November, 1923
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1183
Determination of Total Sulfur in Soils and Silicate Rocks' By W. M. Shaw and W.H. MacIntire AGRICULTURAL EXPERIMENT STATION, UNIVERSITY OF TENNESSEE, KNOXVILLE, TSNN.
occlusion of sulfate in the €€Edetermination of A rapid and eficient method is offeredfor the determination of hydrated ferric oxide. sulfates involves a total surfur in large charges of soils by fusion at a relatioely low Hart and Petersone comnumber of chemical temperature and without use of platinum. Complete oxidation pared the official (Hilgard), considorations which have and absolute disintegration is assured. Aqueous extraction and modified Van Bemmelen been studied in particular filtration of the melt insures remooal of aluminium, iron, manaqua regia, and Osborne soby Allen and Johnston,lI* ganese, and alkali earths, and eliminates any interference of natioe dium peroxide methods. Blumenthal and Guernbarium which oitiates results obtained by the generally followed They concluded that the ~ e y , a~ n d Hirst a n d hydrochloric acid extraction. Necessity for silica dehydration is latter was superior, even Greaves. obviated and a minimum of potassium salts is insured. without elimination of ferMuch difficultv has been ric and aluminium chlorides encountered in" making speedy and accurate determinations of total sulfur in soils. and silica from solution prior to precipitation of barium At its 1919 meeting the Association of Official Agricultural sulfate. The Osborne14 method, applied to soils, calls for Chemists directed the junior author, as Referee for Soils, the solution of the sodium hydroxide-peroxide melt with to study the problem. I n connection with that study,lo,llJz hydrochloric acid, and addition of an indefinite amount of ammonia to neutrality. The original Fraps5 method using the present mode of procedure was developed. Most soils are of low sulfur content and large charges are potassium nitrate ignition after a preliminary digestion with necessary to insure sufficient analytical quantities of barium nitric acid, and its subsequent modificationg calling for sulfate. Assurance of complete combustion and absolute calcium nitrate instead of potassium nitrate when applied disintegration of the mass must be had. When these two to soils, both called for hydrochloric acid solution of ignited requiwments are met there is the further difficulty of insuring residue. Hart and Petersons made a hydrochloric acid complrte removal of the oxidized sulfur from the insoluble solution of the peroxide melt without elimination of iron residuo. I n studying methods the authors endeavored to and a t a volume sufficient to inhibit precipitation of silica. secure complete oxidation and removal of sulfates from the Shedd16also obtained a strong hydrochloric acid solution of the insoluble siliceous residue by boiling with concentrated peroxide melt, leaving iron and silica in the solution and nitric acid and filtering through a 10-cm. Buchner, the use guarding against inclusion of silica in the barium sulfate of which facilitated washing of a thin layer of soil. No by hydrofluoric plus sulfuric acid ignition. I n the A. 0. A. C. assurance was obtained that complete oxidation of native official method2 the 1-gram soil-charge fusion with 5 grams sulfur materials was effected. It was also found by the of sodium carbonate is extracted with hydrochloric acid and authors and their collaborators12 that with longer periods of made to volume, 0.2 or 0.4-gram equivalent aliquots being boiling of soil with nitric acid and subsequent replacement used for the determination of barium sulfate. I n using magneof nitric acid by hydrochloric acid, lower sulfate recoveries sium nitrate to effect oxidation, Swanson and Latshawle made were obtained. It appeared that this inverse relationship a hydrochloric acid solution of the pulverized, ignited residue. I n all the foregoing it will be observed that the melt was of time to recovery was due, in part at least, to the interference of barium. The common occurrence of barium in dissolved in hydrochloric acid. This,the authors believe to soils was shown by F a i l ~ e r . ~It is obvious that this element be a serious disadvantage, if not an error, for several reasons: will be dissolved by hydrochloric acid extractions of the first, the excess of acid necessary to effect solution is stronger than permissible for the precipitation of barium sulfate; melts secured by the several fusion methods. The presence of iron and aluminium in hydrochloric acid secondly, native barium and strontium present as carbonates solution has been considered detrimental to the accuracy are dissolved and during the prescribed digestion some of barium sulfate determinations. Williamslg showed that barium sulfate is occluded in the acid-insoluble portion of sulfate determinations in soils were consistently