July 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
show that minerals such as epidote and zoisite require intense ignition for complete removal of water of hydration and that this water is not removed in the standard method for determination of ash in coal by which the coal is ignited to constant weight a t a temperature between 700" and 750" C. No information is available as to the amount of such minerals that may be present in coal. ACENOWLEDGMENT The writers gratefully acknowledge the assistance of W. H. Ode, assistant chemist, Pittsburgh Experiment Station, U. S. Bureau of Mines, in making a number of the tests included in this paper.
241
LITERATURE CITED (1) Am. SOC. Testing Materials, Standards, Pt. 11, Non-Metala, pp. 689-726. 1930. (2) Fieldner, A. C., and Selvig, W. A., Trans. Am. Inst. Mining Met. Engrs., Coal Division, 597-613 (1930). (3) Fieldner, A. C., Selvig, W. A., and Gibson, F. H., Ibid., 224-35 (1932). (4) Fish, F. H., and Addlestone, 3. A,, IND.ENG.CHEM., Anal. Ed., 3, 155-8 (1931). (5) Parr, S. W., Eng. Expt. Station, Univ. Illinois, BUZZ.180 (1928). (6) Parr, 8. W., Illinois Coal Mining Investigations, Bull. 3 (1916). RECEIVEDJanuary 17, 1933. Presented before the Division of Gas and Fuel Chemistry a t the 86th Meeting of the American Chemical Society, Washington, D. C., March 26 to 31, 1933. Published b y permiesion of the Director, U. 8.Bureau of Mines. (Not subject to copyright.)
Determination of Small Amounts of Manganese in Salt Solutions NORMAN ASHWELLCLARK,Department of Chemistry, Iowa State College, Ames, Iowa
S
INCE the introduction in 1917 by Willard and Greathouse (9) of potassium periodate for the oxidation of manganese to permanganate, the method has come into general use. The color from 0.01 mg. of manganese in 50 cc. has been reported as easily checked against known standards. Recently a still more delicate color reaction has been suggested by Stratton, Ficklen, and Hough (8) using benzidine hydrochloride, with an optimum concentration for color comparison between 0.001 and 0.0001 mg. per 100 cc. Both methods have been used in these laboratories for the determination of traces of manganese in salt solutions.
POTASSIUM PERIODATE METHOD The potassium periodate method depends upon the oxidation of the manganese in acid solution to the colored manganese ion, and comparison with a standard similarly prepared, Willard and Greathouse, for 100 cc. of solution containing 2.5 or more mg. of manganese, advised 10 to 15 cc. of concentrated sulfuric acid, 20 cc. of concentrated nitric acid, 5 to 10 cc. of 85 per cent phosphoric acid, or mixtures of two or more acids. The amount of acid was shown to vary widely without influencing the results. This method was used by Bartow and Thompson ( I ) without the sulfuric acid, for manganese in potable waters. I n 1930 Richards ( 6 ) reported results on biological material that showed the desirability of controlling the acidity very accurately in order to get satisfactory results with small quantities of manganese. Sulfuric acid was used and, with the acidity from 5 to 6 per cent, no fading of the color was observed. Excess acidity prevented development of the full color or caused it to fade rapidly. I n the same year Skinner and Peterson (7) published a paper on the determination of manganese in animal tissue. They showed that 10 to 20 grams of material, when ashed in a muffle furnace a t cherry red, could be extracted with 5 cc. of 85 per cent phosphoric acid and 30 to 50 cc. of water and, after filtering, could be oxidized directly with 0.3 gram of potassium periodate without loss of color. They found that 0.01 mg. of manganese in 50 cc. gave a readable color when compared in Nessler tubes against an inverted V of white paper. I n these laboratories it became necessary to know the content of manganese fairly accurately in a number of salt solutions both separated and together, and in concentrated
