T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y oil, a product t h a t dries t o a harder, less porous, a n d more fully saturated film. T h e value of t h e “sanded surface” method is shown b y comparing t e s t No. 2 7 with test No. 1 5 , t h e paints used being of t h e same composition. T h e sanded surface of No. 2 7 gave much better insulating values a n d i t s bonding strength was double t h a t of t h e unsanded specimen. Another instance of t h e value of t h e “sanded surface” method is shown b y a comparison of specimens Nos. 2 8 a n d 14. U S E O F PIGMEXTS-It is quite likely t h a t t h e nature of t h e pigment used i n a paint designed t o prevent electrolysis of embedded metal will have some bearing upon t h e results obtained from its use. T h e addition of a pigment t o a n oil usually increases t h e resistance t o moisture a n d makes a more impermeable film. Theoretically, pigments which are of a non-conducting n a t u r e should be preferable. T h e inert pigments are examples of this t y p e (asbestine, china clay, silica, etc.). There should, however, be present in a paint a sufficient q u a n t i t y of rust-inhibitive pigment (basic pigments or pigments of t h e chromate t y p e ) t o produce a passive condition of t h e steel, similar t o t h e condition t h a t is produced b y t h e basic lime compounds in cement. Prominent among t h e protective compounds t h a t gave t h e best results are Nos. 2 a n d 9, composed of processed a n d heat-treated t u n g oil (Chinese wood oil). These compounds dried t o a h a r d , non-porous film of a saturated nature. H a d t h e y been sanded, t h e y undoubtedly would have given still better results. Distinction should also be accorded t o Nos. 17, 2 7 a n d 2 8 , although t h e good results obtained with t h e last t w o should be largely credited t o t h e sanding of t h e surfaces during drying. CONCLUSIONS
T h e corrosion of metal embedded in concrete structures, b y s t r a y currents of high voltage, is often productive of serious effects. T h e use of properly made paints upon such metal constitutes a safeguard t h a t should not be neglected b y t h e engineer. Such paints m a y be prepared as follows: I-The vehicle should contain: I-Boiled or bodied oils or products which d r y t o fairly s a t u r a t e d films. 2-Oils which d r y b y semi-polymerization rather t h a n b y oxidation. 3-Oils which d r y t o a flat r a t h e r t h a n a high gloss surface. 11-The solid portion should contain a percentage of : I-Pigments which are coarse a n d which therefore t e n d t o form films having a rough surface. 2-Pigments which a r e inert a n d which d o n o t a c t as conductors of electricity. 3-Pigments which are either basic or of t h e chromate type. 111-The painted metal should be “sanded” if possible. T h e writer wishes t o acknowledge t h e very valuable assistance throughout these tests of Mr. Leland P. H a r t , INSTITUTE OF INDUSTRIAL RESEARCH WASHINGTON, D. C .
Vol. 7, No. 6
PAINT VEHICLES AS PROTECTIVE AGENTS AGAINST CORROSION‘ B y MAXIMILIAN TOCH
A careful search of t h e literature of t h e past t w e n t y years has failed t o reveal anything like a systematic investigation of t h e relative value of different vehicles used in t h e manufacture of paints for structural steel a n d t h e prevention of corrosion. There are a few isolated cases in which boiled linseed oil,* K a u r i linseed oil varnish3 a n d spar varnish as protective coatings o n structural steel were studied. For many years past much has been written a n d many investigations have been made on t h e protective quality of t h e pigments, b u t no one has apparently made a n y s t u d y of t h e vehicles. I t is quite obvious t h a t without a rehicle a pigment is useless, a n d I know of no instance where a pigment could be used alone, with perhaps t h e single exception of Portland cement, if t h a t m a y be classed as a pigment; even then, Portland cement would be useless unless water were used a s a vehicle. I hardly need make t h e experiment of taking a dry pigment a n d using water as a vehicle t o show you t h a t when t h e water evaporated i t