Determination of Manganese in Caustic Soda DWIGHT WILLIAMS
AND
R. V. ANDES
Research Department, Wertvaco Chlorine Ptoducts Corporation, South Charleston,
W. Va.
The manganese content of caustic soda may be determined colorimetrically as the permanganate b y oxidation with periodate after addition of sufficient phosphoric acid to give a p H of 4. Chlorides and silica in the concentrations normally found in commercial caustic soda do not interfere in this method. The limit of uncertainty of
the method under the best conditions (LUJ was found to be *0.18 part per million in a sample analyzing 0.37 per million. The LU, was reduced to t0.03 part per million b y extracting the manganese as the oxinate with chloroform prior to the development of the permanganate color.
A
A dispensing buret such as Scientific Glass Apparatus Co. Catalog No. 5-814 is desirable for dispersing hydrochloric acid. Several 1000-ml. Squibb separatory funnels are required. Sodium sulfite, 100 grams of anhydrous C.P. sodium sul6te dissolved in sufficient water to make 1 liter of solution. Phenolphthalein, 10 grams per liter in isopropanol. Oxine, 10 grams of 8-hydroxyquinoline (oxine), Eastman reagent 794, dissolved in 1 liter of isopropanol. Chloroform, technical grade. Perchloric acid, c . P . ,70 to 72%. Hydrogen peroxide, c.P., 3oa/,. Potassium periodate, C.P. Sodium phosphate. Dissolve 500 grams of sodium dihydrogen phosphate monohydrate in sufficient water to make 1 liter of solution. If turbid, filter through Whatman 40 filter paper by means of a Buchner funnel. Baker's C.P. sodium phosphate was found to be sufficiently low in manganese for this purpose. Standard manganese solution. Dissolve 0.3077 gram of manganous sulfate monohydrate in water, add 1 ml. of concentrated sulfuric acid, and dilute to 1 liter in a volumetric flask. One milliliter of this solution contains 100 micrograms of manganese.
SUMBER of years ago a need arose in this corporation for a precise method for determining manganese in caustic soda. The concentration of manganese in caustic soda is extremely low, usually less than one part per million. A large sample and a sensitive method are required for the precise estimation of manganese at these low levels. The periodate method of Willard and Greathouse ( 5 ) appeared likely to be the most useful in this application. In preliminary work the oxidation was carried out in a sulfuric acid solution. This acid 4ad numerous disadvantages in this application. Because the relatively high concentrations of sodium chloride present in commercial caustic soda used up a large amount of periodate, it was necessary to remove chlorides by evaporation prior to the oxidation. Turbidity due to the precipitation of silica invariably developed during this evaporation and haiu to be removed by filtration. Thus, the procedure w w time-consuming and, in addition, the results were not always reliable. Wllard and Greathouse found that the permanganate color could be developed in solutions containing sulfuric, nitric, or phosphoric acid. Clark (1) reported that phosphoric acid is more satisfactory than other acids for the development of the color. This suggested the possibility of oxidizing the manganese after adding an excess of phosphoric acid. Experimentation showed that the permanganate color developed readily if the caustic soda were acidified with sufficient phosphoric acid to reduce the pH to 2 and that a considerable variation in the amount of phosphoric acid was permissible. Turbidity formation is entirely eliminated by this procedure and the permanganate color develops and is stable in the presence of the chloride ion. While this simple direct procedure proved useful, subsequent work indicated that it was not sufficiently precise for some purposes. Further, while it could be applied in the presence of low concentrations of chloride such as those found in commercial caustic soda, it was not applicable t o cell liquor from chlorinecaustic soda cells where sodium chloride equaled or exceeded sodium hydroxide. The best solution for these problems appeared to be a concentration 2rocedure. Precipitation of the manganese as the hydrated oxide along with ferric hydroxide as a carrier was tried but found unsatisfactory. Moeller's proposal (8) to extract a number of metals as the oxinate by means of a chloroform solution of oxine suggested the possibility of extracting manganese by a similar procedure. After the manganese is extracted from a large sample, the oxine is oxidized by means of hydrogen peroxide and perchloric acid to a form which does not interfere with the subsequent analysis and the manganese is determined in the usual manner. APPARATUS AND REAGENTS
An Eimer and Amend photoelectric colorimeter was used. All meaaurements were made using Wratten filter 58 or 58.4 and a rectangular optical cell having a capacity of somewhat more than 100 ml. and measuring 40 X 40 mm. inside on the base. A scoop having a volume of 0.15 ml. was used for adding the potassium periodate crystals. A cylindrical glass cup sealed to a short length of glass rod makes an ideal scoop. I
Predent sddreu, Brown and Bigelow, 8t. Paul. Minn.
