Methyl purple solution

Replace the inlet hose of the flask with a rubber bulb containing water (panel b). Insert the glass outlet tube of the flask into a beaker of indicator such as slightly alkaline, commercial methyl purple solution, which is green above pH 5.4 and purple below pH 4.8. Squirt 1 mL of water fiom the rubber bulb into the flask to create a vacuum and draw a colorful fountain of indicator solution into the flask (Color Plate 3). 

Chondrofoline, CgjHgjOjNj, 2H2O, crystallises from methyl alcohol in triangular plates, m.p. about 135°, [ajffg — 280-6° (dry base N/10. HCl). It is a phenolic base, contains three methoxyl groups, does not give a Millon reaction, but in methyl alcoholic solution gives a faint, pink-purple colour with ferric chloride. The nitrate forms needles m.p. 225° (dec.). 

Add 2.54 g. (0.02 mole) of 2-[(methylamino)methyl]piperidine (obtained by reduction of 2-[(methylamino)methyl]pyridine with sodium and alcohol ) to 10 ml. (0.01 mole) of nickel nitrite. The bluish-purple solution is concentrated and left overnight. The resulting blue crystals are recrystallized in absolute ethanol, filtered, washed with cold ethanol and dry ether, and dried in vacuo. The compound melts at 223-225°. Yield is 2.43 g. (59%). Anal. Calcd. for C14H32NiN604 C, 41.29 H, 7.92 N, 20.64. Found C, 41.23 H, 7.66 N, 21.24. 

Accurately weigh 1-2 g of sample and make up a sample solution of 100 ml. Mix 20 ml of the sample solution with 2 drops of methyl purple indicator and 50 ml of freshly prepared 1 Ig 1 periodic acid solution. Shake well. 

Add 100 ml of distilled water and 3 drops of methyl purple indicator to the sample solution and the blank. Titrate with 0.1 N NaOH to neutral pH. 

Note that between the first and second end points, a gradual decrease in pH due to the HC03"/C02 buffer system will occur. This will give a poor visual end point, unless the buffer couple is destroyed. In practice, the visual titration used for standardization is continued until the methyl purple end point is reached, at which time the solution is gently boiled to remove the CO2, leaving only the remaining HCOs", which is then titrated to completion (see Chapter 8 for a more detailed discussion). 

The acid phase was divided into several portions that were used to make the following measurements. The total acidity (reported as percent sulfuric acid) was determined by titration with a standard sodium hydroxide solution using a methyl purple indicator. Total sulfuric acid was determined in a second sample in a similar manner after the nitric acid and lower oxides of nitrogen were expelled by evaporation on a steam bath. Lower oxides of nitrogen were determined by a redox method. Total nitric acid was determined by difference. 

Well prepared purple solutions have a homogeneity scarcely less than that of the crystalloidal dyestuffs. The behavior toward mercury is another evidence of the correctness of the view upheld by Berzelius. Metallic mercury dissolves gold very easily, while the gold in purple is not attacked by mercury. The electrolysis of purple solutions resembles closely that of a complex, or that of a dyestuff dissolved by an electrolyte, such as Methyl Orange. Just as in the case of the last salt dimethylamidoazobenzolsulfonic acid is deposited on the anode in crystalline form, so is purpuric acid precipitated on the anode in form of a gel. 

Transfer a 25 ml aliquot of the standard ammonium chloride solution (1 ml = 10 pg N) into the distillation tube of the nitrogen apparatus and 10 ml boric acid solution (1 %) and 2 drops methyl purple (bromo-cresol green/methyl red) solution (0.1 %) into the receiving flask the tip of the outlet should be just above the solution in the receiving flask. 

Quantitatively transfer the absorber contents to a 500-mL Erlenmeyer flask, using ion-free water for rinsing. Add 2 drops of Fleisher s methyl purple indicator solution to this solution and titrate to a faint green end point with 0.03 M NaOH solution (Note 12). Add 1 mL more of the 0.03 M NaOH solution to the solution and reduce the volume to 2 to 3 mL by evaporation on a hot plate in sulfate-free environment. (Warning—see Note 11) DO NOT BOIL DRY. Cool the solution to room temperature and measure its volume in a 10-mL graduate (Note 12). Adjust the volume to 3.0 mL by adding ion-free water. 

Although litmus paper, cabbage juice, and phenolphthalein can indicate whether a substance is acidic or basic, they have limitations in that they cannot determine an exact pH. To do this, an acid-base indicator called universal indicator can be used. Universal indicator is actually a mixture of several different acid-base indicators (usually phenolphthalein, methyl red, bromthymol blue, and thymol blue). This mixture produces a wide range of colors to indicate different pHs. Under very acidic conditions, universal indicator is red. It turns orange and then yellow between the pHs of 3 to 6. It is green at neutral pH and turns greenish-blue as a solution becomes more alkaline. In very basic conditions, universal indicator turns a dark purple color. 

Bromcresol green, 0.1%, and methyl red, 0.01%, each in 96% ethanol mix in a volume ratio of 1 2. [A stronger methyl red stock solution (2) is difficult to prepare.] The working solution, which is stable indefinitely, is made by adding 12 ml of the mixed indicator per 100 ml of the boric acid. Only exceptionally need adjustment to the desired reddish-purple color be performed, by adding drops of acid or alkali. 

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