would leave t h e pigment, a n d t h e pigment in t u r n would leave t h e metal; a n d yet, t o t h e best of m y knowledge, nobody has paid a n y attention t o t h e very important role t h a t is played by t h e vehicle itself. There is a n old proverb which says, “ O n e h a n d is useless, for one h a n d washes t h e other,” a n d i t strikes m e t h a t t h e same is t r u e with reference t o vehicle a n d pigment, for one is of little value without t h e other, a n d if a n y value is t o be attached t o either of t h e m t h e vehicle has b y f a r t h e advantage, because, as I will show you, there are some vehicles which protect for a considerable length of time. With this end in view exposure tests were made in 1913, in which fifty-two steel plates (in duplicate) were carefully freed from grease b y washing with benzol, dried, sanded, a n d rubbed clean with pumice, a n d t h e n coated with all t h e paint vehicles or protective vehicles t o t h e extent of fifty-two in number, many of which, of course, are seldom, if ever, used alone, a n d some of which are failures a short time after t h e y are p u t on. However, I wanted t o d o this thing thoroughly, a n d for this purpose I selected t h e same quality of steel, known as cutlery steel, which I have been using for many years for m y exposure tests. I t is a steel which rusts very rapidly a n d of which I have several samples here. I must eliminate those plates which have shown no rusting in t h e year a n d five months t h a t t h e y have been exposed. These were coated with t h e paraffin or machinery oil compounds, a n d i t would be poor advice t o a n y engineer t o coat steel with paraffin compounds, for t h e method of cleaning before t h e application of a n y good paint would have t o be very carefully followed o u t , since n o protective paint would hold on steel t h a t retained t h e least trace of a paraffin 1 P_aper ptesented before the New York Section of the Society of Chemical Industry, Chemists’ Club, April 23, 1915. 2 C. Vdn Kreybig, Farben Zfg., 17, 1 7 6 6 8 ; J. N. Friend. Carnegie Scholarhsip Report, Iron and Steel Inst., May, 1913, Pp. 1-9. 8 Address of Prof. A. H. Sabin before American Society of Civil Engineers, Nov. 4, 1896, reported in Engineering News, July 28, 1898.
T H E JOL-R,VdL O F I S D I ' S T R I . 4 L
June, 191j
coat. Then the paraffin. or non-drying oils, all collect a great deal of dirt, which showed t h a t this would have t o be entirely removed before a n y paint could be applied. Plate No. 41 showed excellent resul'ts, and a material of this kind would not be so very expensive where engineers demand t h a t steel be coated with a clear liquid in the shop so t h a t the steel may be inspected in t h e field. This was composed of half spar varnish a n d half stand oil. Stand oil is practically a polymerized linseed oil. Linseed oil when heated t o j j O o with a drier like Japanner's Brown or borate of man-
A S D E Y G I SE E RI S G C H E M I S T R Y
jII
a Tungate drier, and then thinned Kith r j per cent of benzine. This made a material which is hardly more expensive t h a n good, boiled linseed ,oil, and left a most excellent surface for repainting. In fact, this has proved itself the equal of Plates No. 2 2 and N o . 2 3 , with the addition of a better surface for repainting. Plate No. 46 was coated with kettle-boiled linseed oil, and is very good, but this material might be regarded by some engineers as too expensive for application, as it took all day t o make this oil. A carefully selected linseed oil was chosen t o start with, t o which
INSPECTION REPORTO N STEELPLATESEXPOSED DECEMBER 8. 1913 No. NATURE O F COATING 1 Raw linseed oil 2 R . 1. oil f 10% linoleate drier 3 R. 1. oil 107, Japan drier
+ 4 R. I. oil + 107, Japan drier 5 J a p a n drier 6 Menhaden oil + 10% Japan drier Menhaden oil 8 Soya bean oil 9 S. h. oil + 10% light drier i n S b. oil + 1Oyo linoleate drier R a w oil, menhaden oil + lOYo liqoleate drier 11 R a w oil, '!z soya bean oil + lOyolinoleate drier 12 13 Perilla oil 14 Perilla oil + lOyo Japan drier 15 Petrolatum
CORROSION April 14, 1914 Complete About 1/a Slight
i
1/z
I/?
