英语翻译12.titration,back-titration,standard solution,primary st

英语翻译
12.titration,back-titration,standard solution,primary standard,standardization,end point,titration error,indicators
A titration is a process wherein the capacity of a substance to combine with a reagent is quantitatively measured.Ordinarily this is accomplished by the controlled addition of a reagent of known concentration to a solution of the substance until reaction between the two is judged,to be complete; the volume of reagent is then measured.Occasionally it is convenient or necessary to carry out a volumetric analysis by adding an excess of the reagent and then determining the excess by titration with a second reagent of known concentration.The second titration is called a back-titration.
The reagent of exactly known composition used in a titration is called a standard solution.The accuracy with which its concentration is known sets a definite limit upon the accuracy of the method; for this reason,much care is taken in the preparation of standard solutions.Commonly the concentration of a standard solution is arrived in either of two ways:(1) a carefully measured quantity of a pure compound is titrated with the reagent and the concentration calculated from the weight and volume measurements; or (2) the standard solution is prepared by dissolving a carefully weighed quantity of the pure reagent itself in the solvent; this is then diluted to an exactly known volume.In either method,a highly purified chemical compound - called a primary standard is required as the reference material.The process whereby the concentration of a standard solution is determined by titration of a primary standard is called a standardization.
The goal of every titration is the addition of standard solution in such amount as to be chemically equivalent to the substance with which it reacts.For example,the equivalence point in the titration of sodium chloride with silver nitrate is attained when exactly one formula weight of silver ion has been introduced for each formula weight of chloride ion present in the sample.In the titration of sulfuric acid with sodium hydroxide,the equivalence point occurs when two formula weights of the latter have been introduced for each formula weights of the former.
The equivalence point in a titration is a theoretical concept.In actual fact we can only estimate its position by observing physical changes associated with it in the solution.The point in titration where such changes manifest themselves is called the end point,it is to be hoped that the volume difference between the end point and the equivalence point will be small.Differences do arise,however,owing to inadequacies in the physical changes and our ability to observe them.This results in an analytical error called a titration error.