Titration is used in analytical chemistry to determine the amount or concentration of a substance, known as the analyte. Titration is a quantitative measurement of an analyte in solution by its complete reaction with a reagent, In a titration, one reagent(the titrant) is slowly added to a solution containing the species being measured(the analyte). As it is added, a chemical reaction occurs between the titrant and analyte. The point at which the reaction is complete and an equivalent quantity of titrant and analyte are present ( a stoichiometric equivalent) is called the equivalence point. This can be determined by a chemical indicator that is also present in the solution, or by a measureable physical change in the solution, like pH, electrode potential, conductivity, or light absorption(color). In practice, an abrupt change of this physical property signals the end of titration, called the endpoint.
The purpose of titration is to determine the quantity or concentration of an analyte with a known concentration and volume of a titrant. Titrations are based on chemical reactions which must fulfill four requirements.
** The reaction between the analyte and the titrant mus occur quickly, without a secondary reaction.
** The reaction must go to completion.
** The reaction must have well-known stoichiometry(reaction ratio)
** Must have a convenient method of endpoint detection.
Titrations are highly precise and can provide many advantages over alternative methods. Titrations are quickly performed and require relatively simple apparatus and instrumentation.
Automatic Titration
Automatric titration is done with instrumentation that delivers the titrant, stops at the endpoint and calculates the concentration of the analyte automatically. Automatic titrators are best for accurate and repeatable results, as an electrochemical measurement is used to determine the endpoint as opposed to a subjective color indication.
Analytes that can be performed by potentiometric automatic titrators include.
** Acid-base titrations.
** Oxidation reduction titrations.
** Complexometric titrations.
** Precipitation titrations.
** Non-aqueous titrations.
** Argentometric titrations.
** pH, ORP and ion selective measurements.
Analyses performed by bivoltammetric automatic titrators include :
** Coulometric Karl Fischer titration(trace amounts of water determination)
** Volumetric Karl Fischer titration(greater than one hundred part per million water determination)
The required equipment for automatic titration include an automatic titrator equipped with a burette, a standardized titrant, a volumetric pipette (to measure the sample volume) or analytical balance(to measure or weigh a sample), a beaker, a sensor, and a stirring mechanism.
The automatic titrator must have an accurate liquid-dispensing system. In high accuracy systems, this is typically a motor-driven piston burette, a valve system to switch between titrant inlet and outlet, and a titration tip to dispense the titrant into the sample solution. These three main subsystems must be as accurate as possible, with very low gear backlash in the burette drive mechanism, low piston seal flexing, accurate burette glass cylinder diameter, low dead volume in the valve, minimal evaporation/permeation and chemically resistant building.
Standards and Standardization
One of the substances involved in a titration must be used as a standard for which the amount of substance present is accurately known. The standard can be present either in the form of a pure substance or as a solution. The titrant solution can be standardized in two ways ; using a primary standard, or more commonly, titrating it against a previously standardized solution.
ไม่มีความคิดเห็น:
แสดงความคิดเห็น