Chemistry, Iowa State College, Ames, Iowa
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. In 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 per
manganate of known normality, or from recrystallized potassium permanganate as suggested by Richards (6‘). In 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 phos
phoric 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 man
ganese 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 oxida
tion, 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 commer
cial 85 per cent phosphoric acid contained from 0.002 to 0.003 mg. in 5 cc. B y recrystallizing this could be elimi
Vol. S, No. 4 nated, but usually a fresh bottle was checked and the correc
tion made.
A dilute solution of the previously mentioned recrystallized salts was tested for its manganese content. This solution as concentrated. Some precipitation which occurred at 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 pre
cipitate, and the oxidation with the potassium periodate was earned 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 the concentrated solution. Six recrystallizations subse
quently removed all trace of manganese from the magnesium sulfate. 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. In 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
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
Addedas crude iron precipitated K eil’s
Solution B W ater citrate iron citrate solution
Mg. Mg. Mg. Mg. Mg,
0 0 .0 0 2 0 .1 0 0 0 .1 1 0 0 .0 0 3
0 .0 0 2 . . . . . . . . . 0 .0 0 5
Be n z i d i n e Me t h o d
The benzidine reaction with manganese was investigated by Feigl (2), 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 ele
termination 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 solu
tion 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 permanganate ion, a brilliant blue-green color is produced which is more favorable for colorimetric estimation than the permanency of the permanganate color produced by potas
sium periodate, the blue fades rapidly; Stratton therefore suggested a permanent standard made from copper sulfate and picric acid.
In 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
from water or benzene and dissolved to saturation in différent 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 fewr drops of all the benzidine solu
tions, 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.
A 50-cc. solution containing 0.025 mg. of manganese as man
ganese 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 drop of the benzidine acetate solution, showed a strong blue which faded only after several hours; with one drop of the chloride the blue was similar in color, but there was no fading even after several days. When the amount of man
ganese was reduced to 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 gradu
ally. 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 per
manganate with 33.5 cc. of a diluted (1 to 1) 3 per cent hydrogen peroxide. The colloidal solution of manganese dioxide so pro
duced 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 precipitate on the filter to give a blue color with one drop of the hydroxide, gave a similar color on the filter paper. The limiting factor in this case seemed to be the quantity of manganese and not the dilution. The test failed with 0.0016 mg. of manganese as manganese sulfate when the solution was boiled with alkali 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 it is the dilution factor which causes the failure. Any adaptation to a quantitative determination by this method did not seem feasible.
Solutions of manganese sulfate, boiled with sodium hydroxide and made neutral to methyl orange but not filtered, showed the
Solutions of potassium permanganate gave almost the same results. At a dilution of 1 to 100,000 for the manganese, a solution containing 0.05 mg. gave the blue color with 1 drop of the benzidine hydrochloride. This gradually faded and a slight brown precipitate was produced. With 0.005 mg. of man
ganese at a dilution of 1 to 1 million the deep blue color faded
243 gradually. A t increasing dilution the color produced was successively fighter and faded more quickly.
An attempt wras 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, at a concentration of 1 to 100,000, the addition of a salt to the solution, either before or just after the benzi
dine 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 1 to 1 million (0.005 mg. of manganese) no precipitate was formed, but the blue of the solution was the same color as the pre
cipitate at the higher concentration, and this blue was also stable for many hours. Potassium nitrate was particularly good, but ammonium chloride, magnesium chloride, am
monium nitrate, and others produced the same effect. More salt was needed at the higher dilution— about 0.5 gram in
tative analysis (3, 4), will be of value only under very special conditions for a quantitative determination. recommended by Feigl and Olszewski for qualitative analysis of manganese, and as a quantitative method by Stratton, use of the method for quantitative determinations.
Li t e r a t u r e Ci t e d associates have recently revealed chemical methods of detecting writing ink in places w’here 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, though being colorless is invisible to the human eye. Doctor Türkel, 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 up; six months, small writing illegible; one to two years, entirely illegible.
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.