Elementary operations carrying out

Balances and weighing

Chemical, pharmaceutical and analytical balances are usually used in chemical laboratories.

Chemical and pharmaceutical balances allow to determine the mass to a precision of 0,01 g and analytical one – that of 0,0001 g. Balances are equipped with the set of weights called the set of fractional weights. Weights from the set are taken with pincers.

For weighing the object put it on the left scale pan and put the weight on the right one. Chemical balances have the device called arrester supporting the scale pans in the non-working state. To bring the balances in the working state the arrester has to be put down. Take and put objects and fractional weights only when the arrester is lowered (non-working state). Weighing can be considered as completed if the deviation of pointer to the right and to the left from medium line becomes equal. After weighing put immediately the fractional weights back to the set.

Keep the following rules when weighing:

1. Do not put hot or wet objects on the scale pans. Work with liquids; avoid spilling of liquid on balances and fractional weights.

2. Do not put the substance being weighed directly on the scale pans, use special glassware (clock glass or weighing bottle) or filter paper.

3. Take fractional weights only with pincers and do not use the weights of another set of fractional weights.

4. After weighing put balances in the non-working state and do not leave anything on the balances.

Measurement of liquid reagents volume

Measurement of liquid reagents volume is carried out with the measuring vessels. Measuring graduates, measuring tubes, and volumetric flasks are used for the relatively rough measurement of liquid volume.

Burettes allow to measure the volume of flowing out liquid or the evolving gas to a precision of 0,1%. Volumetric flasks are used for preparation of solutions with necessary concentration or for diluting of solution to given extent. The precision of volume measurement is 0,12 - 0,20 %.

Measuring (Mohr’s pipettes) and transfer pipettes are used in order to take precisely the volume. Measuring pipettes can be used to measure the total volume or its part.

Heating

Heating is often used in laboratories at conducting the chemical reactions; incineration, dehydration, melting of solid substances; evaporation and boiling of solutions.

For heating different devices are used: electrical plates, drying chambers, electrical furnaces (muffles), spirit burners, bathes. Depending on the substance properties and the purpose of heating, the different methods are used.

1. When substances are heated in test tubes, porcelain and metal crucibles for short time, and the open fire is used. Heating is done in the upper part of flame, fixing the test tube in the wooden holder. Liquids in the thermally sound glass or porcelain ware are heated on spirit burners through the asbestos net.

2. If the long-term heating at high temperature is necessary (incineration, melting etc.) the electrical plates (temperature up to 300⁰C) and electrical furnaces (temperature 300 – 1000⁰C) are used. Incinerated substances are put to the porcelain, metal or alundum crucibles or boats.

3. If the heating of solutions (evaporating) is to be done in the narrow temperature interval, the bathes (water, sand, glycerine, oil or silicon) filled with liquid (sand) to 2/3 its volume are used. The vapors of boiling water produce heating; if glycerin or oil is used, the liquid produces the heating itself. Sand bath is used for long heating. Bathes allow getting the temperature up to 300⁰C.

4. Drying and the heating in order to remove hygroscopic moisture at temperature to 110⁰C are made in the drying chambers. Hygroscopic substances and substances decomposed at heating are dried in desiccators with the help of certain substances, taking water.

Drying is considered as completed if the crystals of substance taken with glass rod are poured from it when knocked slightly.

Separation of solid components from liquids

For separation of solid components from liquid ones the filtering and decantation are used. Filtering is the process of liquid separation from solid matter through the filtering material with the simultaneous detaining of the sediment on its surface.

When the aim of filtering is the obtaining of solid precipitate the plain filter is used. If it is necessary to get pure liquid the folded filter is used. For separation of fine-crystalline precipitate and for the acceleration of filtering the vacuum filtering or filtering with removal by suction is used. For the separation of heavy precipitate the decantation – pouring the liquid off the precipitate – is used.

If it is necessary to filter a hot solution, the special double-wall funnel with electric heating is used. It is the metal funnel with double wall, the heating element inserted between them. Ordinary funnel is inserted in this double-wall funnel and is heated. The filtering with heating by vapor is widespread operation in chemical laboratory. The funnel for such filtering is the glass double-wall one. The jet of steam is passed between its walls at filtering.

Filtering materials are: paper, cotton wool, glass wool, and fabric. Paper filters can be ordinary and quantitative (ashless). Filter is quantitative if the weight of ashes forming when it burns is less than 0,0001 g. Funnels and Shott’s crucibles (funnels with soldered pore glass partition) are used for aggressive liquids filtering.

Characteristics of paper filters are given in the Table 9.