16 17 18 19 20 21
22
23
24 25 26
27
28 29 30 31 32 33
34
35 36 37
38 39 40 41 42 43 44 45 46 47
48
49 50 51 52
Spar varnish Turpentine substitute Anhydrous pine oil Benzine Turpentine Paraffin oil Bodied linseed oil Turpentine substitute thinner Pyroxylin lacquer Celluloid solution No. 38 China wood oil varnish No. 5 copal, China wood oil varnish No. 6 copal, wood oil varnish No. 39 China wood oil varnish Steam cylinder oil Gear case oil turpentine Blown linseed oil Kerosene oil Zy0 cobalt drier R a w oil Raw China wood oil Raw oil, 1,'z raw China wood oil Blown linseed oil, turpentine substitute, 1Oyo linoleate drier R a w oil, lOf7, paraffin oil, l0Tc J a p a n drier China-wood-oil-varnish mixture 11%heavy boiled linseed oil, '/z raw oil 1 / 2 Spar varnish, 1 / z stand. oil 80% raw oil, 15% spar varnish, 5 % linoleate drier
+
I
+
+
t
Almost complete Complete Complete Almost complete Complete rlbout '11 hbout ' / n About 1/1 About I/z Complete .4lmost complete Slight (much dirt) K-one Complete Complete Complete Complete Complete Kone xone Complete Complete Slight None Slight Slight Complete Slight (much dirt) Slight Complete About Ahout '12 About l,'z Slight About ',z h-one None None h-one
Raw oil, 1 / z No. 6 Copal wood oil varnish, 57, J a p a n drier None 1:~ Heavy boiled linseed oil, VZmenhaden oil, 5 X Japan drier Sone 1/2 Heavy boiled linseed oil, 1 2 soy? bean oil, 5 % J a p a n drier Sone Kettle-boiled oil Kone Pale refined varnish oil Complete Pale refined linseed oil Complete 8 0 R Wood oil, lOYc No. 3 8 China wood oil varnish, lOyo Japan drier h-one 90% raw oil, 10% paraffin oil, 1% cobalt drier None 957, spar varnish, 5 % petrolatum in turpentine subs. Kone China wood oil. heated with t o x tungate a n d thinned with benzine Sone
ganese will produce a very thick viscous liquid, mrhich is largely used as a patent leather finish. This can be reduced with jo per cent of thinner and still have the fluidity or viscosity of ram linseed oil. and is, therefore, inexpensive. Plate No. 50 was coated with a material containing I O per cent of paraffin oil, which might be classed as a n adulterated linseed oil, a n d while i t showed up very well, i t could not be recommended because, on a n exposed structure like a bridge, a coat of good protective paint would not adhere very thoroughly. Plate No. j 2 has taught us a valuable lesson with regard t o t h e use of raw China wood oil which is heated t o a sufficient degree of heat t o take I O per cent of
CORROSION December. 1914 Complete Bad along edges: partly protected in middle (11 Slight along edges: fairly protected over rest of film ( 2 ) Bad along edges: partly protected in middle .4lmost complete Complete Complete C om p 1et e Complete Much on upper half: none on lower half Much on upper half; none on lower half Much on upper half: none on lower half Bad 3 '4 way down; fairly good on lower fourth Complete Complete Slight (collected much dirt) Slight Complete Complete Complete Complete Complete None Very slight Complete Complete Appreciably under whole film None Much over whole plate Under whole film t o small extent Complete Slight (much dirt) Slight in few spots. Rad over all Complete Bad 1/2 way down Almost complete Bad 8/a way down
CORROSION April 15, 1915 Complete Complete Complete
P a r t in quite a few isolated spots Bad 3/a way down None (vert, eood) k o n e (very good) None (very good) A little along upper edge and in several other spots
Complete Complete Bad S o n e (still good)
3 5
Rad
3
Kone (very good)
Bad
3
Very slight
Bad
3
&*one (very good) Practically none Complete Complete
Bad Vnchanged (very Complete Complete
Practically none Somewhat on t o p . Perfectly good a t bottom Slight underneath film along edges
Complete Slight (still good) Slight (still good)
5
None (very good)
Kone (very good)
5
VALUE 1 1B 1B
Complet? Complete Complete Complete Complete Complete Complete Complete Complete Complete Comolete Sligh: (very good) Slight (very good) Complete Complete Complete Complete Complete Xone (very good) Very slight (very good) Co m p 1et e Complete Rather had Rather bad Complete Complete Complete Slight (still protected)
1
1 1 1 1 1B 1B 1B lB 1 1 4
5 1 1 1 1 1
5 5
1 2 2 2 1
4
L
Complete Complete Complete Complete
R ~ a d_
_
1 1B 1 1
7
5
was added j per cent of litharge and no other drier. This oil dried very badly, but when it did dry produced a good flexible film which lasted. This must not be confounded with the average boiled linseed oil of commerce. The various coatings used in these exposure tests have been divided according t o their protective value i n t o five classes: I and IB-Those vehicles which have little or no value for the prevention of rusting. (a) The raw and refined drying and semi-drying vegetable oils. (Plates Nos. I , 7, 8, 13, 3j, 36, 47, 48.) ( b ) The same oils t o which I O per cent of drier has been added. (Plates Nos. 2, 3, 4, 6, 9, I O , 11, 12, 14,34.)