higher if the melt; thirdly, the interfering elements iron, alhminium, the iron was removed by ammoniacal precipitation. It is not and possibly manganese are dissolved; and fourthly, the clear, however, whether the higher results obtained from the alkali and alkali-earth silicates are dissolved and maximum ammoniacal eliminations by Williams are due to (a) pre- silica content is carried by the hydrochloric acid solution, vention of the solvent action of ferric and aluminium chloride, I n the procedure advanced by the writers, the melt is (b) obviating the loss of sulfate from a double iron barium disintegrated by aqueous digestion and filtered before sulfate-a point advanced also by Talbot,l' or (c) to sulfate acidifying. By this process all iron, barium, and some impurities in the ammonia. No mention was made by aluminium are retained as silicates or carbonates in the Williams of blanks upon ammonia. The writers have found water-insoluble residue, while much of the silica is eliminated that the amount of ammonia used must be definite and as silicates. I n only the Hillebrand' method, where potassium nitrate corrected for, even with the best high-grade chemical. MacIntire and Shawl2have further pointed out that precipitation is used, does it appear that the melt is dissolved in water of barium sulfate from solutions of equal ferric chloride and filtered from alkaline solution. Practically the identical content is influenced by concentration and by temperature. technic used by Hillebrand has been advanced recently It was shown further that, after applying empirical blanks, by Thomas,l* and by Mahin and Carr.13 I n the present three ammoniacal precipitations are necessary to prevent method the minimum of undesirable potassium salts is insured and the advantageous features of several different 1 Received August 20, 1923. procedures have been incorporated. * Numbers in text refer t o bibliography at end of article.
T
1184
INDUSTRIAL AND ENGINEERING CHEMISTRY PROCEDURE
The instructions and notes sent out to A. 0. A. C. collaborators are here given. PREPARATION O F MIXTURE-Place a 5 to 10-gram charge of 0.5-mm. fineness into a 100-cc. nickel crucible; add equal weight of C. P. anhydrous sodium carbonate, and mix well with a stout nickel stirring rod of such length as to permit introduction into the furnace to be used in the fusion. Pipet carefully 4 cc. of water into each 10 grams of soil; stir well to a stiff paste, adding more water if necessary, a few drops a t a time. Immediately add successive portions of about 1 gram of sulfur-free spdium peroxide, stirring well after each addition to obviate excessive frothing and overflow. Continue to add peroxide until the mixture becomes dry and granular, and then add, as a surface coating, enough to make the total peroxide addition 25 grams per each 10 grams of soil. Place the mixture in an electric furnace. FUSION OF MIxTURE--Maintain a temperature of between 400" and 500' C. during the first half hour, then raise the .temperature rapidly to bright red heat (about 900' C.) and continue the fusion at this temperature for about 10 minutes. Withdraw the crucible from the muffle quickly, manipulate so as to cause the melt to spread out in a thin sheet over the interior of the crucible, and cool rapidly by contact with some good conductor in cool atmosphere. DISINTEGRATION OF MELT-Place the chilled crucible sideways in 600-cc. beaker; immerse in distilled water. Add about 5 cc. of ethyl alcohol to decompose sodium manganate. Cover beaker with watch glass, place on a cold electric hot plate, and apply heat. Boil briskly until all the melt is disintegrated (30 minutes are ordinarily sufficient). When the suspension has assumed a flesh-colored, flocculent appearance, with no glassy green lumps of the melt in the interior of the crucible, remove the crucible and rod from the beaker and wash any flaky particles back into beaker with the aid of a policeman, rinsing several times with hot water. (Should small glassy particles still cling to the inside of the crucible, disintegrate by boiling water in the crucible over the hot plate or a small flame and add crucible content to main volume.) Filter immediately by suction through a 9-em. Buchner-a liter beaker placed under a bell jar being the most convenient arrangement. FILTRATION AND WASHING-When no more of the liquid can be drawn through the filter, return residue together with the filter paper to the original beaker, washing any adhering particles carefully from the funnel. Add about 1 gram of sodium carbonate, macerate with the stirring rod, add 75 to 100 cc. of water, and bring to a brisk boil while stirring vigorously. Again throw on to a Buchner filter, suck nearly dry and wash three or four times, with about 20 cc. of hot water, to a volume of 500 cc., or 700 cc., for the 5 and 10-gram charges, respectively. ACIDULATION OF FILTRATE AND