化学专业基础英语.ppt

The point in titration where such changes manifest表明, 证明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不充分in the physical changes and our ability to observe them. This results in an analytical error called a titration error. One of the common methods of end-point detection employed in volumetric analysis involves the use of supplementary chemical compounds that exhibit changes in color as a result of concentration changes occuring near the equivalence point. Such substances are called indicators. 5. mean (arithmetic mean, average), median（平均值（数学平均，平均数）,中值 ） The mean, arithmetic mean, and average are synonymous同义的terms that refer to the numerical value obtained by dividing the sum of a set of replicate重复measurements by the number of individual results in the set. The median of a set is that value about which all others are equally distributed, half being numerically greater and half being numerically smaller. If the set consists of an odd number of measurements, selection of the median may be made directly; for a set containing an even number偶数 of measurements, the average value of the central pair is taken as the median. We shall see that in the ideal case the mean and median are numerically equal; this fails to be true more often than not, however, when only a small set of measurements has been taken. 6. precision, absolute deviation, relative deviation, standard deviation（精密度，绝对偏差，相对偏差，标准偏差） The term precision is frequently used to describe the reproducibility of results. It can be defined as the agreement between the numerical values of two or more measurements that have been made in an identical fashion. Absolute deviation is simply the difference between an experimental value and that (of) which is taken as the best for the set (usually the arithmetic mean). Re