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Senin, 02 Agustus 2010

GLASSWARE


The most commonly used pieces of apparatus in titrimetric (volumetric) analysis are graduated flasks, burettes, and pipettes. Graduated cylinders and weight pipettes are less widely employed. Each of these will be described in turn. Graduated apparatus for quantitative analysis is generally made to specification limits, particularly with regard to the accuracy of calibration. In the United Kingdom there are two grades of apparatus available, designated Class A and Class B by the British Standards Institution. The tolerance limits are closer for Class A apparatus, and such apparatus is intended for use in work of the highest accuracy: Class B apparatus is employed in routine work. In the United States, specifications for only one grade are available from the National Bureau of Standards at Washington, and these are equivalent to the British Class A.

Cleaning of glass apparatus. Before describing graduated apparatus in detail, reference must be made to the important fact that al1 such glassware must be perfectly clean and free from grease, otherwise the results will be unreliable. One test for cleanliness of glass apparatus is that on being filled with distilled water and the water withdrawn, only an unbroken film of water remains. If the water collects in drops, the vesse1 is dirty and must be cleaned.

Various methods are available for cleaning glassware. Many commercially available detergents are suitable for this purpose, and some manufacturers market special formulations for cleaning laboratory glassware; some of these, e.g. 'Decon 90' made by Decon Laboratories of Portslade, are claimed to be specially effective in removing contamination due to radioactive materials. 'Teepol' is a relatively mild and inexpensive detergent which may be used for cleaning glassware. The laboratory stock solution may consist of a 10 per cent solution in distilled water.
For cleaning a burette, 2 mL of the stock solutiondiluted with 50 mL of distilled water are poured into the burette, allowed to stand for 4 to 1 minute, the detergent run off, the burette rinsed three times with tap water, and then several times with distilled water. A 25 mL pipette may be similarly cleaned using 1 mL of the stock solution diluted with 25-30 mL of distilled water. A method which is frequently used consists in filling the apparatus with 'chromic acid cleaning mixture' (CARE), a nearly saturated solution of powdered sodium dichromate or potassium dichromate in concentrated sulphuric acid, and allowing it to stand for several hours, preferably overnight; the acid is then poured off, the apparatus thoroughly rinsed with distilled water, and allowed to drain until dry. [It may be mentioned that potassium dichromate is not very soluble in concentrated sulphuric acid (about 5 g per litre), whereas sodium dichromate (Na2Cr20,,2H20) is much more soluble (about 70 g per litre); for this reason, as well as the fact that it is much cheaper, the latter is usually preferred for the preparation of 'cleaning mixture'. From time to time it is advisable to filter the sodium dichromate-sulphuric acid mixture through a little glass wool placed in the apex of a glass funnel: small particles or sludge, which are often present and may block the tips of burettes, are thus removed.

A more efficient cleaning liquid is a mixture of concentrated sulphuric acid and fuming nitric acid; this may be used if the vessel is very greasy and dirty, but must be handled with extreme caution. A very effective degreasing agent, which it is claimed is much quicker-acting than 'cleaning mixture' is obtained by dissolving 100 g of potassium hydroxide in 50mL of water, and after cooling, making up to 1 litre with industrial methylated spirit.

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