DEHYDRATION OF SILICACover the beaker containing the filtrate with a watch glass and (important) place a supported funnel with its stem bent so it just reaches into the lip of the beaker. Through the funnel pour gradually about 80 cc. of concentrated hydrochloric acid per each 10-gram charge of soil, taking care that the acid runs down the side of the beaker. Slightly lift the cover glass and gently stir with a stirring rod, watching for strong effervescence. If effervescence does not ensue, add more acid immediately until effervescence occurs. If the Jiltrate remains clear when concentrated t o a volume of 400 cc., the barium sulfate precipitation may be effected immediately by the addition of barium chloride solution, with the silica remaining in solution.7
Vol. 15, No. 11
However, to cover all cases, proceed as follows: Transfer acidulated filtrate to a shallow porcelain evaporating dish of 1000-cc. capacity as soon as effervescence has ceased. Place the dish 1 to 2 inches above an electric hot plate a t full heat. When crystallization begins, raise the dish about 3 inches from the hot plate. Permit evaporation and dehydrate without stirring, breaking the surface crust occasionally to expedite evaporation. PRECIPITATION OF BARIUM SULFATE-TO the dehydrated mass add 0.5 cc. hydrochloric acid and 200 cc. water. Dissolve the salts completely by warming. Filter off the silica on a Hirsch funnel using No. 2 Whatman filter, or paper of equivalent texture. Wash six times with hot water to a volume of about 400 cc. Heat the filtrate, add slowly 10 cc. of 5 per cent barium chloride solution, and allow to stand over night. Filter barium sulfate on a Gooch asbestos filter, ignite in an electric furnace (or place Gooch in a porcelain crucible over a burner flame), cool, and weigh. NOTESAND SUGGESTIONS FUSION MIXTURE-sodium carbonate serves both to moderate the speed of the oxidative process and to eliminate barium as barium carbonate in the first extraction. Optimum proportions of charge and sodium carbonate have been determined empirically by means of charges of pure quartz. The addition of sodium peroxide should be so regulated as to permit the comfortable handling of the crucible with the fingers, which will obviate excessive frothing and loss from overflow. Time is conserved by the simultaneous mixing of a number of charges. Should solidification of the mixture occur a t any time during the mixing process, it may be reverted to the pasty consistency by immersion of crucible into hot water for a few minutes. FUSION OF MIXTURE-speed is insured by the use of a large electric furnace, but any sulfur-free heat may be utilized. It is important that the specified temperature be not exceeded until the preliminary oxidation process has been insured, after which complete disintegration should be effected a t a higher temperature. The formation of a thin layer of the melt and its rapid crystallization on the interior of the crucible are essential to maximum efficiency. DISINTEGRATION OF MELT-T'he melt is usually completely disintegrated by the procedure outlined. Aqueous digestion over night offers no advantage. Furthermore, the successful use of the hot plate is conditioned upon the fact that the disintegration is started with a clear liquid in the beaker and with all the melt contained in the crucible. The presence of suspended material in the beaker a t the beginning will make the use of the hot plate impossible on account of severe bumping, and a steam bath or water bath will have to be employed with a consequent delay of 2 or 3 hours in the process. The disintegration is completed by the action of the boiling water alone without any other mechanical agitation. FILTRATION AND WASHING-The filtration of the alkaline aqueous extract removes iron, manganese, a good portion of the aluminium, all the barium, and earthy bases-mostly in the form of carbonates and silicates. ACIDULATION OF FILTRATE AND DEHYDRATION O F SILIcA-The covering of the beaker and addition of acid as directed, rather than pouring in and stirring with a rod, is most important. Ofte? n o silica precipitation takes place, particularly if the solution is left to come to room temperature prior to addition of acid, even after subsequent concentration to st volume far below that from which the barium sulfate precipitation is made. By this procedure the dehydration process is also greatly facilitated and expedited. The silica usually comes out as coarse crystals after some sodium chloride has crystallized and the full evaporation and dehydration is accomplished in about 4 hours. The use of inverted tripods on the hot plate affords a very convenient means of adjusting the distances for the evaporation and dehydration of the silicic acid. Barium chloride additions show no detrimental effect upon sulfate recoveries and sulfates added t o the melt as barium sulfate are completely recovered.
