Vessel semimicro

When volumes of 0 2-2 ml. of solutions are to be analysed using a mercury pool reference electrode, the semimicro-vessel depicted in Fig. 20b can be used. The wide arm of the vessel serves to keep the level of mercury constant in the narrower part, where the solution to be analysed is placed. This vessel is equipped with a side arm for deaeration of the solution. The atmospheric oxygen is removed from the solution to be electrolysed by bubbling an inert gas (nitrogen, hydrogen or carbon dioxide) through the solution. The outlet of the gas is in the upper right section of the vessel and is usually connected to a water seal. 

As a general rule, when working with semimicro quantities, it is best to employ pear shaped vessels (Fig. XII, 1, 1, a) and centrifuge tubes (Fig. XII, 1, 1, 6) as containers for liquids convenient capacities are 6, 10 and 20 or 26 ml. for flasks, and 1, 3, and 6 ml. for centrifuge tubes. Round-bottomed vessels of similar volumes... 

Test for nitrate (andI or nitrite) If ammonium is found, transfer the solution from test 5 with the aid of 0-5 ml water to a semimicro boiling tube (or crucible), add 0 5 ml sodium hydroxide solution and evaporate down to a volume of about 0-2 ml - this treatment completely decomposes the ammonium salt. Transfer the residue to a semimicro test-tube, rinse the vessel with 0-5 ml sodium hydroxide solution and add this to the contents of the test-tube. Then add 10 mg of Devarda s alloy (or of aluminium powder or thin foil), introduce a Pyrex filter tube provided with a loose plug of cotton wool at the lower end and containing a strip of red litmus paper or mercury(I) nitrate paper (Fig. II.29a), and place the assembly in the hot water rack. If frothing occurs, remove the apparatus from the water bath until the vigorous reaction has subsided. 

Note The macro method is not recommended for beginners, due to the danger involved in handling bromine in a separatory funnel over a reaction vessel. The macro experiment is suitable for advanced work and for demonstration by the instructor. The semimicro method involves the use of 3 ml of bromine, and with suitable precautions the experiment can be assigned to beginners. 

Transferring of precipitates in some cases precipitates can be transferred from semimicro test-tubes with a small spatula (two convenient types in nickel or monel metal are shown in Fig. 2.20). This operation is usually difficult, particularly for centrifuge tubes. Indeed, in semimicro analysis it is rarely necessary to transfer actual precipitates from one vessel to another. 

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