and dilute solutions. The salts in which the manganese was determined included potassium nitrate, primary calcium phosphate, magnesium sulfate, ferric chloride, and later ferric citrate. I n all cases 50-cc. Nessler tubes were used for the color comparison. The tubes were carefully matched for color of glass; for this reaction, tubes with a trace of brown in their composition were found to be less delicate than others. With a good frame and milk-glass reflector, and with sky light, the comparison was as easily made as with the inverted V paper used by Skinner and Peterson. It was usually possible to detect the presence of 0.001 mg. of manganese in the 50-cc. tube; 0.002 mg. could always be seen, and there was a distinct difference in additions of 0,001 mg. Colors produced by the potassium periodate oxidation were remarkably stable; in some cases standard solutions which had been kept for 7 months in the dark showed no trace of fading. The standard solution was made from potassium permanganate of known normality, or from recrystallized potassium permanganate as suggested by Richards (6). I n both cases the solutions were reduced by sodium sulfite and sulfuric acid, and the sulfur dioxide boiled off. Various combinations of sulfuric, nitric, and phosphoric acids for the oxidation with potassium periodate showed that phosphoric acid alone was the most satisfactory. The remainder of the oxidations were made in 50 to 100 cc. of a solution containing 5 cc. of 85 per cent phosphoric acid and 0.3 gram of potassium periodate. A few minutes' boiling usually brought out the full color; the solution was then cooled and made up to 50 cc. in the Nessler tubes. A further solution was made up from the same standards and reduced with the sulfur dioxide as before, or by Bureau of Standards sodium oxalate, diluted until the content was 0.002 mg. of manganese per cc. and, when freshly oxidized by potassium periodate, used as a check on the diluted standard solutions. A few cubic centimeters of this (solution B+), before oxidation, were used with separate samples of unknowns to test for interfering substances. A liter of the distilled water used was evaporated to 50 cc., but showed no trace of manganese after oxidation. By varying the amounts of potassium periodate, this salt also was shown to contain no manganese. Samples of commercial 85 per cent phosphoric acid contained from 0.002 to 0.003 mg. in 5 cc. By recrystallizing this could be elimi-
ANALYTICAL EDITION
242
nated, but usually a fresh bottle was checked and the correction made. A dilute solution of the previously mentioned recrystallized salts was tested for its manganese content. This solution contained per liter 0.6 mg. of iron (as ferric chloride), 16 mg. of calcium, 24 mg. of manganese, and 312 mg. of potassium. The result, along with a check on the water, is shown in Table I. After subtracting the 0.002 mg. of manganese in the 5 cc. of phosphoric acid, it will be seen that no manganese can be detected at this dilution. A solution of the same salts was then made up 200 times as concentrated. Some precipitation which occurred a t this concentration was removed by filtration or centrifuging. Checks were made by addition of solution B before and after precipitation to determine any possible loss or interference. The volume was made small before the removal of the precipitate, and the oxidation with the potassium periodate was carried out in a more dilute solution. The oxidation in some cases caused further precipitation; Skinner and Peterson (7) recommend filtration and reoxidation, or allowing the precipitate to settle in the Nessler tube and transferring the supernatant liquid to a second tube. The solution evidently contains 0.006 mg. of manganese in 100 cc. TABLEI. MANGANESE IN MIXEDSALTSOLUTIONS MANQAXESE FOUND MANQANE~~E ADDEDAS SOLUTION B Jfg.
0
0.002
0.004
0.008
In 50 cc.
In 50 cc. dilute solution 5 CC. Hapoi
Mo.
In 100 cc. concentrated solution 6 cc. Hapoi Mg.
0.002
...
0.002
0.008
0:oio
0:oos
+
water 5 GO. Hap04 Mg.
+
0.004
o:oiz
...
The separate salts were then made up in the same dilution as the concentrated solution in Table I. Table I1 shows the figures, The total manganese in the separate salts checks very closely the amount found when determined in the concentrated solution. Six recrystallizations subsequently removed all trace of manganese from the magnesium sulfate. TABLE11. MANGANESE IN SEPARATE SALT SOLUTIONS MANQANE~~E MANQANESE FOUND IN 100 cc. SOLUTION WITH 5 cc. &PO4 ADDEDAR SOLUTION B Water FeCls Ca(HzP04)z MgSOi KNOs Mg. MO. MO. Mg. MQ. MQ. 0 0.002 0.003 0.002 0.008 O.OOl+ 0.002 0.005 0.004 0.004 . I .