EXPERIMENTAL Since a perfectly clear melt is obtained by the fusion technic, it is apparent that complete disintegration of the soil
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Kovember, 1923 TABLE I-EFFECT
SOLUBLE AND INSOLUBLE SULFATE ADDITIONS TO SOILS UPON THE SULFATE RECOVERY OBTAINED BY
OF
SOIL Tennessee 4893 Tennessee 4893 Kansas 1 Kansas 1 Kansas 1
Weight of Sample G. 5 10 5
5 5
BaSOi Equivalent of Addition as FeSOa BaSOa G. G. None None 0.1851 None None None 0.0755 None None 0.1000
-Bas04 A
OF
Determined-
G. 0.0115 0.2101 0.0224 0.09'80 0. 12'17
mass xs assured. It is next essential (1) that complete removal of sulfate be secured, and ( 2 ) that barium, and possibly strontium, exert no interference in the removal of sulfates formed during the oxidative and disintegrative reactions utilized in obtaining a perfect melt. Since all barium and strontium are converted into carbonate form and a large excess of carbonate is present in the aqueous solution of the melt, those elements may be considered as eliminated as silicates along with iron and aluminium by the proposed procedurc!. To assure attainment of these two essentials, two soils, one from Tennessee and one from Kansas, were fortified with soluble and insoluble sulfates. The use of barium sulfate for the latter purpose provides both an insoluble sulfate and the presence of the interfering element barium. Full recoveries of both added soluble and insoluble sulfates are shown by the data of Table I, which gives the differences between the two soils before and after fortification. The further fact that the presence of either soluble or insoluble barium in the soil before sodium carbonate-peroxide fusion is of no effect upon the recovery of sulfate, is also shown by the data of Table 11. TABLE 11-EFFECT
.I185
SOLUBLEA N D INSOLUBLE BARIUMADDITIONS TO
SOIL UPON SULFATE RECOVERY B Y THE PROPOSED METHODa
Variation in Bas04 Equivalent Recovery as InBas04 Equivalent of --BaSOI Determined--fluenced by BaAdditions of Barium as A B Average rium Additionsb BaClr Bas04 c, G. G. G. G. G. - .. None None 0.0224 0.0222 0.0223 .... 0.0250 None 0.0230 0.0211 0.0221 -0.0002 None 0.1000 0.1217 0.1225 0.1221 -0.0002 A 5-gram sample of Kansas 1 soil was used in each determination. B I3aCla and BaSOa additions may be considered as additions of BaCOs and EaCOs plus NazSOc, since the NasCOs-NapO~fusion converts all barium t o t h e carbonate form in the melt. (1
The removal of iron and aluminium in granular form by an aqueous extraction and filtration of the melt is very desirable. The primary consideration in mind, however, was that of elimination of the barium, for the hydrochloric acid solution of the melt brings into play the undesirable preliminary reaction of soluble barium chloride and soluble
B
Average
G. 0.0125 0.2085 0.0222 0.0992 0.1225
0.0120 0,2083 0,0223 0.0886 0.1221
G.
THE PROPOSED METHOD Recovery of BaSO4Equivalent of Sulfate Addition Actual Variation G. G.
....
...... ......
0,1883
0.0002
0.0763 0.0998
0.0008 -0.0002
itated from a hydrochloric acid solution of the melt, without the removal of silica by dehydration, an excess of acid exerts a solvent action upon the barium sulfate formed after the addition of the barium chloride solution. Again, this excess of hydrochloric acid may not be killed off by ammonia, because of (1) the undesirability of ammonium salts, ( 2 ) the necessity for an absolute blank upon the amount of ammonia used, and (3) the necessity for sufficient excess of acid to care for the dissolved iron, aluminium, 'and manganese. As stated previously, the aqueous extract has additional advantages over the acid solution of the melt. The insoluble residue from the melt not only removes all barium, strontium, iron, manganese, and phosphorus pentoxide, but it leaves these elements for separate determination. In addition, it takes from solution more silica than does the acid solution. The recoveries of sulfate by aqueous and hydrochloric acid extractions for soil and soil