Titan Yellow Qualitative Test for Magnesium EDNA BISHOP OTTO AND CARL E. OTTO University of Maine, Orono, 3Iaine
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tions by both the drop and impregnated paper methods. No conditions could be found that did not give the same results whether magnesium was present or not. This is consistent with Eilers’ (3) report that he obtained the test with magnesium-free filter paper.
HE use of titan yellow as a qualitative reagent for mag-
nesium was first proposed by Kolthoff (6). Further studies of qualitative applications and interference were made by Barnes ( I ) , Eilers (3) Eegriwe (d), Kolthoff (7), van Nieiiwenburg (9), and Feigl (4),and i t is reported in Organic Reagents (IO) and in Tables of Reagents ( 5 ) . However, as the information concerning interference is incomplete and contradictory, this investigation was undertaken to make further study of the interferences of various ions with the magnesium-titan yellow reaction. It was first necessary to study the conditions required for the reaction.
Effect of Other Ions The cation solutions were chlorides, nitrates, or sulfates and contained 10 mg. of the respective cation per ml. The anion solutions were sodium or potassium salts, except the oxalate which was ammonium, and contained 0.2 gram of salt per 10 ml. of solution. Tests were made by the Kolthoff and the modified spot plate methods on the solution with and without the addition of amagnesium solution containing 0.1 mg. er ml. For the Kolthoff method, 10 ml. of solution and 2 ml? of magnesium were used; for the spot plate, one drop of each.
Test-Tube Method Kolthoff (6) used 10 ml. of solution, 0.2 ml. of 0.1 per cent aqueous titan yellow solution, and 0.25 t o 0.5 ml. of 4 N sodium hydroxide, and reported the detection of 0.0002 mg. per ml. and an increase in intensity of color with an increase in concentration of magnesium. This was found to be true up to a concentration of 1 mg. of magnesium per ml. Further increase in the concentration of magnesium produced a decrease in the color intensity and, at a concentration of 50 mg. per ml. a white precipitate was formed and the solution remained yellow. On the addition of more sodium hydroxide the precipitate became light orange, then pink. Adding sodium hydroxide slowly from a buret to magnesiumtitan yellow solutions, the color was found to change gradually from yellow to orange to red. The exact point was hard t o determine, but a pH of about 12.5 was necessary to produce the red color. Reversing this change by the addition of acid, the point of color change, again difficult to detect, was around 11.5. Hence, concentrated solutions of magnesium fail to give the test unless an excess of sodium hydroxide is added, as all the hydroxide is used to precipitate the magnesium and the pH is too low to give the red color. Therefore, if a white precipitate and no red color are obtained, either an excess of sodium hydroxide should be added or a dilution made and the test repeated. The dilution is preferable, as sodium hydroxide itself at high concentration gives a brownish-orange color that may be confused with the magnesium color.
Results and Discussion The reaction of titan yellow with magnesium is not specific, but is shown by many cations. As with magnesium, each cation has its characteristic pH of reaction and is further affected by the concentration of the ion and of titan yellow. Silver, mercurous, and mercuric ions react in neutral or slightly acid solution to give a red color with titan yellow if both the cation and titan yellow are of high concentration. Cadmium, calcium, cobalt, copper, lead, lithium, manganese, and nickel react under the conditions used in testing for magnesium. I n the test tube, only the blue copper hydroxide precipitate could be seen, b u t on the spot plate two precipitates, a blue and a red-lavender, were distinctly visible. All these ions gave red or pink precipitates at 10 mg. per ml., none at 0.1 mg. per ml. Copper, cadmium, cobalt, manganese, and nickel reacted at 1 mg. per ml. while calcium, lead, and lithium did not. It is possible t h a t other cations may react under different conditions-for example, barium was found t o react on standing. Some of these that did react were so near the critical pH t h a t slight differences in the pH of initial solutions as well as differences in concentration of solutions used by different investigators may account for some of the contradictory results reported. The intensification of color produced by barium, calcium, and cadmium (6, 7 ) is probably due t o their coprecipitation with magnesium under conditions where they did not precipitate alone. The following ions, because of their color or t h a t of compounds formed with hydroxide, mask the magnesium-titan yellow color-chromium, copper, ferrous, ferric, mercurous, mercuric, silver, and permanganate. With the modified spot plate method, aluminum, lanthanum, and zinc prevent the formation of the magnesiumtitan yellow red color. With the Kolthoff procedure, aluminum, ammonium, antimony, arsenic, lanthanum, stannous, stannic, and zinc prevent the formation of the magnesiumtitan yellow red color. I n each case, however, the p H was below 12.5 which is the minimum necessary t o produce the red color, and the addition of excess concentrated sodium hydroxide did give the red color. This high concentration of sodium hydroxide, however, is not advisable in testing procedures, as i t alone gives a color t h a t can be confused with the magnesium test. Kolthoff (6) attributes the interference of tin and aluminum to their adsorption of magnesium. However, regardless of adsorption, the presence of these ions lowers the pH below t h a t necessary t o produce a red color. I n general, anions had no effect on the reaction, but tartrate and ferricyanide did decrease the sensitivity and permanganate masked the color. The following ions did not interfere with the test by either the Kolthoff or modified spot
Spot Plate Method Feigl (4) placed one drop each of solution, 0.1 per cent titan yellow, and 0.1 N sodium hydroxide on a spot plate and was able to detect 1.5 micrograms of magnesium a t a concentration of 0.03 mg. per ml. Since Kolthoff (6, 7) reported that calcium and barium do not interfere but intensify the color, saturated solutions of calcium and barium hydroxides were substituted for sodium hydroxide. Slightly better color was obtained, barium being preferable, and magnesium was detected a t 0.02 mg. per ml. As with the Kolthoff method, the intensity of color increased to a maximum at 1mg. per ml., then decreased to none at 50 mg. per ml. Further difficulties were encountered when studying interference by this method, as only a slight lowering of pH prevented the formation of the red color and the concentration of titan yellow was so reat that an increase of hydroxide produced a color hard to Astinguish from the magnesium color. Various concentrations were tried, and satisfactory results were obtained by the use of 1 drop each of solution, 0.01 per cent titan yellow, and 2 N sodium hydroxide. I n the absence of magnesium, the color is yellow to taupe and in the presence of magnesium, pink. The difference in color is sufficiently distinct to cause no confusion. Magnesium can be detected a t a concentration of 0.01 mg. per ml., the intensity of color increases continuously with increasing concentration of magnesium through 50 mg. per ml., and much interference is eliminated. Titan yellow solutions are more stable at higher concentrations; so 0.1 per cent solutions, which will keep for a t least 11 months, may be prepared and dilutions made as needed.
