Methylene blue, solution preparation

Match the color developed in one of the sulfide standard aliquots (after adding the reagents) to that of methylene blue solution, either by diluting the latter, or by adding more dye. For example, select the 1 ppm sulfide standard and match the color developed in it with the color of the prepared methylene blue solution I. 

Fabrication Process of Dichromated Gelatin Sensitized by Methylene Blue 1 Preparation of dye-sensitized solution ... 

Procedure. A drop of the weakly acid test solution, in a micro test tube, is mixed with four drops of 0.0012 % aqueous methylene blue solution and about 20-30 mg of hydrazine sulfate. A parallel test is prepared with the same mixture and a drop of water. Both test tubes are placed in boiling water. With quantities exceeding 0.5 y molybdenum, the color of the methylene blue is discharged within 3 minutes, whereas the blank remains unchanged. When smaller amounts of molybdenum are present, the fading of the blue is distinctly evident after 10 minutes heating and on comparison with the blank. 

Several variations of the chemical method are in use. In the one described below, a freshly prepared Fehling s solution is standardised by titrating it directly against a standard solution of pure anhydrous glucose when the end-point is reached, I. e., when the cupric salt in the Fehling s solution is completely reduced to cuprous oxide, the supernatant solution becomes completely decolorised. Some difficulty is often experienced at first in determining the end-point of the reaction, but with practice accurate results can be obtained. The titrations should be performed in daylight whenever possible, unless a Special indicator is used (see under Methylene-blue, p. 463). 

A solution of buffered methylene blue (pH 3.6) may be used as a vital stain for the examination of fresh specimens for protozoa. The wet mount is prepared as described above, with buffered methylene blue substituted as diluent and 5 to 10 min allowed for the dye to become incorporated in the organisms before examination. Organisms become overstained in 20 to 30 min. 

Measurements show some variation depending upon the staining solution used and the method of application. In dried and fixed smears, the cell wall and slime layer do not stain with weakly staining dyes such as methylene blue but do stain with the intensely staining pararosaniline, new fuchsin, crystal violet, and methyl violet. The great majority of bacteria have been measured in fixed and stained preparations. In some instances dried, negatively stained smears have been used. Therefore, the method employed should be specified when measurements of bacteria are reported otherwise the results will be of doubtful v alue. 

Experiment 12. Schardinger s Reaction.—Of two 25 c.c. portions of fresh milk one is boiled for a short time and cooled then 1 C.c. of the formaldehyde (prepared in an earlier experiment) and a few drops of an aqueous solution of methylene blue are added to each portion. When the two samples are now warmed to about 50° the dye in the unboiled milk is very rapidly decolorised and the same happens to further amounts added. In the boiled milk the colour remains unaltered. 

Reduction of diaryltellurium dichlorides with samarium diiodide (typical procedure). Diaryl tellurium dichloride (1 mmol) was added to the deep blue solution of Sml2 (2.2 mmol) in THF (22 mL) at room temperature under nitrogen with stirring. The deep blue colour of the solution disappeared immediately and became yellow. The resulting solution was stirred at room temperature under nitrogen for 30 min. To the solution was added dilute hydrochloric acid, and the mixture was extracted with ether. The ethereal solution was washed with brine and dried over MgS04. The solvent was evaporated in vacuo, and the residue was purified by preparative TLC on silica gel (petroleum ether-methylene dichloride as eluent). 

The sodium salt of these acids may be used to prepare aqueous solutions of indicators. Other examples of redox indicators include starch, potassium thiocyanate, methylene blue, and phenosafranine. Some selected general indicators in redox titrations are listed in Table 1.6.3. The properties of starch as an indicator in iodometric titration are discussed in the following section. 

Preparation of a standard calibration curve. A series of calibration standards are prepared from LAS. The concentrations of the calibration standards should range from 10 to 150 pg/L, which corresponds to a mass of 1 to 15 pg LAS, respectively, in 100 mL solution. Each LAS standard solution in 100 mL quantity should be treated with methylene blue reagent. 

Note A masked phenolphthalein indicator may be used with off-color materials. Prepare the indicator by dissolving 1.6 g of phenolphthalein and 2.7 g of methylene blue in 500 mL of alcohol, and adjust the pH with alcoholic alkali solution so that the greenish blue color is faintly tinged with purple. The color change, when going from acid to alkali, is from green to purple. 

Before the preparation of methylene blue is started, solutions of the necessary reagents are prepared so that the materials can be added rapidly and at the correct temperature. 

The moist, crude methylene blue, as obtained in the above preparation, is mixed with 50 cc. water and 20 grams of 62 per cent nitric acid (40° Be) and 5 grams of sodium nitrite, dissolved in the minimum amount of water, is added at 25°C. The temperature is then raised to 50° with continuous stirring, and held at this point for 2 hours. The mixture is then diluted with 200 grams of saturated salt solution and filtered after 12 hours. The crude dye is redissolved in 1 liter water at 60° (not higher), the solution is filtered, and the dye is reprecipitated by the addition of 150 grams of salt and 50 grams of 50 per cent zinc chloride... 

Methylene blue, a colorant used in preparations for external use, imdergoes photooxidation (11) and photoreduction in the presence of ethylenediamine tetraacetic acid (EDTA) (12). The photodecomposition of vanillin solutions in ethanol is accompanied by the formation of a yellow color and the development of a slightly bitter taste (13). 

It is prepared by oxidation of dimethylparaphenylenediamine-thiosulphonic acid (see Methylene Blue) with ethyl-methylaniline, and boiling the resulting insoluble green compound with zinc-chloride solution. A leuco-compound is formed, and is converted into dyestuff by oxidation. The commercial product forms a reddish-brown powder. 

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