fortified with barium chloride and barium sulfate are shown by the data of Table 111. It is apparent that the recovery of sulfate by the acid extraction is consistently lower than that obtained in the aqueous alkaline extraction. It is also apparent that a constant of sulfate and an increase in barium result in an increased discrepancy between the two methods of extraction. It is still further emphasized that, with high sulfur content, sulfate unrecovered by the acid extraction is practically equivalent to the amount added. But in the aqueous extraction of the fortified soil the recovery was equivalent to the amount of insoluble sulfate added. These data demonstrate the fallacy of hydrochloric acid extraction of soil melts; this point has not been stressed by investigators, so far as the writers are aware. The data of this table afford further proof that normal and abnormal occurrences of barium are of no effect in hindering sulfate recovery from soil melts. Variations between 5 and 10-gram charges had no effect upon the accuracy of the method. In comparisons between these two charges it was found that the proportionate recoveries were obtained with a constant ratio of soil to sodium carbonate-peroxide. Comparisons are therefore absolute when the results from the more feasible 5-gram charges of
TABLE111-COMPARISON BETwEBN AQUEOUS A N D HYDROCHLORIC ACIDSOLUTIONS O F MELTS O F S O I L AND S O I L FORTIFIED WITH BARIUMCHLORIDEAND BARIUMSULFATE BaSOa DETERMINEBas04 Equivalent of Aqueous Alkaline Variation between Acid Barium Addition as Extraction Y H C 1 ExtractionExtract and Aqueous BaCh Bas04 A B Average A B Average Alkatine Extract SoILa G. G. G. G G. G. G. G. G. Kansas4 None None 0.0182 0.0200 0.0191 0,0170 0.0154 0,0162 -0.0029 None Kansas 5 None 0.0180 0.0186 0 0183 0.0124 0.0140 0.0132 -0.0051 Kansas 6 None None 0.0130 0.0124 0.0127 0.0102 0.0107 0.0104 -0.0023
--
e
Kansas 1 None Kansas 1 0.0250 Kansas 1 None A 5-gram sample used.
None None 0.1000
0.0224 0.Q211 0.1225
0.0222 0.0230 0.1217
sulfate. With increased concentration of the hydrochloric acid solution of the melt this reaction between native barium and sulfur may be minimized during the preliminary digestion prior to filtration. However, if the hydrochloric acid extract is evaporated to effect removal of silica, as prescribed inmost of the acid extractions of melt, still further opportunity is afforded for barium sulfate precipitation and loss along with the dehydrated silica. Or, if the barium sulfate is precip-
0,0223 0.0221 0.1221
....
0.0140 0.0224
....
0.0130 0,0285
....
0.0135 0.0254
......
-0.0086 -0.0967
soils of high organic matter content are compared with analyses of 10-gram charges from soils of low organic matter and sulfur content.
REFEREXCES 1-Allen, J . A m Chem Soc , 32, 588 (1910) 2-Assoc Official Agr. Chem., Methods, 1920, p. 317. 3-Blumenthal and Guernsey, Iowa Expt. Sta , Research Bull. 26
(1915)
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1186
a-Failyer, U.S . Dept Agr., Bur. Soils, Bull. 7Z. 5-Fraps, J . A m . Chem. Soc., 24, 346 (1902). 6-Hart and Peterson, Wisconsin Agr. Expt. Sta., Research Bull. 14, (1911). ;-Hillebrand, U.S Ceol. Survey, Bull. 232 (1919) 8-Hirst and Greaves, Sod Sczence, 1S, 231 (1922) 9-Lomanitz, Texas Agr Expt. Sta., Bull. 302 (1922). IO--MacIntire, J . Assoc. Oficial Agr. Chem., 6, 52 (1922). 11-MacIntire, Ibid., 6 , 405 (1922).
Vol. 15, No. 11
12-MacIntire and Shaw, Ibid., 6, 320 (1923). 13-Mahin and Carr, “Quantitative Analysis.” p. 262. McGrawHill Book Co , New York. 14--Osborne, J . A m . Chem. SOG.,24, 142 (1902). 15-Shedd, Kentucky Agr. Expt. Sta , Bull. 174, 278 (1913). 16-Swanson and Latshaw, Soil Sczence, 16, 421 (1922). 17--Talbot, “Quantitative Chemical Analysis,” 4th ed., 1902, p. 33. I&Thomas, Soil Science, 15, 7 (1923). 19-Williams, J . A m . Chem. Soc., 24, 658 (1902).
Determination of Dissolved Oxygen in t h e Presence of Iron Salts’ By A. M. Buswell and W. U. Gallaher STATEWATERSURVEYDIVISION,URBANA, ILL.