PROCEDURE
DIRECTMETHOD. Weigh 20 grams of 50% sodium hydroxide (or the equivalent amount of other concentrations) on a Harvard trip balance, add 50 ml. of water, and swirl to mix. Add 50 ml. of 1 to 1 phosphoric acid and 0.4 gram of potassium periodate, heat to boiling, and boil for 20 minutes. Cool to room temperature and dilute to 100 ml. in a graduated cylinder. Measure the transmittancy on a photoelectric colorimeter using a Wratten 58A filter. Read the micrograms of manganese from a calibration curve. Calculate the parts of manganese per million parts of sodium hydroxide by dividing the micrograms of manganese by 10. To prepare the calibration curve measure out portions of the standard manganese solution, containing 0, 10, 20, 50, and 100 micrograms of manganese, by means of a I-ml. Mohr pipet. Proceed as described above, beginning with the addition of 50 ml. of water. Plot micrograms of manganese against per cent transmittancy EXTRACTION METHOD. Weigh 100 mams of 50% sodium hydroxide (or the equivalent ;mount d other conce6trations) into a 1-liter beaker and add sufficient water to give a total volume of 500 or 600 ml. Add 1 ml. of sodium sulfite solution (100 grams per liter), acidify with 12 N hydrochloric acid, and add 5 ml. in excess. I t is advisable to add about 90 ml. of the acid, then 3 to 5 drops of phenolphthalein, and finally to acidify and add the 5ml. excess. Mix the acidified solution aud let stand 3 to 5 minutes. Into another beaker introduce q00 ml. of water and 5 ml. of hydrochloric acid to be used as a blank. Carry this blank through the same procedure as the sample. Add with stirring 5 mi. of oxine solution followed by 16 ml. of ammonium hydroxide (specific gravity 0.90), cool to room temperature in running water, and transfer to a 1-liter Squibb separatory funnel. Rinse the beaker twice with a stream of water from a wash bottle and add to the solution in the separatory funnel. Add 30 ml. of chloroform, shake vigorously for 60 seconds, and allow the two liquids to se arate. Draw off the chloroform layer into a 250-ml. wide-moutged conical flask. Add 10 ml. of additional chloroform to the separatory funnel, shake 15 seconds, and draw off and combine this chloroform with the fimt extract. Make a third extraction with 10 ml. of chloroform. Tf the third extract shows appreciable color, make further extractions until a colorless extract is obtained. To the combined extracts add 5 ml. of 7070 perchloric acid and 5 ml. of 30% hydrogen peroxide, cover with a Fisher Speedyvap watch glass, and evaporate slowly to a volume of 1 to 2 ml. Do not evaporate t o dryness, as this may result in a mild ex-
.
January, 1945
ANALYTICAL EDITION
plosion. Evaporations with perchloric acid are dangerous and, when appreciable quantities of organic matter are present, will result in violent explosions. For this reason, the procedure must not be altered so as to increase the amount of organic matter present during the evaporation. Cool, rinse off the watch g!ass, and add 10 ml. of sodium diiiydrogen phosphate solution, 100 ml. of water, 0.4 gram of potassium periodate, and a glass bead. Boil to a volume of 90 to 100 ml. (10 to 20 minutes’ boiling), cool in running water, and dilute to 100 ml. in a graduated cylinder. Measure the transmittancy and read the micrograms of manganese from the calibration curve. Calculate the manganese concentration in parts per million parts of sodium hydroxide by dividing the micrograms of manganese (after subtracting the blank) by the weight of sodium hydroxide (grams) in the original sample. To prepare the calibraton curve introduce into a series of 250ml. flasks portions of the standard manganese solution containing 0, 25, 50, 100, and 150 micrograms of manganese, using a 1-ml.’ Mohr pipet. Add 2 ml. of 70% perchloric acid and proceed as described above, beginning with the addition of sodium dihydrogen phosphate. Plot the data obtained, using a scale such that the smallest division on the graph paper corresponds to 1 microgram of manganese and 0.2% light transmittancy. Table 1.