512
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
( c ) The more or less volatile paint thinners.
(Plates Nos.
17,
18, 19, 2 0 , 33.)
(d) Solutions of celluloid and pyroxylin. (Plates Nos. 24,25.) (e) The liquid (at room temp.) paraffin oils. (Plates Nos. 21,
30.1
2-Those vehicles which showed some degree of protection, though not very much a t best. ( a ) Wood-oil varnishes containing, a certain percentage of rosin. (Plates Nos. 26, 29.) (b) Copal-wood-oil varnishes. (Plates Nos. 2 7 , 28.) (c) Varnishes made from linseed oil which have been thickened and oxidized by blowing with air, oxygen or ozonized air. (Plates Nos. 32, 37.) This compared with t h e results obtained below with cooked-oil varnishes proves conclusively t h a t t h e film yielded b y a blown oil is not nearly as waterproof a n d resistant t o severe weather conditions as t h a t formed b y a boiled or polymerized oil. 3-Varnishes or varnish mixtures which protected t h e steel very nicely as long a s weather conditions were not severe a n d temperature changes n o t very rapid a n d pronounced. (Plates Nos. 39, 40, 42. 43, 44, 45, 49.) 4-The semi-solid a n d solid paraffin oils. These show a very high degree of protection from rusting. (Plates NOS. I j , 31.) 5-Those varnishes a n d vehicles which afford a high degree of protection against corrosion. T o be set down in this class a material must be extremely water-proof; it must d r y with a film which is very elastic a n d yet tough in order t o be able t o withstand “weathering.” A film which can not remain intact against condensed moisture, snow a n d ice a n d despite comparatively wide a n d sometimes rapid changes in temperature (as between d a y a n d night even in rather warm climates), will of necessity afford very little protection for t h e steel t o which i t is applied. As the Table on page 5 1 1 shows, this class comprises: ( a ) Spar varnish. (Plate No. 16.) ( b ) Varnishes made from linseed oil, or China wood oil, which have been thickened by a heat process. (Plates Nos. 2 2 , 23, 5 2 . ) (c) Open kettle-boiled oil. (Plate No. 46 ) I n Plate No. j o we find a rather anomalous case. I t seems t h a t raw linseed oil which has been dried with a small percentage of a liquid paraffin oil proved t o be a n excellent coating for rust prevention. The addition of a n y paraffin or non-drying oil, even in such a small quantity as is shown in Plate No. 50, is dangerous in case repainting becomes necessary. Although t h e matter is not settled in m y mind as t o whether linseed oil a n d paraffin oil dissolve in each other m y idea a t present is t h a t , although t h e y apparently make a clear solution, separation takes place. I have conducted several experiments, and find t h a t a film of linseed oil which contains paraffin oil in some quantities when apparently dry shows minute globules of paraffin oil in wet condition when t h e film is heated over 100’ C. A film of linseed oil containing t e n per cent of paraffin oil after it is six months old can be extracted with n a p h t h a and shows uncombined paraffin oil. These experiments prove conclusively t h a t i t is dangerous t o mix a paraffin
Vol. 7 , No. 6
oil with linseed oil for a n y purpose, excepting where i t is not necessary, or not t h e intention, t o repaint subsequently. NOTE: All t h e photographs submitted (see page 513) were taken during December, 1914. ABSTRACT OF DISCUSSION
Dr. Thompson, t h e chairman, asked M r . Toch whether t h e oil which showed u p so well was a kettleboiled oil or a commercial boiled oil, and what was its gravity. Mr. Toch replied i t was a kettle-boiled oil which he h a d made himself, a n d while he did not know its exact gravity from memory he was quite sure i t was over 0.980. Mr. Gaines said t h a t in his laboratory a sample of a heavy paraffin oil was submitted b y t h e Standard Oil Company for t h e purpose of protecting steel against rust, a n d this sample while it protected against rust collected dirt a n d cinders in t h e meantime. Mr. Toch referred t o sample panel I j , which he said was coated with a n analogous material, which he h a d thoroughly cleaned off to show t h a t a paraffin compound, if i t were heavy enough, protected steel from corrosion, b u t i t h a d t o be completely removed before a n y paint would adhere t o t h e metal. Mr. Low asked whether Mr. Toch h a d a n y d a t a on t h e viscosity of these oils previous t o their being applied, for it appeared t o him t h a t t h e value of t h e oil depended upon t h e thickness of t h e coating, a n d t h a t was probably why t h e kettle-boiled oil (panel No. 46) a n d t h e boiled China wood oil with t h e Tungate drier (panel No. j2) produced such good results. Mr. Toch replied t h a t there was no question t h a t M r , Low was correct in his premises. The thickness of t h e coating is in direct ratio t o its protective quality, provided i t has a