...
Ferric citrate has been used as a source of iron both for plants and for animals, and it became desirable to know the amount of manganese in the commercial product and whether reprecipitation would decrease it. The citrate, however, interfered with the oxidation, as no color was produced on adding solution B and heating with phosphoric acid and potassium periodate, Willard and Greathouse suggested boiling with nitric acid to free from reducing agents, adding persulfate if carbon was present. Skinner and Peterson ashed their samples in a muffle furnace. I n this case it was sufficient to heat the iron citrate in porcelain crucibles until the carbon was removed and the ferric oxide formed. Several methods were checked for taking up the residue; 0.1 gram of the finely ground, ashed citrate, digested for several hours on the water bath with 50 cc. of water and 5 cc. of 85 per cent phosphoric acid, usually gave a solution which could be satisfactorily oxidized with potassium periodate. Ferric citrate has not been obtained in crystalline form. The solution can be concentrated to a sirup and precipitated as a yellow amorphous mass by the addition of ethyl alcohol. If this precipitate is dissolved in water, and the solution allowed to evaporate to dryness in thin layers, red-brown
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laminas are obtained which are slowly soluble in water. These laminas were treated in the same way as the crude ferric citrate, but the manganese content increased slightly rather than decreased. A citrate solution supplied by H. L. Keil showed much less manganese. This had been made from iron wire precipitated as ferric hydroxide, with recrystallized citric acid. The solution was copper-free and contained 1.21 mg. of iron per cc. Five cubic centimeters of this were evaporated to dryness in a double crucible with a few drops of nitric acid, then heated to redness and treated in the same way as before. The solution was found to contain 0.0002 mg. of manganese per cc. The ferric citrate results are shown in Table 111: TABLE111. MANGANESE IN FERRIC CITRATE MANQANESE ADDEDAS SOLUTION B Mo. 0
0.002
+
MANQANESE FOUNDIN 50 cc. SOLUTION 5 cc. Hap01 0.1 gr,am 0.1.$ram 5 90. crude iron precipitated Keil:s Water citrate iron citrate aolution MQ Mg. Mo Mo. 0.002 0.100 0.110 0.003 ... 0.005
.
...
.
...
BENZIDINE METHOD The benzidine reaction with manganese was investigated by Feigl (d), who used it as a colorimetric method for the detection of the presence of that element. Feigl showed that the intense blue color was also given by other oxidizing elements-e. g., titanium, cerium, and bromine. When manganous sulfate was boiled with alkali and filtered, and the precipitate on the filter touched with a drop of benzidine in acetic solution, the blue color which appeared was usually stable, although with small amounts it soon faded. The reaction was sensitive under the best conditions to 1 in 125 million, and was explained as the result of an autoxidation in which the manganese participated. The reaction was adapted to microtechnic ( 3 ) . Olszewski (4) applied the method to the qualitative determination of manganese in drinking water, and preferred an acetic acid solution of the benzidine to a hydrochloric solution. He did not advise the reaction as a quantitative method, but recommended instead the persulfate oxidation. Olszewski also investigated the reaction of free chlorine with benzidine chloride and used the blue color produced in solution for a semiquantitative determination of the chlorine in drinking water ( 5 ) . He showed that this reaction was influenced by the presence of carbonates and organic matter, by the quantity of the benzidine chloride added, and to some extent by the pH. He states that in a solution containing free chlorine and small amounts of manganese, the bluegreen color due to the manganese can be avoided by shaking the solution a t once, to prevent the acid in the reagent from reacting with the manganese. Stratton, Ficklen, and Hough (8) find that if benzidine chloride is added to a very dilute soIution containing the permanganate ion, a brilliant blue-green color is produced which is more favorable for colorimetric estimation than the natural color of the permanganate ion. They recommend 2.3 grams of benzidine base in 100 cc. of 5 per cent hydrochloric acid. One drop of this reagent, added to 100 cc. of a solution containing from 0.001 to 0.0001 mg. of permanganate ion, gives a blue-green color which can be matched against a known standard. I n contrast to the remarkable permanency of the permanganate color produced by potassium periodate, the blue fades rapidly; Stratton therefore suggested a permanent standard made from copper sulfate and picric acid. I n solutions containing small amounts of manganese, the speed with which the color fades made it difficult to obtain quantitative results. The reaction was therefore examined further. Commercial benzidine base was recrystallized