Effect of Phosphoric A c i d Concentration and p H on Recovery of Manganese from Sodium Hydroxide (100 microgram of manganese added)
85% H I P O I
pH
Transrnittancy
4.1 2.2
82: 5 75.7 75.3 75.2
7”
M1. 15 20
30 35
1.5 1.2
Manganese Found Microoroma 69 99 100 101
EXPERIMENTAL
DIRECT METHOD. To determine the effect of varying concentrations of phosphoric acid, there were added to 20-gram portions of Soy0 sodium hydroxide 100 micrograms of manganese followed by various amounts of phosphoric acid and the color was developed by boiling with potassium periodate. The data in Table I indicate that color development is incomplete when 15 ml. of 85% phosphoric acid are used but that essentially complete color development is obtained by using 20 ml. The further apparent slight increase in manganese for larger amounts of phosphoric acid, while within the precision of the method, may be due to the manganese content of the acid. The color of the standards, to which no sodium hydroxide was added, developed readily when using 5 to 40 ml. of 85% phosphoric acid. These data indicate that the pM must be maintained below 4 and possibly aa low aa 2 for the maximum color development. The colors developed under these conditions showed no fading during 3 days and indeed the color intensity increased slightly during this period. Of the filters tested Wratten 58A gave the greatest spread in transmittancy per unit of manganese, although filter 58 was almost as good. The calibration curve with either of these filters is almost linear over the range 0 to 150 micrograms and has a slope of approximately o.25y0 transmittancy per microgram of manganese. The colorimeter which was used in this work is no longer manufactured but any other colorimeter that utilizes a two-cell balanced-bridge circuit and a 40-mm. light path and the same filter would doubtless give similar results. The time required for the appearance of the permanganate color may vary from a few seconds to almost 20 minutes after boiling is started. A longer boiling time is required for the appearance of the color when the amount of manganese is very small. Color appeared in a sample in which 17 micrograms of manganese were found after boiling for 10 minutes. A portion of the same sample to which 60 micrograms of manganese were added required 4 minutes and intermediate concentrations required intermediate boiling periods. A 20-minute boiling period was used in all cases before reading the transmittancy on the colorimeter. Consistent results were obtained by boiling 20 minutes, which indicates that longer boiling is unnecessary.
Table
29
II.
Preciiion of Direct Method for Manganese in 50% Sodium Hydroxide Test No. ,Manganese Deviation 1 2
3
4 5 I3
7 8 9 10 Av. U I of group L U I of method
P.p.m. 0 30 0 35 0 40 0 45
P.p.m.
-0 07 - 0 02 4-0 03 +o 08 -0 02
0 35 0 40 0 35 0 40
+ O 03 -0 02 + O 03 + O 03
0 40
-0
0 25
12
0 37 * O 055
* O 180
No turbidity has ever been observed when following this procedure. The method has been applied in the presence of 6000 parts per million of added silica and to caustic soda from all the leading American manufacturers. The precision of the method was determined according to the procedure given by Moran (3). The limit of uncertainty under the best conditions was found to be +0.18 part per million parts of sodium hydroxide for a sample analyzing 0.37 part per million (Table 11). The precision of the measurement of the color of a transsingle colored solution has been shown to be +=o.35y0 mittancy. Based on the slope of the calibration curve of 0.25% transmittancy per microgram, the precision of the method of +0.18 part per million is equivalent to +o.45y0transmittancy. Thus, the precision of the method is of the same order of magnitude as the precision of the measurement of the color and is as good as can be expected. The precision of the method was also determined under routine conditions by four chemists who made a total of 20 analyse8 during several days. The sample used for this test analyzed 0.96 part per million and the limit of uncertainty was found to be +0.36 part per million. While these precisions are sufficient for some purposes, they are obviously not satisfactory for detecting small changes in manganese content. CONCENTRATION METHOD. An attempt was made to concentrate the manganese by precipitation as the hydroxide or hydrated oxide. The precipitate was filtered, dissolved in hydrochloric acid, and the manganese determined colorimetrically in a phosphate-buffered solution. The samples were compared with a calibration curve which was prepared by adding known amounts of manganese to hydrochloric acid and developing the color in a phosphate-buffered solution in the same manner as the sample. Since the amount of manganese was small, ferric iron was added as a gathering agent. Recovery of added manganese by this procedure was erratic and usually low, varying from 50 to 1 0 0 ~ o of the amount added. A study of the effect of the temperature of precipitation, the time between precipitation and filtration, the amount of iron added, and the addition of hydrogen peroxide failed to indicate any set of conditions which would result in the consistent recovery of manganese. Moeller’s (a) method for the extraction of iron, aluminum, indium, cobalt, and .nickel involved adjusting the pH of the aqueous solution to a predetermined value and extracting with a chloroform solution of oxine. Qualitative tests indicated that manganese could be extracted a t a pH of about 9. However, recovery was not always quantitative. It was shown that it is necessary to acidify the sample before any manganese can be recovered. Only 2 micrograms of manganese were recovered from a sample of cell liquor to which 100 micrograms had been added, ?hen the extraction was made without first neutralizing the sodium hydroxide. From 50 to 80 micrograms of manganese were recovered from similar samples which were acidified ‘with hydrochloric acid and then made alkaline with ammonia before being extracted. This indicated that the manganese is completely converted in sodium hydroxide solutions to a form which does not react readily with the chloroform solution of oxine. It
