Weighing Bottles

Take approximately 0 5 g. of the finely powdered mannitol, using either a weighing bottle, or else weighing directly in the flask A as before. Heat on the water-bath for 60 minutes. During the titration with the NaOH solution, a fine precipitation of the hexacetyl-mannitol may occur, but will not affect the titration. The results are excellent. 

Continue precisely as for triacetin, but since hexacetyl-mannitol (p. 142) is a crystalline compound, weigh out i o-i 2 g. of the finely powdered substance, either from a weighing-bottle, or by direct weighing in the flask A. The results are excellent. 

Hydrolysis (or saponification) of n-butyl acetate. Boil 4-5 g. of n-butyl acetate (Section 111,95) with 50 ml. of 10 per cent, sodium hydroxide solution under reflux until the odour of the ester can no longer be detected (about 1 hour). Set the condenser for downward distiUation and coUect the first 10 ml. of distillate. Saturate it with potassium carbonate, aUow to stand for 5 minutes, and withdraw all the Uquid into a small pipette or dropper pipette. AUow the lower layer of carbonate solution to run slowly into a test-tube, and place the upper layer into a small test-tube or weighing bottle. Dry the alcohol with about one quarter of its buUr of anhydrous potassium carbonate. Remove the alcohol with a dropper pipette and divide it into two parts use one portion for the determination of the b.p. by the Siwoloboff method (Section 11,12) and convert the other portion into the 3 5-dinitrobenzoate (Section III, 27) and determine the m.p. 

The most convenient form of weighing bottle is one fitted with an external cap and made of glass, polythene or polycarbonate. A weighing bottle with an internally fitting stopper is not recommended there is always the danger that small particles may lodge at the upper end of the bottle and be lost when the stopper is pressed into place. 

Woodward and Redman 6c have described a specially designed weighing bottle which will accommodate a small platinum crucible when a substance has been ignited in the crucible, the crucible is transferred to the weighing bottle and subsequently weighed in this. This device obviates the need for a desiccator. 

If the substance to be weighed is a liquid, it is placed in a weighing bottle fitted with a cap carrying a dropping tube. 

If necessary refer to Section 3.5 dealing with the operation of a chemical balance, and to Sections 3.25 and 3.22 which are concerned with the use of weighing bottles and desiccators respectively. 

The material, prepared as above, is usually transferred to a weighing bottle which is stoppered and stored in a desiccator. Samples of appropriate size are withdrawn from the weighing bottle as required, the bottle being weighed before and after the withdrawal, so that the weight of substance is obtained by difference. 

Procedure. Weigh out accurately from a weighing bottle about 0.2 g of the pure sodium carbonate into a 250 mL conical flask (Note 1), dissolve it in 50-75 mL of water, and add 2 drops of methyl orange indicator (Note 2) or preferably of methyl orange-indigo carmine indicator (Section 10.9), which gives a very much more satisfactory end point (Note 3). Rinse a clean burette three times with 5 mL portions of the acid fill the burette to a point 2-3 cm above the zero mark and open the stopcock momentarily, in order to fill the jet with liquid. Examine the jet to see that no air bubbles are enclosed. If there are, more liquid must be run out until the jet is completely filled. Re-fill, if necessary, to bring the level above the zero mark then slowly run out the liquid until the level is between the 0.0 and 0.5 mL marks. Read the position of the meniscus to 0.01 mL (Section 3.12). 

Notes. (1) For elementary students, an approximately 0.05 M solution of sodium carbonate may be prepared by weighing out accurately about 1.3 g of pure sodium carbonate in a weighing bottle or in a small beaker, transferring it to a 250 mL graduated flask, dissolving it in water (Section 3.28), and making up to the mark. The flask is well shaken, then 25.00 mL portions are withdrawn with a pipette and titrated with the acid as described above. Individual titrations should not differ by more than 0.1 mL. Record the results as in Section 10.30. 

Procedure. Weigh out accurately from a weighing bottle 0.4-0.5g of pure sodium tetraborate into a 250 mL conical flask (Note 1), dissolve it in about 50 mL of water and add a few drops of methyl red. Titrate with the hydrochloric acid contained in a burette (for details, see under A) until the colour changes to pink (Note 2). Repeat the titration with two other portions. Calculate the... 

Procedure. Weigh an empty stoppered weighing bottle, add about 2 g of syrupy phosphoric(V) acid and re-weigh. Transfer the acid quantitatively to a 250 mL graduated flask, and then proceed as detailed for sulphuric acid, but using the phenolphthalein-1-naphtholphthalein mixed indicator. 

Method A. Weigh out accurately about 3.5 g of potassium cyanide from a glass-stoppered weighing bottle, dissolve it in water and make up to 250 mL in a graduated flask. Shake well. Transfer 25.0 mL of this solution by means of a burette and NOT a pipette to a 250 mL conical flask, add 75 mL water, 5-6 mL 6M ammonia solution, and 2 mL 10 per cent potassium iodide solution. Place the flask on a sheet of black paper, and titrate with standard 0.1 M silver nitrate. Add the silver nitrate solution dropwise as soon as the yellow colour of silver iodide shows any signs of persisting. When one drop produces a permanent turbidity, the end-point has been reached. 

Procedure. To 100 mL of distilled water, add 5mL of concentrated sulphuric acid, cool and then add 5 g of pure boric acid when this has dissolved cool the mixture in ice. Transfer gradually from a weighing bottle about 0.5 g (accurately weighed) of the sodium peroxide sample (handle with care) to the well-stirred, ice-cold solution. When the addition is complete, transfer the solution to a 250 mL graduated flask, make up to the mark, and then titrate 50 mL portions of the solution with standard 0.02 JVf permanganate solution. 

Prepare a stock solution of nitrobenzene by dissolving about 0.25 g (accurately weighed in a weighing bottle) in 50 mL of 95 per cent ethanol, and adding this to 450 mL of the buffer solution contained in a 500 mL graduated flask. Rinse the weighing bottle with two successive 5mL portions of 95 per cent ethanol and add each rinsing to the 500 mL flask finally make the flask up to the mark with buffer solution. 

Procedure. Weigh out accurately about 5 g of the oil sample, dissolve in a small volume of white spirit and transfer to a 50 mL graduated flask use the same solvent to wash out the weighing bottle and finally to make up the solution to the mark. 

The drying takes place in a drying oven, an oven that can be set to temperatures up to 100°C and beyond. The weighing bottle, with sample contained, is usually placed in the oven, with lid ajar, in a small beaker, and the beaker covered with a watch glass, as in Figure 3.6. 

FIGURE 3.6 A weighing bottle with sample, lid ajar, in small beaker, ready to be placed in a drying oven for the purpose of drying the contained sample. 

Handling a weighing bottle with a rolled-up piece of paper towel to avoid fingerprints. 

While dispensing and weighing a small amount of the dried sample contained in the weighing bottle, it is prudent to avoid contacting the sample with laboratory utensils or weighing paper in order to avoid loss of a portion of the sample, which might adhere to the utensil or paper. In such instances, weighing by difference is recommended. 

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