T
HE determination of
A review of the literature on the determination of dissolved oxygen
one of duplicate samples he
dissolved oxygen is is given. The modified Winkler method is shown to give erroneous ~ ~ u n d 0 u b t e d 1Y the results in the presence of iron salts. These errors cannot be corrected alkaline with ammonium most frequently used and by applying a factor based on the amount of iron present. The hydroxide and then acid; to the most important of all Letts and Blake modification of the Levy method is found to giue the other he added acid the chemical methods results which check the gasometric determinations in the presence available for the investigaof iron salts and organic matter. both with standard pertion of water pollution probmanganate. The difference lems. Itmeasuresthe capacbetween the two titrations is ity of a body of water for receiving organic matter without proportional to the amount of oxygen in the water. Some investigators have declared t h a t this method gives low results, causing nuisance; it measures one of the important factors while others say the results are too high. affecting fish life; it is one of the important steps in the biologiLevy8 modified Mohr’s method using potassium hydroxide cal oxygen demand test for measuring the important portion and a bulb with a stopcock a t both ends and a funnel a t one end of the oxidizable substances in the sample; and it furnishes to introduce the reagents. Letts and BlakeQused a separatory valuable information concerning the biological and bio- funnel and sodium carbonate as the alkali. Thresh’s3 method consists in the oxidation of hydriodic acid chemical reactions going on in the stream. Naturally, a by the oxygen in the water to give free iodine. This is done in determination of such importance has received considerable the presence of nitrites, which by forming nitric oxide furnish attention in the past-in fact, no less than thirteen distinct a carrier for the oxygen. A blank is run to correct for the nimethods have been proposed. trite added and that present in the water. The released iodine To be successful, a method for determining dissolved oxygen is titrated with standard thiosulfate in a current of coal gas or under a layer of petroleum. This method is accurate but too must meet two requirements. First, on account of the small cumbersome for field use. amount of substance to be determined (a few milligrams per Direct titration of the dissolved oxygen was accomplished by liter), it must be exact; second, it must be carried out in com- Linossier. l o He avoided precipitation of iron by the presence pact, light apparatus capable of field operation, since the of sodium potassium tartrate and titrated the free oxygen with samples cannot be transported without undergoing chemical standard ferrous sulfate solution with phenolsafranine as a n indicator. change.
~~~~~~$i.&~~n~~~~~~~
PREVIOUS WORK The method least subject to chemical errors and probably the first to be proposed is that of Bunsen. l, * In this method the gases are boiled out either under atmospheric pressures or diminished pressures as practiced by Adeney,2 T h r e ~ h and , ~ others.* The gas collected is then determined by ordinary absorption methods. It is too cumbersome for field work (although Birge6has designed a metal apparatus for field use) and requires considerable skill for accurate manipulation, but it is still the standard by which all new volumetric or colorimetric methods are judged. ShutZenbergera made use of the absorption of oxygen by sodium hyposulfite. The determination was made in a current of hydrogen with indigo as an indicator. The hyposulfite solution was standardized against a n ammoniacal copper sulfate solution. Investigators differed on the reliability of this method. It was claimed that hydrogen peroxide was formed during the titration and the results were said to be too low, with variations of 0.5 cc. from gasometric methods. Mohr? made use of the oxidation of ferrous iron to ferric iron by the dissolved oxygen in a n alkaline solution. To 1 Presented before the Division of Water, Sewage, and Sanitation a t the 65th Meeting of the American Chemical Society, New Haven, Conn., April 2 to 7, 1923. Numbers in the text refer to the bibliography a t end of the article.
*
A few colorimetric methods have been proposed. Ramsay11 allowed the oxygen in the water to oxidize a cuprous salt to a cupric salt, thus going from a colorless to a blue solution. The intensity of the color was proportional to the oxygen present. He used standard tubes made from sodium chloride and copper sulfate reduced with sulfur dioxide gas as comparates. His method was never successful, since the color faded from the standards on standing, and the color of the water interfered as also did the nitrites. Mackay and Middletonlz absorbed the oxygen from a portion with alkaline pyrogallol. They boiled another equal portion to get rid of the oxygen and after the addition of pyrogallol to the boiled sample added air until the color matched the first sample. From the amount of air added the dissolved oxygen was calculated. The method was inaccurate because the air added could not be easily measured and the apparatus required was cumbersome. The method commonly used a t present for the determination of dissolved oxygen in water is that of Winkler.13 It depends on the oxidation of bivalent manganese (Mn++)by the dissolved oxygen in an alkaline solution. The oxidized manganese oxidizes iodide in an acid solution, giving free iodine. The liberated iodine is titrated with standard sodium thiosulfate solution. This method has been found to check very well with gasometric determinations. I t gives too high results in the presence of nitrites. Winkler13 recognized this fact and proposed to correct by titrating a separate sample with a weak manganese chloride solution. This means is out of the question for field work.