INDUSTRIAL AND ENGINEERING CHEMISTRY
30 Table
Ill. Effect of Time of Shaking with Chloroform on Recovery of Manganese from Cell Liquor (100 microgram of manganese added) Time Mn Found Second8
Micrograms
93
15 30 60
96
104 105
120 Table
IV. Effect of Excess Hydrochloric A c i d and Ammonium Hydroxide on Recovery of Manganese from Cell Liquor (100 micrograms of manganeae added) Excess HCl Excess “&OH Mn Found M1.
MI. 10 10
105
106
15
15
~~~
V.
73 94 93
15
2 10 20
Table
Micrograms
0 5 50
10 .~
~
60
~~~~
Number of Extractions Re uired for Recovery of Manganese from C e l l l i q u o r (100 micrograms of manganese added) Mn Found Extract No. Microerama 1 2 3 4
102 3
1
3
is probable that a variable portion of the manganese is converted to a similar form in ammoniacal solution. This suggested that the manganese oxinate should be formed by adding the oxine to the acidified aqueous solution and then making this solution ammoniacal. Chloroform, rather than a solution of oxine in chloroform, would be used for the extraction in this caae. Quantitative recovery of manganese was obtained consistently by this procedure. The time required to extract the manganese oxinate by means of chloroform waa determined as follows: To 500 ml. of cell liquor were added 1” m i ~ r o ~ a m ofs manganese (as the chloride or sulfate). Five mdhhters exwas of 12 N hydrochloric acid waa added, followed by a solution of 0.1 gram of oxine in 5 ml, of 12 N hydrochloric acid. Ten milliliters excess of w o m u m hydroxide (specific gravity 0.90) waa added and the manganese oxinate extracted with a single Wml. portion of the solvent. Table 111 indicates that a 60-second shaking period ie desirable when extracting with chloroform. Chloroform and trichloroethylene were found to be about equally efficient. Carbon tetrachloride appeared to be somewhat less efficient, although the difference is probably not significant. Using a similar procedure i t waa found that variations between 5 and 50 ml. in the exceas ammonium hydroxide had no si&cant effect on the recovery but that recover waa low when the solution was just neutralized. The w e of 2- to IO-ml. exof hydrochloric acid gave about the same recovery, b$ 13 larger excess mused low results (Table IV). The low results for the larger e x c w of hydrochloric acid were probably due to the partial oxidation of the oxine by the chlorate from the cell liquor. This oxidation could doubtless have been prevented by the addition of a reducing agent, such aa d u m d t e , as described below. Variations in the volume of solvent between 10 and 100 ml. did not give a significant variation in the recovery. Essentially constant recovery was obtained using 0.02 to 0.2 gram of oxhe. Several other metals including iron are precipitated by oxhe under the conditions used here and it is m t i a l to add sufficient oxine to react with all metals which are precipitated under them conditione. Moeller recommended the me of chloroform containing alcohol. Amounts of iaopropanol up to 15 ml. did npt
Vol. 17, No. 1
cause any variation in recovery, Isopropanol is a convenient solvent for oxine and was subsequently used for this purpose in place of hydrochloric acid. The data in Table V indicate that most of the manganese is extracted with a single 30-ml. portion of chloroform. Extraction with chloroform is as effective aa extraction with a l-gram-perliter solution of oxine in chloroform. Samples which were acidified, treated with additional oxine, and then made alkaline before each extraction did not yield any greater amount of manganese than those which were not so treated. Three extractions with chloroform were adopted for the h a 1 procedure to provide for possible variations in technique. An occasional sample of cell liquor was found which contained manganese dioxide in a form that could not be dissolved in hydrochloric acid by the usual procedure. Sulfites have been suggested as an aid for the dissolution of manganese dioxide ( 4 ) . The addition of a small amount of sodium d t e at the time the sample waa acidified increased the recovery of manganese from these refractory samples. The manganese content obtained in this manner agreed with that obtained by boiling the insoluble material with concentrated hydrochloric acid and adding this solution to the main body of the sample. A large excess of sodium sulfite should be avoided, since it retards the recovery of manganese. The effect of increasing amounts of sodium sulfite on the recovery of manganese from two typical samples of cell liquor is shown in Table VI. Sample 1 contained some “insoluble” manganese dioxide and a small amount of sodium sulfite is rBquired for the recovery of all the manganese. Additional amounts of sodium sulfite did not affect the recovery, presumably because of reaction of most of the sulfite with oxidizing agenta in the cell liquor. Sample 2 waa free from “insoluble” manganese dioxide. A small amount of sodium sulfite had no s+cant effect on the recovery but large amounts caused low results. While no explanation could be found for these low results, they were observed rather consistently and appear to be
Table VI.
