Usp volumen

Páginas: 3 (605 palabras) Publicado: 14 de septiembre de 2010

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Most of the volumetric apparatus available in the United States is calibrated at 20 , although the temperatures generallyprevailing in laboratories more nearly approach 25 . To minimize volumetric error, the temperature should be the same for the volumetric apparatus, the material being prepared, the solvents being usedto prepare the volumetric solutions, the area in which they are prepared, and the final volume adjustment.   Use— To attain the degree of precision required in many Pharmacopeial assays involvingvolumetric measurements and directing that a quantity be “accurately measured,” the apparatus must be chosen and used with care. A buret should be of such size that the titrant volume represents not lessthan 30% of the nominal volume. Where less than 10 mL of titrant is to be measured, a 10-mL buret or a microburet generally is required. The design of volumetric apparatus is an important factor inassuring accuracy. For example, the length of the graduated portions of graduated cylinders should be not less than five times the insidediameter, and the tips of burets and pipets should restrict the outflow rate to not more than 500 µL per second.   Standards of Accuracy— The capacity tolerances for volumetric flasks, transfer pipets, and burets are those accepted by the National Institute of Standards andTechnology (Class A), as indicated in the accompanying tables. Use Class A volumetric apparatus unless otherwise specified in the individual monograph. For plastic volumetric apparatus the acceptedcapacity tolerances are Class B.
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The capacity tolerances for measuring (i.e., “graduated”) pipets of up to and including 10-mL capacity are somewhat larger than those for the corresponding sizesof transfer pipets, namely, 10, 20, and 30 µL for the 2-, 5-, and 10-mL sizes, respectively. Transfer and measuring pipets calibrated “to deliver” should be drained in a vertical position...
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