Volumetric determination after distillation

2 Volumetric determination after distillation General remarks 

On taking the sample, convert the free, dissolved carbon dioxide and hydrogen carbonates into carbonates by adding sodium hydroxide. In closed apparatus in the laboratory, add acid to the weakly alkaline sample to 

First neutralize the excess sodium hydroxide solution by adding acid until the transition point of the phenolphthalein/naphtholphthalein mixed indicator is reached (pH 8.6) as carbonate is converted into hydrogen carbonate. After adding the bromocresol green/methyl red mixed indicator (pH 4.3), 

360 mg/1) of total carbon dioxide. For a sample volume of 23 ml, this 

Sulphides and H2S have an interfering effect. Before the acid is added, 

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If the water is coloured or turbid and does not become clear after filtration, the boron may be separated off as boric acid methyl ester (see "Volumetric determination following distillation. Section 3.5.1.3). 

The weighing methods belong to the oldest and most frequently used techniques in the past. They are based on the extraction of oil products from a sample of water with a solvent and the subsequent determination of the weight of extracted substances after distillation [13-1-5, 66]. The volumetric methods are based on distillation of the volatile oil substances from water and measuring their volume after condensation [67, 68]. 

Fig. 9.10. Apparatus for the determination of molecular weights of air-sensitive compounds by the isothermal distillation technique. Temperature fluctuations of the two solvents are minimized in this illustration by a Dewar filled with water. This apparatus is used in the following manner. In a dry box the sample is placed in a tared tube through sidearm A, the sidearm is then sealed off, the tube and remnant of the sidearm are weighed, and the tube is attached to the apparatus by glassblowing at B. A weighed portion of a standard is introduced into the other bulb, and the filling tube is sealed off. After evacuation, opening of the break-seal, and reevacuation, a measured portion of solvent is distilled into both arms of the apparatus. The process of equilibration is followed by periodic removal and measurement of the solvent from one arm. The solvent may be measured volumetricly in the liquid or gas stales, or by weight.

B) Qualitative Determination. Exactly 25 c.c. of the distilled spirit are boiled with about 30 c.c. of water, 0 5 c.c. of syrupy phosphoric acid and 0-5 c.c. of aniline under a reflux condenser for half an hour. After cooling,the bulk of the liquid is distilled and the distillate collected in a 50 c.c. measuring flask and made up to volume with water. The ketones are then determined (see section 2), 20 c.c. or 4 c.c. being used according as the gravimetric or volumetric method is employed. 

Adjust the pH of the sample to between 11.5 and 12. To 20 mL of sample in a 50 mL volumetric flask, add 5 mL of phosphate buffer and 2 drops of EDTA solution. Add 2 mL chloramine-T solution and stir well. Test for residual chlorine using Kl-starch paper. If required, add more chloamine-T so that there is enough residual chlorine in the solution. Allow the solution to stand for exactly 3 min. After this, add 5 mL pyridine-barbituric acid and mix well. Dilute to 50 mL mark. Let the solution stand for 8 min. Measure the absorbance at 578 nm in a 1 cm cell against distilled water. Determine the GNL concentration from the calibration curve. 

Pentose sugars and pentosans may be quantitatively estimated by conversion into furfural by distillation with hydrochloric acid. The amount of furfural is determined gravimetrically after precipitation with phloroglu-cinol, barbituric acid, or thiobarbituric acid, or volumetrically by titration with bromine or phenylhydrazine. Approximately theoretical yields of furfural are obtained if the furfural is removed rapidly from the reaction mixture by steam distillation ... 

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