July 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
from water or benzene and dissolved to saturation in different concentrations of hydrochloric, nitric, acetic, citric, and other acids. When manganese sulfate was boiled with sodium hydroxide as described by Feigl, the precipitate after filtering gave a blue color with a few drops of all the benzidine solutions, except with high acidities. A solution of commercial benzidine hydrochloride (powder) in water produced a blue which in general was more lasting than one from a saturated solution of benzidine in 5 per cent acetic acid. With very small amounts of manganese, the color from any combination faded.
243
gradually. At increasin dilution the color produced was successively lighter and faded more quickly. An attempt was made to stabilize the intense blue color of the higher concentrations by shaking with various organic solvents, such as ether, chloroform, etc., but without success. However, a t a concentration of 1 to 100,000, the addition of a salt to the solution, either before or just after the benzidine chloride was added, caused the formation of a deep blue flocculent precipitate. This precipitate was quite stable when left in the solution or when filtered out. At 1to 1million (0.005 mg. of manganese) no precipitate was formed, but the blue of the solution was the same color as the precipitate a t the higher concentration, and this blue was also stable for many hours. Potassium nitrate was particularly good, but ammonium chloride, magnesium chloride, ammonium nitrate, and others produced the same effect. More salt was needed a t the higher dilution-about 0.5 gram in 5 cc. At still higher dilutions the addition of the salt failed to prevent the fading, and when the precipitate from the 1 to 100,000 was diluted to 1 to 20 million the color disappeared in a few minutes. Unless some method can be found to prevent this quick change of color in the dilute solutions, the benzidine hydrochloride or acetate test for manganese, while useful in qualitative analysis (3, d), will be of value only under very special conditions for a quantitative determination.
A 50-cc. solution containing 0.025 mg. of manganese as manganese sulfate (approximately 1 in 2 million) was boiled with 2.5 cc. of 0.1 N sodium hydroxide. The brown precipitate on the filter, with one dro of the benzidine acetate solution, showed a strong blue which Zded only after several hours; with one dro of the chloride the blue was similar in color, but there was no Fading even after several days. When the amount of rnanganese was reduced t o 0.0025 mg. before boiling with the alkali, no brown precipitate could be seen on the filter paper, but both acetate and chloride produced traces of blue which faded gradually. The blue from the acetate solution seemed somewhat the brighter. The precipitate was also obtained by treating 100 cc. of a solution containing 0.3 gram of recrystallized potassium permanganate with 33.5 cc. of a diluted (1to 1) 3 per cent hydrogen peroxide. The colloidal solution of manganese dioxide so produced was stable and could be diluted without causing immediate precipitation. Five cubic centimeters of this colloidal solution before dilution, when passed through a filter paper, left sufficient SUMMARY precipitate on the filter t o give a blue color with one drop of the benzidine acetate or chloride. On diluting, 5 cc. containing Oxidation of manganese in mixed salt solutions, with approximately 0.0016 mg. of manganese failed to give any color on the filter paper; when this amount was boiled with 1 cc. of potassium periodate in phosphoric acid, has given good 0.1 N sodium hydroxide, cooled, and filtered, the filter paper colorimetric comparisons; a difference of 0.001 mg, of