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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. (5)
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., 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.
T .
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 an 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.
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MICROMETER
OPAL
OF UPPERPART OF MICROPHOTOMETER FIGURE 2. DETAILS
The details of the upper part of the instrument are shown in Figure 2. There is nothing critical about the dimensions; actually, the distance from the axis of the hinge to the micrometer is about 5.5 inches (140 mm.). The wood pieces on which the optical parts are mounted are drilled to take the microscope tube and the 0.375-inch (10-mm.) rod supporting the slit. A longitudinal saw cut is then made, so that transverse bolts can hold the wood tightly around the microscope tube and the rod. The micrometer can be made by cutting off the frame of a 25-cent micrometer caliper. (Almost any screw will do, but it is convenient to have the scale of the micrometer caliper). The prism can be held on with wax or adhesive tape. The slit assembly is mounted on a 0.375-inch (10-mm.) rod which is held in the wood piece as described above. It is convenient to be able to rotate the slit in order to make it parallel to the image of the spectral line. Care should be taken to have the slit short enough so that light will not strike the walls of the tube before reaching the photocell. A small piece of opal glass is placed in front of the photocell in order to spread the light over its surface. The photocell, opal glass, and slit are held together with rubber bands. A hood of black paper may be placed over the slit and prism in order to keep stray light off the slit. The light source for reading galvanometer deflections is a Mazda No. 63, 3-candlepower, 6- to %volt automobile light, mounted horizontally just below a transparent celluloid scale. The scale is clamped between two pieces of wood cut to the proper curvature to keep all parts of the scale at the same distance from the galvanometer. The lamp and scale are so placed that an image of the filament of the lamp is formed on the scale. No attempt has been made to choose the most suitable equipment. However, the parts listed below are satisfactory. Microscope, Bausch & Lomb, Model H, No. 31-21-50-06 (32mm. objective). Mechanical stage, Bausch & Lomb, No. 3159-59. Voltage-regulating transformer, Raytheon, No. VR 107--4; output volts 6 to 7.5. Prism, Bausch & Lomb, No. 31-90-01-016. Slit, Gaertner, KO.M710 (a slit from a spectrometer can be used). Photocell, General Electric, S o . 4,120,833 G1. Galvanometer, Leeds & Northrup, Type R, No. 2500-f. Sensitivity ampere per mm. Galvanometer scale, Gaertner, No. M675.
Operation In measuring a spectrogram, the photographic plate is clipped on the mechanical stage and focused on the slit. The image of a spectral line is brought near to the slit by adjusting the mechanical stage and the slit is turned until it is parallel to the
image of the line. The image of the line is now centered on the slit roughly by adjusting the mechanical stage and precisely by turning the micrometer screw until the galvanometer deflection is a minimum. The density of the line is computed fromkthis galvanometer deflection and that through the clear plate.
Performance The magnitude of the random fluctuations in the galvanometer deflection was determined by taking 25 readings, one each 10 seconds. This was done several times and it was found that, with a deflection of from 45 to 50 em., the average deviation of a n individual reading from the mean never exceeded 0.1 em., approximately 0.2 per cent of the reading. These fluctuations were probably caused by slight voltage fluctuations which the voltage regulating transformer failed to eliminate. Since the light reaching the photocell strikes different portions of its surface as the micrometer is turned from one position to another, it is necessary to see that this causes no error. Tests show that no appreciable error is introduced by turning the micrometer a half turn either way from its normal position. I n use, it need never be turned more than this. A comparison of the calibration of a neutral-tint wedge made using this instrument with that furnished by the maker shows that the deflections are proportional to the light reaching the photocell for deflections from 4 to 40 cm., the range tested. I n order to show what precision may be expected in measuring actual spectral lines, the ratio of the deflections obtained with the instrument set alternately on a line of density 0.36 and on a line of density 0.60 was calculated. The average deviation of a single ratio from the mean of five was 0.5 per cent. The time required to center the image of a spectral line on the slit and to read the galvanometer deflection is approximately one minute. The instrument has been in use intermittently for over a year and, although it is rather slow, it has been found t o yield entirely satisfactory results.
