Platinum crucibles

Benzoic acid [65-85-0] M 122.1, m 122.6-123.1, pK 4.12. For use as a volumetric standard, analytical reagent grade benzoic acid should be carefully fused to ca 130 (to dry it) in a platinum crucible, and then powdered in an agate mortar. Benzoic acid has been crystd from boiling water (charcoal), aq acetic acid, glacial acetic acid, C6H6, aq EtOH, pet ether (b 60-80 ), and from EtOH soln by adding water. It is readily purified by fractional crystn from its melt and by sublimation in a vacuum at 80. ... 

Neodymium oxide [1313-97-9] M 336.5, m 2320°. Dissolved in HCIO4, ppted as the oxalate with doubly recrystd oxalic acid, washed free of soluble impurities, dried at room temperature and ignited in a platinum crucible at higher than 850° in a stream of oxygen [Tobias and Garrett J Am Chem Soc 80 3532 1958]. 

Glass in the form of fibres is relatively inexpensive and is the principal form of reinforcement used in plastics. The fibres are produced by drawing off continuous strands of glass from an orifice in the base of an electrically heated platinum crucible which contains the molten glass. The earliest successful glass reinforcement had a calcium-alumina borosilicate composition developed... 

Filter on the porcelain funnel with the pump and wash three or four times with small quantities of cold water. Press the piecipitate down and dry on a porous plate in the vacuum-desiccator. When thoroughly dry, weigh out about 0-5 to i gram of the compound into a porcelain or platinum crucible, and heat gently with the lid on, and then moie strongly until the organic matter is completely burnt away. Cool the crucible in the desic-ca.or and weigh. 

A mixture of 1 c.c. of benzaldehyde and 5 grams of lime are placed in the bottom cf a platinum crucible about 3 ins. in depth, a layer of lime placed on the top and the crucible covered and gently heated a con-... 

Gold is stable in most strong reducing acids, whereas iron corrodes rapidly, yet finely divided gold can be quickly dissolved in oxygenated cyanide solutions which may be contained in steel tanks. A mixture of caustic soda and sodium nitrate can be fused in an iron or nickel crucible, whereas this melt would have a disastrous effect on a platinum crucible. 

Solid carbon, however produced, presents a hazard. It may be burnt off at low temperatures, with free access to air, without harm to the crucible, but it should never be ignited strongly. Precipitates in filter paper should be treated in a similar manner strong ignition is only permissible after all the carbon has been removed. Ashing in the presence of carbonaceous matter should not be conducted in a platinum crucible, since metallic elements which may be present will attack the platinum under reducing conditions. 

All platinum vessels must be handled with care to prevent deformation and denting. Platinum crucibles must on no account be squeezed with the object of loosening the solidified cake after a fusion. Box-wood formers can be purchased for crucibles and dishes these are invaluable for re-shaping dented or deformed platinum ware. 

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. 

Naturally, the flux employed will depend upon the nature of the insoluble substance. Thus acidic materials are attacked by basic fluxes (carbonates, hydroxides, metaborates), whilst basic materials are attacked by acidic fluxes (pyroborates, pyrosulphates, and acid fluorides). In some instances an oxidising medium is useful, in which case sodium peroxide or sodium carbonate mixed with sodium peroxide or potassium nitrate may be used. The vessel in which fusion is effected must be carefully chosen platinum crucibles are employed for... 

For the preparation of samples for X-ray fluorescence spectroscopy, lithium metaborate is the preferred flux because lithium does not give rise to interfering X-ray emissions. The fusion may be carried out in platinum crucibles or in crucibles made from specially prepared graphite these graphite crucibles can also be used for the vacuum fusion of metal samples for the analysis of occluded gases. 

A. Benzoic acid (C6H5COOH R.M.M. = 122.12). Analytical grade material has a purity of at least 99.9 per cent. For work demanding the highest accuracy, the acid should be dried before use by careful fusion in a platinum crucible placed in an oven at about 130°C, and then powdered in an agate mortar. 

Another example is the determination of pure silica in an impure ignited silica residue. The latter is treated in a platinum crucible with a mixture of sulphuric and hydrofluoric acids the silica is converted into the volatile silicon tetrafluoride ... 