again showed blue with a drop of the acetate. This is about 1 manganese in 50-cc. Nessler tubes can be observed. in 3 million. The same quantity of manganese, when diluted The blue color formed by manganese with benzidine, to 1 in 100 million or 1 in 200 million before heating with sodium hydroxide, gave a similar color on the filter paper. The limiting recommended by Feigl and Olszewski for qualitative analysis factor in this case seemed to be the quantity of manganese and of manganese, and as a quantitative method by Stratton, not the dilution. The test failed with 0.0016 mg. of manganese Ficklen, and Hough, has been investigated. The deep blue as manganese sulfate when the solution was boiled with alkali color can be stabilized both as a precipitate and in solution at dilutions from 1 in 100 to 1 in 200 million, although the same amount gave a clear blue coloration at 1 in 3 million. Evidently with a concentration around 1 in 1 million, with the amount it is the dilution factor which causes the failure. Any adaptation of manganese from a few hundredths to thousandths of a to a quantitative determination by this method did not seem milligram. At very dilute concentrations the color is much feasible. Solutions of manganese sulfate, boiled with sodium hydroxide lighter and fades rapidly. No way was found to prevent and made neutral to methyl orange but not filtered, showed the this fading, and the loss of color interferes greatly with the blue color with both the chloride and acetate of benzidine; the use of the method for quantitative determinations. color was stable only if a precipitate formed. With 0.025 mg. LITERATURE CITED of manganese at a concentration of 1 in 200,000, with one drop of the saturated benzidine hydrochloride, a bright blue preBartow and Thompson, Proc. Iowa Acad. Sci., 36, 245-50 (1929). cipitate was produced which was comparatively stable. At a Feigl, Chem.-Ztg., 44, 689-90 (1920). concentration of 1 in 400,000 there was a fine pale blue preFeigl, Mikrochemie, 1, 74-8 (1923). ci itate formed which was stable for some hours. At higher Olszewaki, Chem.-Ztg., 47, 273-4 (1923). difutions a light blue color appeared in the solution but began Olszewski, Ibid., 47, 649-50 (1923) ; Pharm. Zentralhalle, 68, to fade at once. A change in the pH value of the solution did 733-5 (1927). not $top the disappearance of the color. Richards, Analyst, 55, 554-60 (1930). Solutions of potassium permanganate ave almost the same Skinner and Peterson, J. Biol. Chem., 88, 347-51 (1930). results. At a dilution of 1 to 100,000 for the manganese, a Stratton, Fioklen, and Hough, IND.ENG.CHEM.,Anal. Ed., 4, solution containing 0.05 mg. gave the blue color with 1 drop of 2 (1932). the benzidine hydrochloride. This gradually faded and a slight Willard and Greathouse, J.Am. Chem. Soc., 39,2366-77 (1917). brown precipitate was produced. With 0.005 mg. of manganese at a dilution of 1 t o 1 million the deep blue color faded RECEIVEDSeptember 26, 1932.
FORGERIES DETECTED BY CHBMICAL TEST. Siegfried Turkel, research chemist for the police administration of Vienna, and associates have recently revealed chemical methods of detecting writing ink in places where it does not belong. The Vienna experts observed that all writing inks contain chlorides, but in widely variant quantity. The chloride slowly spreads in paper, thoagh being colorless is invkible to the human eye. Doctor Turkel, by a simple chemical reaction, replaces the chloride in a document by metallic silver, deposited as a black image like that of a kodak print. In the same operation he bleaches out the normal ink dye. The document, photographed to yield a
“chloride print,” takes on a new appearance, depending on its age: one hour old, clear black writing; one day, clear but with broadened lines; four days, margin of pen stroke hazy; ten days, quite fuzzy; sixty days, small loops in letters filled u six months, small writing illegible; one to two years, entirely iliiible. With certain inks the chloride spreads so effectively that the actual pen-stroke line is largely freed of that substance. In such a case white lines will be seen on a fuzzy black background when the silver treatment is given. The tell-tale spreading of chloride is caused by the slight condensation of moisture from the air upon the paper fibers.
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