INDUSTRIAL AND ENGISEERING CHEhIISTRY
714 T.4BLE
VI.
COMPARISON O F OXYGEN DEPLETIOS~
(On l/250 dilutions oi river mud obtained b y old procedure and by analysis of clarified portions after continuous rotation during incubation) -Oxygen DepletionsOld Continuous Rotation procedure, during Incubation no 41um clarifiCentriflocculaIncubation Period cation iuge tion Hours P . p . nc. P . p . 111. P . p . 71L. Immediate (apparent initial depletion) 5 35 1.88 1.07 3 6 26 5.22 5.13 4
5 9 i i.01 i.64 8 33
6 42 6 70 7 03 7.02
5 6 7
6.14
6.75 I .75 S 39
TABLEv-11. COMPARISON O F OBSERVED AND CAISXJLATED VALUESOF y FOR DILUTIONS OF SCIOTO RIVERRICD 1/250 Dilution a t 20’ C . Incubation ObCalcutime served lated Hours 1 2 3 4 5 6
7
..
.. .. a
2.60” 3.90Q 5.13 6.14 6.75 7 i 5 s.39
... ... ...
2.06 3.72
5,06 6.13 7.00 7.70
8.26
..
..
1/2000 Dilution a t 20’ C . Incubation ObCalcutime serred lated Days 2 3 50 3.10 3 4
4.04 4.9s
5 6 7 8
5.46 5,48
9 10 11
6.70 6.52a 6.76 7.02 7.19
4 23
5.01 5.61 lj.05 ti.39
6.65 6.84 6.99 7 10
Interpolated graphically.
The results of this experiment, as given in Table VI, show how misleading the values might be if the oxygen demand was determined b y the usual method alone. From the old procedure one would conclude that the oxygen demand of the mud was 1750 p. p. m. in 7 hours, whereas the alum flocculation method together with continuous rotation shows a n oxygen demand of 2100 p. p. m. in the same period. It is recommended, therefore, t h a t the following steps be observed in the determination of B. 0. D. on river mud suspensions : (1) calculation of the initial dissolved oxygen; (2) maintenance of the solids in suspension during the incubation period; and (3) determination of the dissolved oxygen upon the rotated incubated sample after clarification b y alum flocculation. I n another experiment a l/2000 dilution of river mud was rotated and depletions were determined by the alum flocculation procedure, b u t the period of observation was extended to 11 days. Upon the assumption that this oxidation follows a unimolecular reaction, the Thomas (6) slope method of derivation was applied to the data of the last tn-o experiments and the following values for L and k were obtained: Dilution Used in Experiment 1/250
l/2000
Period of Observation 3 t o ihours 2 t o 11 days
k (per Day) 2.25 0.1214
L 2,650 14,890
When the values of these constants are substituted in the equation y = L (l-lO-kt), and the values of y for each observation time are calculated, i t is found, as shown in Table VII, t h a t the observed and calculated values of g check on the whole remarkably well. From these results it must be concluded t h a t there is a change in the rate of oxidation of the mud from the first few hours (as the reaction velocity is independent of the dilutions) to the period following the first d a y of incubation. The reaction data for the entire period will not fit the unimolecular formula. The course of the oxidation can be fairly well represented, however, as the sum of two unimolecular reactions as first suggested b y Theriault and RlcKamee ( 3 ) . I n fact, the similarity of these results to those of Theriault and McSamee may be taken as
VOL. 12, NO. 12
confirmation of the results these authors obtained when aerating sludge.
Summary The apparent oxygen loss from the calculated initial dissolved oxygen, obtained when a dilution of mud is immediately examined by the Kinkler method, is due to an interference with the analytical procedure. This interference occurs largely either in the preliminary acid treatment with the Rideal-Stewart or azide procedures, or during the final acidification and titration in the short or regular Kinker methods. Aeration of the diluted mud oxidizes the interfering materials, though 10 per cent of the initial interference may remain after 48 hours of this treatment. Removal of the solids from the sample reduced the interfering materials and the initial apparent dissolved oxygen loss. Solids may be removed by centrifuging the sample in completely filled glass-stoppered bottles or flocculating with alum without interfering with the dissolved oxygen content. The alum flocculation procedure gave the better results 1% hen applied to river muds. The B. 0. D. determined by the regular dilution procedure as applied to water and sewage is not applicable to river muds because of the interference noted. A procedure for more accurate determination of the biochemical oxygen demand on river mud dilutions has been proposed.
Literature Cited Health Assoc., “Standard Methods for the Examlnation of Water and Sewage”, 8th ed., New York, 1936. Stephenson, H. F., Analyst, 64,344-6 (1939). Theriault, E. J., and McNamee, P. D., ISD. EXG. CHEX.,22, 1330-6 (1930) ; Pub. Health Repts., 46, 1301-19 (1931), Reprint 1480. Theriault, E. J., and McNamee, P. D., ISD. ENG.CHEM.,Anal. Ed., 4, 59-64 (1932). Theriault, E. J., McNamee, P. D., a n d Butterfield, C. T., Pub. HeaZth Rept., 46, 1084 (1931), Reprint 1475. Thomas, H. A . , Jr., Sewage Works J., 9, 424-31 (1938).
(1) din. Pub. (2) (3)
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PRESENTED before the Division of Water, Sewage. and Sanitation Chemistry a t the 99th lIeeting of the American Chemical Society, Cincinnati, Ohio,
Correspondence-New Development in Thermionic Relays In the article on “A h-ew Development in Thermionic Relays” by Waddle and Saeman [IND.ENG.CHERI.,A4nal.Ed., 12, 225 (1940)], the following statement is made: “If there is danger of grounding the circuit through the control leads, P I or P,,another wsistor, Ra, may be inserted to avoid this danger.” The foregoing is, in my opinion, a dangerous understatement of facts. I t leaves the installation of the resistor, Ra, to the option of the user of the device and fails to mention that this resistor has any value as regards personal safety. LEE KETTING 2 4 4 1 RUSSELLST.
BERKELEY, CALIF.
In view of AIr. Nutting’s suggestions, it would be well to point out the fact that, in the absence of resistor Ra, as indicated on the circuit diagram, severe shocks may be received by the operator if he happens to ground the control circuit through his body. WALTER S A E V A N FLORENCE, ALA.