Reaction Paper I n Tables of Reagents (6) it is stated that the test may be carried out on reaction paper. No specific directions for its use were found. Tests were made over a wide range of concentra65
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plate method-barium, bismuth, potassium, sodium, strontium, acetate, arsenate, arsenite, borate, bromide, carbonate, chlorate, chloride, chromate, cyanide, cyanate, ferrocyanide, iodide, nitrate, sulfate, sulfite, thiocyanide, and thiosulfate.
Comparison with Other Tests When carried out under similar conditions, the limit of detection of magnesium by titan yellow, pnitrohenzeneazoresorcinol, or p-nitrobenzeneazo-m-naphthol is about the same. There may he a matter of personal preference between a red and a blue color. Titan yellow has the advantage of developing its color rapidly, whereas in dilute solutions the others, especially the resorcinol compound, develop their colors slowly, so that i t is necessary t o wait 5 or 10 minutes. Mehlig and Johnson (8) show that traces of aluminum, barium, calcium, and strontium present through faulty separations will not interfere with the p-nitrobenzeneazoresorcinol test, whereas they do interfere with phosphate. The present work shows that the same is true with titan yellow for traces of barium, calcium, and strontium hut that aluminum will prevent the test. These color tests are quicker, more easily recognized, and require less solution than the phosphate test.
Summary The titan yellow test, although not specific for magnesium, is useful and efficient when properly applied. It is suitable
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for test tube or spot plate hut not reaction paper. An improved spot plate procedure is given. A pH of about 12.5 is necessary for the production of the red color. Interference may he caused by ions which undergo a similar reaction, by ions which because of their color or that of compounds formed with hydroxide mask the magnesium-titan yellow color, or by ions which keep the p H too low. Ions which interfere, however, will be removed in the usual qualitative procedure before testing for magnesium and hence cause no trouble.
Literature Cited
Niiddemah Pu6lishingCo.; 1937.
International Committee of New Analytied Reactions and Reagents, "Tablesof Reagents for Inorganic Analysis", p. 249, Leipuig, Akademische VerliLgsgessllsohaft. 1938. (6) Kolthoff, I. M..Cham. Wsekbfad, 24, 254 (1927); Biochem. Z., (5)
185,334(1927); ~ n o l ~52,430 t. (1927).
(7) Kolthoff, I. M., Mikrochmie, Emich Festschr., 1930, 180; Analyst, 55, 769 (1930). (8) . . Mehliz. J. P.. and Johnson. K. R.. IND.Ewe. CHEM.,And. Ed., (9) Nieuwenburg, C . J. van. Mikrochemia, 9, 199 (1931). (10) "Organic Reagents for Metds and for Certain Acid Radicals". 3rd ed.. London, Hopkin and Williams. 1938.
A Microphotometer for Spectrochemical Analysis EDWARD M. THORNDIKE, Queens Colleee. Flushins. N. Y.
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H E increasing use of quantitative spectrochemical analysis and the accompanying improvements in the reliability of the sources employed have stimulated interest in microphotometers. The instrument companies have improved old models and developed new ones. I n addition, numerous designs have been described in recent literature. This note describes a n instrument which, while not so fast nor so accurate as some, is convenient to use and is satisfactory for much work. It is easily assembled from standard equipment, part of which is probably available in many laboratories; hence, the outlay of time and money required for its construction is small.
Apparatus Figure 1 shows the arrangement. A microscope furnishes a convenient foundation for the instrument. The photographic plate to be measured is mounted on a mechanical stage, thus enabling the observer to bring any spectral line into position quickly and easily. The plate is illuminated by a 32-candlepower singlefilament automobile headlight bulb which receives its power from a voltage-regulating transformer. An enlarged image of the spectral line is directed to a horizontal slit in front of a photocell by a right-angle prism mounted on a strip of metal which is hinged near one end and supported by a micrometer screw at the other. The photocell is connected to a wall galvanometer, the scale of which is mounted over the microscope. This scale is not shown in the photograph. The mechanical stage is easily adapted to hold photographic plates 2 inches (51 mm.) wide. The supports which were designed to hold microscope slides are removed and a plate of 0.125-inch (3-mm.) brass 2.5 X 5 inches (64 X 127 mm.) with a rectangular hole 1.5 X 3.5 inches (38 X 89 mm.) cut in it is screwed on in place of them. The photographic plate (or a piece of 35-mm. motion picture 6lm mounted in a light brass frame) is held on this brass plate with phosphor bronze clips.