protective quality t o s t a r t with, a n d assuming, of course, t h a t i t is a drying oil. Mr. Toch stated t h a t he h a d not made a n y measurements of t h e thickness of t h e various films, b u t those t h a t stood u p well showed a n appreciable thickness of film. Prof. Sabin said t h a t a shop coat of linseed oil for bridge work had not entirely gone out of use. There are several different reasons for doing it. The structural metal, as received from t h e shop, is covered with mill scale. Sooner or later i t will come off, carrying with it a n y paint t h a t has been applied. The shop coat of linseed oil is applied, t h e bridge erected; t h e mill scale is allowed t o rust off, a n d t h e n t h e bridge is thoroughly cleaned a n d painted. On another road a shop coat of linseed oil is applied, the bridge erected and t h e n honestly painted b y the company’s painters. Others say t h a t a shop coat of linseed oil enables t h e inspector t o see whether t h e riveting, etc., is properly done, better t h a n when t h e work is painted. Most engineers believe t h a t they can get better painting done with t h e railroad’s own painters t h a n can be obtained b y t h e bridge company’s painters. The question is whether it is possible t o get a vehicle t h a t will work easier t h a n linseed oil a n d can be sold at a price t h a t will p a y t o use it. The tests for linseed oil are so simple t h a t almost a n y engineer can determine whether he is getting a pure article. Mr. Stevenson said t h a t in t h e erection of steel the
a
5 14
T H E J O U R N A L OF INDCSTRIAL A N D ENGINEERTNG C H E M I S T R Y
shop coat is almost always torn off in t h e rough handling i t gets. I t costs more t o get on a coat of paint in t h e shop t h a n it doe5 in t h e field. Linseed oil gives protection for four or five months, which is as long a s i t is wanted, a n d has given very good results except in one or t w o cases. 320 FIFTHAVENUE,NEW YORK
DETERMINATION OF THE BIOCHEMICAL OXYGEN DEMAND BY THE SALTPETER METHOD IN STOCKYARDS, TANNERY AND CORN PRODUCTS WASTES By ARTHURL E D E R ~ R ? Received April 13, 1915
The saltpeter method described b y me in t h e M a y (1914)issue of t h e Journal of Infectious Diseases a n d in a paper presented a t t h e last December meeting of t h e American Public Health Association ( A m . J . Pub. Health, April, 1915)is based upon experimental work with domestic sewage a n d polluted river water. Briefly, t h e method depends upon t h e denitrification of a sod i u m nitrate solution by t h e sewage bacteria present. It was f o u n d t h a t t h e a m o u n t of saltpeter oxygen absorbed b y t h e organic m a t t e r on incubation is t h e same a s if fresh water oxygen was used. I n preparing a n d adding t h e sodium nitrate solution t o t h e sewage i t was assumed t h a t five atoms of oxygen are available from t w o molecules of sodium nitrate, a n d t h e figures obtained on this basis checked very well with figures obtained by t h e dilution method. By t h e dilution method is meant t h e preparation of sewagewater mixtures in stoppered bottles with t h e addition of a definite q u a n t i t y of methylene blue. T h e dilution m a y or may n o t be carried t o t h e point of absolute stability. Usually t h e mixtures are incubated for t e n d a y s a t zoo C. T e n days’incubation sufficesfor all practical purposes. T w e n t y days’ incubation is preferable when t h e t i m e element is not important. T h e stability is expressed in terms of “relative stability.” Knowing t h e free oxygen content of t h e diluting water a n d t h e t i m e of decolorization, t h e biochemical oxygen demand of t h e sewage can easily be calculated, a n d expressed in milligrams of oxygen p e r liter of sewage or p. p . m. T h e principal objection t o t h e dilution method from a scientific standpoint is t h e impossibility of determination of intermediate points of deoxygenation as is possible with t h e saltpeter method, a n d from t h e working standpoint t h a t it is time-consuming a n d ill adapted for field work. During t h e incubation with t h e saltpeter solution I observe t h e sediment, t h e odor, a n d t h e gas formation. If t h e sediment becomes “septic” (i. e., black color) during incubation, t h e bottle with t h e next higher content of saltpeter is selected for t h e determination of t h e residual nitrate-nitrite oxygen. If t h e saltpeter is present in sufficient quantities there is no putrid odor. If active gas formation does not t a k e place, t h e presence of either free acid or caustic alkali or other germicides may be discovered. This brings me t o t h e subject of t r a d e wastes. A municipal sewage may contain a considerable q u a n t i t y 1 Read in Abstract at the 50th Meeting of the American Chemical Society, New Orleans, March 31, to April 3, 1915. 2 Chemist and Bacteriologist, The Sanitary District of Chicago.