Effect of Sodium Sulfite on Recovery of Manganese from Cell Liquor (100 microgram of manganese added) Sample 1 Sample 2 NatSOa Mn found Mn found NarSOa Gram
Micrograms
Oram
Microgram8
0.0
83
0.0
88 86
97 93 97
0.1
0.6 2.0
0.1
78 67
0.5
2.0
Table VII. Accuracy d Extraction Method for Manganese in 50% Sodium Hydroxide Recovered Found Microgram8 of mangansre
Added
Error
...0
..
26
49 97
-1 -3
Table VIII. Recision of Extraction Method for Manganese in 50% Test No.
Sodium Hydroxide Mangsneae Found Micromama
1 2
3 4 6 6
7 8
9 10
Av.
n of croup LUI of method
Pam.
77
0.77
78 78 78 77 79 77 77 77 77.8
0.78 0.78 0.78 0.77 0.79 0.77 0.77 0.77 0.778
80
r0.m
r3.18
0.80
*0.0098
r0.0318
January, 194.5
ANALYTICAL EDITION
real. Although no evidepce of the presence of insoluble manganese dioxide was found in caustic soda, it appeared desirable to add a small amount of sodium sulfite to prevent the oxidation of the oxine which, as pointed out above, is excessive urder some conditions and doubtleas occurs to some extent under any condition. I n addition, the phenolphthalein is oxidized rapidly if the sulfite is omitted. The accuracy of the method was determined by comparing the recovery of manganese added to caustic soda with a calibration curve which waa prepared without going through the extraction procedure. The maximum error observed (Table VII) is within the precision of the method as shown in Table VIII. It is concluded from this that the extraction procedure is free from constant errors and that the method is accurate. The limit of uncertainty of the extraction method under the
31
best conditions was found to be *0.03 part per million (Table VIII). The ratio of the limit of uncertainty (LUJ of *0.03 part per million by the extraction procedure to *0.18 part per million by the direct procedure is approximately what would be expected from the fivefold increase in sample size. This indicates that the added manipulations of the extraction do not significantly detract from the precision. LITERATURE CITED
Clark, N. A., IND. ENQ.CHEM.,ANAL.ED., 5, 241-3 (1933). (2) Moeller, T..Ibid.. 15,270-2, 346-9 (1943). (3) Moran, R. F.,Ibid., 361-4 (1943). (4) Willard, H.H.,and Diehl, H., “Advanced Quantitative Analysis”, p. 81, Ann Arbor, Mich.. Bloomfield and Bloomfield, 1939. (6) Willard, H. H.. and Greathouse, L. H., J . Am. CAem. soc., 39, (1)
2366-77 (1917).
Determining Oxygen in Hydrocarbon Gases KARL UHRIG, F. M. ROBERTS,
AND
HARRY LEVIN, The Texas Company, Beacon, N. Y.
A
method is described for determining oxygen in concentrations of O x y g e n is reacted with copper wetted with ammonia-ammonium chloride solution, the resulting mixed oxides are dissolved in the same solution and reduced to cuprous form, and copper is determined iodometrically as a measure of oxygen in the
0.001 to 5%.
