 5 s dichromate (1, 2 ) . rVhen silver sulfate mas used, chlorides up to 1.9 mg. per sample, corresponding to 38 p.p.m. iii a 30-nil. stream sample, were 94% oxidized. Increased chloride concentration up t o the equivalent of 300 p.p.ni. resulted in the oxidation of practically a constant amount (1.9 mg.), so that in the strrani studies chlorides above 40 p p.m. mere corrected for this amount only. Without silver sulfate, chloride oxidation \$as quantitative up to the limit of dichromate available. There was a tendency for the C.O.D. values found on some of the organic compounds to decrease 15 ith increased size of sample, both with and aithout silver sulfate. Kith glucose from 1 to 5 mg. and resorcinol from 0.5 to 2 mg., the decrease in C . 0 D. values w w lese than 2y0. The butyric acid decrease mas 2 to 3%; the acetic acid decrease with silver sulfate added !%as i% when the concentration was varied from 1 to 5 mg. Glutamic acid with silver sulfate gave 94.1y0 of theory n i t h a 1-mg. sample, while a 5-ing. sample gave 79.8%. Alanine gave the lowest theoretical percentage recovery for the amino acids. When the reflux time was increased from 2 to 3 hours using silver sulfate, the average per cent of theorj for a 1-mg. sample increased from 5i.3’35 to 63.2%. With 5-mg samples, the values a-ere 45.0 and 51.2%, respectively. The data on the oxidation of organic compounds definitely show that n i t h increased concentration of C.O.D. a decrease in oxidation efficiency sometimes occurred. It is reasonable to suppose that this is due to a decrease in the redox potential in the system n hen 0.025;V potassiuni dichromate is used. To secure uniformity in results, sample sizes should be arranged so so that no more than 50% of the dichromate is reduced. When a sample is sufficiently strong, it is desirable to use an aliquot that gives from 25 to 50% dichromate reduction. On samples of low C O.D. (belon 25 p.p.m.) 70 ml. of sample should be used. I n this investigation, the number of stream samples analyzed A as limited. Changing stream conditions resulted in wide variations in the C.0.D.-B.O.D. ratios. Addition of silver sulfate usually gave slightly higher C.O.D. values in a comparable range m ith duplicate series, where the regular &loore procedure
167 was used. I n both of the Ohio River points sampled, the ratios uere relatively constant, when the river was in pool stage. At point 1, high water may have disturbed unstable bottom sediments, resulting in release of material amenable to biochemical oxidation. The C.O.D. values a t first increased and later dropped to much lower values than those found during pool stage. The B.O.D. values increased during this time, in spite of the high dilution, and continued to remain higher than those found a t pool stage, resulting in the much lower C.O.D.-B.O.D. ratios. Calculation of B.O.D. values from C.O.D. results does not appear feasible on natural stream samples. Seasonal changes, with increased or decreased biological activity, changing stream flows due to rains or melting snow, and changing industrial activities result in widely varying C.O.D. and B.O.D. values, and C.O.D.-B.O.D. ratios which may increase or decrease by several hundred per cent. Results of this investigation agree with the findings of Moore and Ruchhoft ( 3 ) on Lytle Creek and a stream polluted by oil refinery waste. I n normal stream surveys (3)the C.0.D.-B.O.D. ratios from stations located a t representative points on the stream give valuable information on the general condition of the stream, location and probable extent of the pollution load, ability of the stream to oxidize satisfactorily the amount of waste carried, and degree of biological stability. Khere records of C.O.D. and B.O.D. determinations are available for extended periods of time, and seasonal changes and efTect of varying stream flows are considered, it should be possible to predict with reasonable accuracy the probable 5-day B.O.D. at a particular point from the results of the C.O.D. determination. CONCLUSIONS
Results of determinations of chemical oxygen consumed with 0.025 and 0.05N potassium dichromate compared favorably with those reported for 0.2511’ on chlorides and most of the representative organic compounds used. Uniform results in replicate series of oxidation experiments were obtained when all steps in the procedure were carefully controlled. When the chloride content is between 40 and 300 p.p.m. and silver sulfate is used a i t h the regular Moore method, coriection for only 40 p.p.m. should be made on the C.O.D. obtained. Eithout the silver catalyst, chloride corrections are quantitative. Calculation of B.O.D. values from C.O.D. results on stream samples may be unreliable. With weather conditions and stream flows relatively constant, the probable B.O.D. range a t a particular stream point may be estimated from a series of previous representative C.0.D.-B.O.D. ratios. In stream surveys, the C.0.D.-B.O.D. ratios from representative sampling points give information on general stream conditions, locations and extent of pollution, ability of the stream t o oxidize the waste load carried, and relative degree of biological stability. T h e sample size should be so selected that not more than 50% of the 0.025N potassium dichromate is used up during the oxidation. LITERATURE CITED
(1) Moore, W. A., Kroner, R. C., and Ruchhoft, C. C., ; I s a ~ .CHEM. 21, 953 (1949). ( 2 ) Moore, W. A., Ludzack, F. J., and Ruchhoft, C. C., Ibid., 23, 1297 (1951). (3) Moore, W. A., and Ruchhoft, C. C., Sewage and I n d . Wastes 23, 705-12 (1951). RECEIVEDfor review J u n e 20, 1955. Accepted December 17, 1955. Division of Water, Sewage, and Sanitation Chemistry, Symposium on Problems in Stream Pollution, 127th Meeting, ACS, Cincinnati, Ohio, March-April 1955. Other papers in this symposium are published in the February 1966 issue of Industrial and h’ngineerang Chemistry.
