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水质分析概念详解:滴定
Titration is the quantitative measurement of an analyte in solution by completely reacting it with a reagent solution. The reagent is called the titrant and must either be prepared from a primary standard or be standardized versus a primary standard to know its exact concentration.
The point at which all of the analyte is consumed is the equivalence point. The number of moles of analyte is calculated from the volume of reagent that is required to react with all of the analyte, the titrant concentration, and the reaction stoichiometry.
The equivalence point is often determined by visual indicators are available for titrations based on acid-base neutralization, complexation, and redox reactions. and is determined by some type of indicator that is also present in the solution. For acid-base titrations, indicators are available that change color when the pH changes. When all of the analyte is neutralized, further addition of the titrant causes the pH of the solution to change causing the color of the indicator to change.
Manual titration is done with a
buret, which is a long graduated tube to accurately deliver amounts of titrant.
The amount of titrant used in the titration is found by reading the volume of
titrant in the buret before beginning the titration and after reaching the
endpoint. The difference in these readings is the volume of titrant to reach the
endpoint. The most important factor for making accurate titrations is to read
the buret volumes reproducibly. The figure shows how to do so by using the
bottom of the meniscus to read the reagent volume in the buret.
The end point can be determined by an indicator as described above or by an instrumental method. The most common instrumental detection method is potentiometric detection. The equivalence point of an acid-base titration can be detected with a pH electrode. Titrations, such as complexation or precipitation, involving other ions can use an ion-selective electrode (ISE). UV-vis absorption spectroscopy is also common, especially for complexometric titrations where a subtle color change occurs.
For repetitive titrations, autotitrators with microprocessors are available that deliver the titrant, stop at the endpoint, and calculate the concentration of the analyte. The endpoint is usually detected by some type of electrochemical measurement. Some examples of titrations for which autotitrators are available include: