Crucibles cleaning

Prepare crucibles. Clean three porcelain crucibles and covers. Place them over Tirrill burners and heat at the maximum temperature of the burner for 10 to 15 min then place in the desiccator to cool for at least 1 h. Weigh accurately each crucible with its cover. Between heating and weighing, the crucibles and covers should be handled only with a pair of tongs. 

To keep the crucible clean, (i) it should be supported on a glass or porcelain ring with a flange on the water bath during evaporation, (ii) The outer surface of the crucible should be wiped before placing it in the oven for drying. 

The density determination may be carried out at the temperature of the laboratory. The liquid should stand for at least one hour and a thermometer placed either in the liquid (if practicable) or in its immediate vicinity. It is usually better to conduct the measurement at a temperature of 20° or 25° throughout this volume a standard temperature of 20° will be adopted. To determine the density of a liquid at 20°, a clean, corked test-tube containing about 5 ml. of toe liquid is immersed for about three-quarters of its length in a water thermostat at 20° for about 2 hours. An empty test-tube and a shallow beaker (e.g., a Baco beaker) are also supported in the thermostat so that only the rims protrude above the surface of the water the pycnometer is supported by its capillary arms on the rim of the test-tube, and the small crucible is placed in the beaker, which is covered with a clock glass. When the liquid has acquired the temperature of the thermostat, the small crucible is removed, charged with the liquid, the pycnometer rapidly filled and adjusted to the mark. With practice, the whole operation can be completed in about half a minute. The error introduced if the temperature of the laboratory differs by as much as 10° from that of the thermostat does not exceed 1 mg. if the temperature of the laboratory is adjusted so that it does not differ by more than 1-2° from 20°, the error is negligible. The weight of the empty pycnometer and also filled with distilled (preferably conductivity) water at 20° should also be determined. The density of the liquid can then be computed. 

Crucibles fitted with permanent porous plates are cleaned by shaking out as much of the solid as possible, and then dissolving out the remainder of the solid with a suitable solvent. A hot 0.1 M solution of the tetrasodium salt of the ethylenediaminetetra-acetic acid is an excellent solvent for many of the precipitates [except metallic sulphides and hexacyanoferrates(III)] encountered in analysis. These include barium sulphate, calcium oxalate, calcium phosphate, calcium oxide, lead carbonate, lead iodate, lead oxalate, and ammonium magnesium phosphate. The crucible may either be completely immersed in the hot reagent or the latter may be drawn by suction through the crucible. 

If the carbon on the lid is oxidised only slowly, the cover may be heated separately in a flame. It is, of course, held in clean crucible tongs. 

The precipitate, collected in a sintered-glass (porosity No. 4) or porcelain filtering crucible, may be weighed more rapidly as follows. Wash the copper(I) thiocyanate five or six times with ethanol, followed by a similar treatment with small volumes of anhydrous diethyl ether, then suck the precipitate dry at the pump for 10 minutes, wipe the outside of the crucible with a clean linen cloth and leave it in a vacuum desiccator for 10 minutes. Weigh as CuSCN. 

In a clean, dry crucible, mass out approximately 1 g of lithium chloride, LiCl, another typical ionic compound. (The melting point of sodium chloride, NaCl, is too high to observe using classroom laboratory equipment.)... 

Prepare a porcelain crucible and lid by cleaning with hot soapy water and a brush, followed by rinsing and drying with a towel. Then heat the crucible, with lid slightly ajar, in a muffle furnace set at 950°C for 30 min. This step is to ensure that any volatile material will be removed now rather than later, when the weight loss is critical Allow to cool for 10 min on a heat resistant surface and then 10 min more in a desiccator. Then weigh the crucible with lid on an analytical balance. 

Depending on the material of the crucible, various cleaning agents may be used. Whenever possible, water and mild abrasives like sand are preferred. Cleaning with acids is usually applicable to platinum crucibles, but should be avoided in the case of alumina crucibles. Platinum crucibles can be cleaned especially thoroughly in a melt of potassium pyrosulfate. In all such operations it is important for the crucibles... 

The burner is now extinguished, the ammonia shut off, and the pot cover removed by disconnecting at H, H. The crucible is removed from the pot with tongs, and the molten amide is poured into a shallow iron tray, which has been previously heated to remove traces of moisture (Note 8). At this point it is essential to work rapidly to avoid solidification of the amide in the crucible (Note 9). As soon as the product has solidified sufficiently, the tray is transferred to a large desiccator to cool. When cold enough to handle, the tray is inverted on a clean heavy paper the amide is removed by rapping the bottom of the pan and is at once transferred to convenient wide-mouthed bottles and covered with a petroleum fraction (Notes 10 and n). The yields vary from 267 to 282 g. (90-95 per cent of the theoretical amount) (Notes 12,13, and 14). 

Runs of other sizes may be made in the same apparatus. With half the quantity of sodium specified, temperature control demands much more attention. With larger quantities, the nickel crucible is dispensed with, and the carefully cleaned pot used. The checkers used 260-270 g. of sodium and averaged a yield of 94 per cent the reaction time was increased by only one-half hour. By arranging two series of apparatus in parallel, but connected to the same cylinder of ammonia, one operator can prepare twice as much amide in almost the same time. 

Measure the mass of the clean, dry crucible, and record the mass in the data table. 

Measure to the nearest 0.01 g the mass of a clean, dry crucible with a lid. Record the mass. 

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