Attention is directed to the fact that if ignition is carried out in a platinum crucible at a temperature above 1100°C some reduction to the oxide Fe304 may occur, and at temperatures above 1200 °C some of the oxide may be reduced to the metal and alloy with the platinum. This accounts in part for the contamination of the platinum crucible by iron which sometimes occurs in analytical work. This oxide is not produced if silica crucibles are employed for the ignitions. 

Zirconium ( > 100 mg in ca /. M sulphuric acid solution). Add freshly prepared 10 per cent aqueous diammonium hydrogenphosphate solution in 50-100-fold excess. Dilute to 300 mL, boil for a few minutes, allow to digest on a water bath for 15-30 minutes and cool to about 60 °C. Filter through a quantitative filter paper, wash first with 150 mL of 1M sulphuric acid containing 2.5 g diammonium hydrogenphosphate and then with cold 5 per cent ammonium nitrate solution until the filtrate is sulphate-free. Dry the filter paper and precipitate at 110°C, place in a platinum crucible and carefully burn off the filter paper. Finally heat at 1000 °C for 1-3 hours and weigh as ZrP207 (Section 11.51). 

No. 41 or 541 filter paper. Wash the precipitate first with warm, dilute hydrochloric acid (approx. 0.5M), and then with hot water until free from chlorides. Pour the filtrate and washings into the original dish, evaporate to dryness on the steam bath, and heat in an air oven at 100-110 °C for 1 hour. Moisten the residue with 5 mL concentrated hydrochloric acid, add 75 mL water, warm to extract soluble salts, and filter through a fresh, but smaller, filter paper. Wash with warm dilute hydrochloric acid (approx. 0.1M), and finally with a little hot water. Fold up the moist filters, and place them in a weighed platinum crucible. Dry the paper with a small flame, char the paper, and burn off the carbon over a low flame take care that none of the fine powder is blown away. When all the carbon has been oxidised, cover the crucible, and heat for an hour at the full temperature of a Meker-type burner in order to complete the dehydration. Allow to cool in a desiccator, and weigh. Repeat the ignition, etc., until the weight is constant. 

Ignite the filter in a platinum crucible, fuse with 2.0 g of anhydrous sodium carbonate, dissolve the melt in 40 mL of dilute sulphuric acid, and add 1 mL of sulphurous acid solution (about 6 per cent) to reduce any iron(III) salt, etc., formed in the fusion, and filter if necessary. Transfer the solution to a 100 mL graduated flask, dilute to the mark, and mix. This flask (B) contains the acid-insoluble boron. 

Standard solution of silica. Fuse 0.107 g of pure, dry precipitated silica with 1.0 g of anhydrous sodium carbonate in a platinum crucible. Cool the melt, dissolve it in de-ionised water, dilute to 500 mL, and store in a polythene bottle. 1 mL = 0.1 mg Si. Dilute as appropriate, say, to 1 mL = 0.01 mg Si. 

In the laser flash method, a melt of interest is placed between two parallel plates. The upper plate is heated stepwise and the thermal diffusiv-ity is measured from the rise in temperature. The specific design for molten materials and especially slags employed by Ohta et al. is based on the differential three-layer technique utihzing a special cell that can be accommodated in the system. A schematic diagram of the principle of the measurement section is shown in Fig. 31. A laser pulse irradiates the upper (platinum) crucible and the temperature response of the surface of the lower platinum crucible is observed, a liquid specimen being sandwiched between the two. 

Except for dry incineration in a platinum crucible, oxygen-induced decomposition requires special apparatus in order to avoid the loss of volatile substances. Combustion of organic materials may be achieved with the... 

M Platinum crucibles and apparatus are the "silverware" of any good laboratory. Reactions under aggressive conditions can often only proceed in platinum vessels. 

A spirit burner. .., blow-pipe, one platinum crucible, one platinum sheet and 3-4 platinum wires, a test tube stand with 10-12 test tubes, several beakers and flasks, one porcelain dish and a pair of porcelain crucibles, several glass filter funnels in various sizes, a wash-bottle, several rods and watch glasses, one agate mortar, several iron spoons, a pair of steel or brass pincers, a filtration stand made of wood and one iron tripod stand. 

Additional problems arise within fusion methods, however. One is the fact that the flux must be present in a fairly large quantity in order to be successful. The measurement of the analyte must not be affected by this large quantity. Also, while a flux may be an excellent solvent for difficult samples, it will also dissolve the container to some extent, creating contamination problems. Platinum crucibles are commonly used, but nickel, gold, and porcelain have been successfully used for some applications. 

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