Vol. y , No. 6
of t r a d e waste without losing entirely t h e characteristics of a domestic sewage. I n such a case t h e saltpeter method can be employed without a n y modification. However, sewage or t r a d e wastes occur containing caustic alkali or acid, or a germicidal or antiseptic substance. These are t h e wastes which I want t o discuss more closely. I n previous publications I have warned against t h e indiscriminate application of t h e saltpeter method in t h e t r a d e wastes, for t h e reasons already given. I n t r a d e wastes investigations carried on for T h e Sanitary District of Chicago under t h e direction of Langdon Pearse, I have h a d ample o p p o r t u n i t y t o s t u d y t h e application of t h e saltpeter method t o such wastes, including those f r o m t h e stockyards, t a n n e r y , a n d corn products industry. T h e modifications of t h e method required when working with some of these wastes are simple a n d do not complicate t h e application in t h e least. With t h e packing house waste mixed with domestic sewage, t h e method remains t h e same as with domestic sewage. This waste resembles somewhat a concent r a t e d domestic sewage, with a high organic nitrogenous a n d carbonaceous content. When working with such a waste of unknown strength, I have employed 500, 1,000,1,500, a n d 2,000 p a r t s per million of oxygen in t h e form of sodium nitrate. On t h e individual plant outlet higher concentrations might be required. Some concentrated effluents containing blood a n d other complex organic m a t t e r absorb a s much a s ~ j , o o o p . p. m . of oxygen. T h e biologic oxygen consumption of t h e mixed waste obtained from t h e stockyards a n d packing town, with domestic sewage, in Chicago a t Center Ave., fluctuates between goo a n d 1,300 p . p. m . of oxygen during t h e working period of t h e d a y . A little experience is t h e best guide t o t h e q u a n t i t y of sodium nitrate required. It is useless t o a$d methylene blue t o these wastes a s a n indicator on account of t h e adsorption which takes place in t h e presence of colloids. T h e appearance of a black color in t h e sediment or t h e elimination of t h e putrid odor after incubation are not reliable indices, as is t h e case with domestic sewage. Profuse gas formation t a k e s place whether there is a sufficient q u a n t i t y of sodium n i t r a t e present or not. T h e bacterial content of t h e packing house a n d stockyards waste is always very high. Domestic sewage is nearly always mixed in varying proportion, roughly, a b o u t twenty-five per cent. T h e best index of accomplished denitrification is u n doubtedly furnished b y t h e nitrites. For instance, if nitrites are absent or present in traces in t h e 500 p . p . m. oxygen bottle, after incubation, discard t h e bottle a n d select t h e one with t h e higher a m o u n t of oxygen for t h e actual determination of t h e residual nitrite-nitrate. Supposing we find t h a t on t h e fifth d a y of incubation with t h e highest concentration of sodium nitrate there is b u t a weak nitrite reaction ( 5 cc. of t h e liquid is all t h a t is required for this t e s t ) . we have t h e n reason t o assume t h a t we shall be unable t o obtain a n accurate oxygen demand figure unless we a d d some more saltpeter oxygen. This can be done probably without incurring a n appreciable error. A t