Table 9. Characteristics of paper filters

Rules of filtering

1. Correctly chosen filter must be 3 – 4 mm lower than the edge of funnel and fit closely to its walls.

2. Filter is moistened with small amount of distilled water.

3. Let the precipitate settle on the bottom of glassware and carefully, not disturbing it, pour off the liquid along the glass rod on the filter. Only the last portion of liquid is to be mixed with the precipitate.

4. Fill the funnel with liquid so that its level is 2 – 3 mm lower than the edge of filter.

5. When all the liquid flows down, wash the precipitate with washing liquid.

6. Vacuum filtering is carried out with the installation consisting of Bunsen flask, Buchner’s funnel, trap flask and water-jet pump. Filter must cover the whole perforated bottom of Buchner’s funnel and its edge must not rise up. Then turn on the pump and, when the air begins to penetrate through the bottom of funnel, pour off the liquid. Filtering is considered as completed if the drop does not hang from the spout of funnel. The mixture on the filter must not be mixed! The precipitate on the filter is distributed evenly and is concentrated with flat glass stopper. After the operation is finished pass the air in the flask and only then close the water-supply tap. It is necessary to take the substance from the Buchner’s funnel in the following way: disconnect the funnel from the flask, overturn it and knock out (blow out) the substance on the sheet of filter paper by light knocks.

Grinding

For solid substances grinding various mortars: porcelain, metal, agate are used most frequently. Metal mortars are used for rough grinding, porcelain – for finer grinding and agate – for very hard materials.

The very big pieces are firstly broken with hammer. The pieces of the walnut size can be ground in the mortar, filled with the substance to ¼ of its height. Grind carefully so that the substance does not throw out from the mortar. The size of the mortar must be chosen in accordance with the amount of substance. Mortars cannot be used for heating.

Dissolving of solid substances

Preparation of solutions is one of the most important operations of the inorganic synthesis. At preparing solutions the following rules are to be observed.

1. Solutions must be prepared using distilled water.

2. Usually the dissolving is carried out in glass or porcelain beakers, Erlenmeyer (conical) flasks, and other flasks. For the preparation of solutions with certain concentration measuring flasks are used. If the substance dissolves with the great heat production, the solution is prepared in thin-wall porcelain or thermally sound glassware.

The capacity of glassware where solution is prepared must be a little greater than the given volume. Each flask with solution must be provided with an etiquette or inscription.

3. Pour out small quantity of solvent in the glassware then pour the substance, mix and add the solvent in the amount, slightly less then necessary to prepare solution. Add the solvent to the given volume only after solid substance is completely dissolved. After all solvent is added, mix the solution thoroughly and, if necessary, filter before use it.

4. If the substance is hydrolyzed easily, add firstly the substance preventing from the hydrolysis and then dissolve.

Obtaining and drying of gases

In the laboratory conditions gas can be obtained by the solid substance and solution interaction, by thermal decomposition of substances and from the gas storage tank with pressed gas.

The form and design of device for gas obtaining depends on the gas properties.

If the gas is obtained by solid substance decomposition, the latter is placed into the test tube, fixed horizontally so that the emitting moisture does not flow down to the place of heating.

If the gas is evolving when liquid reacts with solid substance, test tubes, Wurtz flasks, Kipp gas generator (Fig. 14) are used. In this case test tube or Wurtz flask is fixed vertically. Kipp gas generator is the device of non-stop action; it is used for obtaining of great amounts of gas.

Methods of gas collecting depend on gas properties. Gases, heavier than air, are collected in the open glassware. If the gas and the air are similar by mass, the gas is collected using the water displacement method. The possibility of gas interaction with water must be taken into account. If it reacts with water, other liquid, for example, the saturated solution of salt, must substitute water. Gases, lighter than air, are collected in open, overturned glassware.

2

The accompanying substances contaminated gases, industrial as well as obtained in laboratory, and gases, evolved from the water solutions, contain water. The purifying and drying of gases is carried out in special glassware: Tyshchenko’s, Wulf’s, Drexel’s flasks, etc.   Figure 14. Kipp gas generator. 1 — upper bulb, 2, 3 — connected vessels, 4, 5 — tubules, 6 — throat.

Cooling

For substances, cooling in dry air after the incineration or melting, desiccators are used. The flowing water, ice, cooling mixtures are used for cooling in order to crystallize the solid substance. Pea-size pieces of ice (snow) are mixed with water to obtain the mess-like mass. To obtain the lower temperatures ice is mixed with Sodium Chloride: 1 part of ice to 0,3 parts of Sodium Chloride (temperature decreases to -21⁰C) or 1 part of ice to 1,43 parts of five-watered calcium Chloride.

Date: 2015-01-12; view: 807