A
STUDY of refinery operations for factors affecting catalyst life made it important to have a method for determining small quantities of oxygen in hydrocarbon gases. The usual methods of Orsat gas analysis employing alkaline pyrogallol, phosphorus, chromous chloride, etc., are obviously unsuitable for the small concentrations of oxygen with which this paper is concerned. Simmons and Kipp (Id) used sodium triphenylmethyl for determining traces of water and oxygen, presuming therefore the a b m c e of one when determining the other. An interesting test is described by Kautsky and Hirsch (4) who determined the time required for destruction of the phosphorescence of trypaflavine as a measure of very low concentrations of oxygen. A nephelometric method based on fume intemity in the reaction between oxygen and phosphorus was mentioned by Wagner (14). A colorimetric method, based on the red color produced by oxygen in a solution of pyrocatechol and ferrous sulfate, was described by Binder and Weinland ( 2 ) . Ambler (1) described a method based on the color imparted by oxygen to alkaline pyrogallol solution. Hofer and Wartenberg (3) based their method on the ready oxidation of sodium hydrosulfite and measured the consumption of the sulfite. Mugden and Sixt (7) determined small amounts of oxygen in gases by comparing the blue color produced in ammoniacal cuprous salt solutions with known cupric salt solutions. The use of manganous hydroxide for determining small amounts of oxygen in gases was suggested by Phillips (8) and h t e r elaborated by Schmid (9). This is a modification of Winkler’s (15) method which was originally proposed for determining oxygen dissolved in water. A method employing the same general principle, but ferrous instead of manganous salts, was described by Shaw (If), who completed the determirdon colorimetrically. A ,method claimed to be fast and practically independent of the composition of the test gas was c!escribed by MacHattie and Maconachie (6) who deposited the oxygen on reduced copper kept moist with ammonia-ammonium chloride solution, dissolved the resulting cop er oxide in the same reagent, and estimated ir, by titrating a glank with standard copper solution to the same depth of blue color. The last method mentioned appeared most promising of all covidered and was investigated extensively. When it was applied to knowns, following in detail the directions of its authors, low results were obtained. This was thought to be due to di5culties experienced in recognizing the end point of the colori-
sample. The method gives accurate results in rbturated r n d unsaturated hydrocarbon gases. Sulfur dioxide, hydrogen sulfide, and mercaptans must be removed and means for doing so are provided, The method is based on well-known reactions but many modifications have been made in technique and equipment.
Table
1.
Copper Determinations ns Measure of Oxygen in Air (Air taken to be 20.9% oxygen by volume)
Air Taken (Diluted to 100 M1. with Pure Nitrogen)
M1. 4.53“ 9.06 11.32 13.61 22.75 5
Copper Calculated from Air Taken Copper Assuming Formation of: CurO CuO F&nd
Me.
MU.
9.8 17.5 24.1 27.5 46.1
10.8 21.5 26.9 32.3 54.0
At 0’ C 760 mm.
b Calculaihd from copper determined
31g.
5.4 10.75 13.45 16.15 27.0
Oxygen Found5 Assuming Formation of:
CurO
R 19.0
17.0 18.7 17.8 17.8
CUO 70
38.0 34.0 37.4 35.6 35.6
microelectrolyeis.
metric copper titration. This method of determining copper was therefore replaced by the more precise microelectrolytic procedure. Those authors (6) believed cuprous oxide was formed but determined from many experiments that 10.45 mg. of copper, instead cf theoretical 11.36,were equivalent to 1.0 ml. of oxygen (0’ C., 760 mm.) and adopted the former empirical value. The present authors have found this to be due to the fact that cuprous and cupric oxides are simultaneously formed. The data given in Table I indicate that mixed oxides are formed and that the ratio may not be constant, the prssent data yielding a copper value of 9.24to 10.33 mg. per cc. of oxygen. i f oxygen be calculated from the copper found in the cell washings, is necessary that it be present either as cupric or cuprous oxide, but not a Variable mixture. &me transformation of cuprous into cupric oxide involves adding oxygen, the cupric oxide must he reduced to cuprous and this is easily accomplished by shaking its ammdniacal solution with copper in the specially designed reaction ce!i described below. io this manner satisfactory results were obtained (Table 11). The rather involved microelectrolytic copper determination was then replaced by simple iodometric procedure without loss in accuracy or precision. To analyze a sample of very low oxygen content such a large portion (up t o 5 liters) must be taken that its passage through the
