Titremetric analysis Dr. Mohammad Khanfar

Concept of Titremetric analysis In general, we utilize certain property of a substance to be analyzed in order to be able to determine its amount or concentration in solution (quantitative) or to identify substance (qualitative). Titration is a procedure used in analytical chemistry to determine the amount or concentration of a substance. In a titration procedure, one reactant (titrant) is added to another (titrand or called analyte, whose concentration or amount to be determined) slowly from a special graduated glass tube called burette.

Concept of Titremetric analysis As the titrant is added to the titrand, the chemical reaction proceeds until no more of the titrand (analyte) is left, this point called equivalence point. We cannot determine the equivalence point of a titration experimentally. Instead, we can only estimate its position by observing some physical change associated with the condition of equivalence. This change is called the end point for the titration. We can determine this end point using other substance called indicator. Typical indicator changes include the appearance or disappearance of a color, a change in color, or the appearance or disappearance of turbidity.

Concept of Titremetric analysis The amount of the analyte based on the facts that we know: 1) the concentration and the volume of titrant consumed in the reaction 2) Volume of the analyte. Therefore, if the reaction between the titrant and titrand occurs in 1:1 ratio, then the number of moles of titrant is equal the number of moles of analyte.

Concept of Titremetric analysis Example: The organic matter in a g sample of a mercuric ointment is decomposed with HNO 3, After dilution, the Hg 2+ is titrated with ml of a M solution of NH 4 SCN, Calculate the percent Hg ( g/mol) in the ointment.

Concept of Titremetric analysis In order to be suitable for titration, the reaction could be of an type as long as the following conditions are met: 1) The reaction must be complete i.e. any amount of the titrant should be able to convert equivalent amount (on molar bases) of the titrand to product. 2) the reaction must be spontaneous (fast). Sometime a catalyst may be used to increase the speed of the reaction. 3) There must be some means of detecting the end points. The most common types of chemical reactions that satisfy the above conditions are: 1) acid-base reactions 2) oxidations reduction reactions 3) complexation reactions 4) precipitation reactions

Concept of Titremetric analysis How do we know the concentration of the titrant? In order to prepare a solution of with known concentration (this is known as Standard solution) we need to take proper weight of the material (contains the required number of moles) and transfer it carefully to a proper volumetric flask, the volume is completed to the mark

Concept of Titremetric analysis The ideal standard solution for a titrimetric method will 1. Be sufficiently stable so that it is necessary to determine its concentration only once 2. React rapidly with the analyte so that the time required between additions of reagent is minimized 3. React more or less completely with the analyte so that satisfactory end points are realized; 4. Undergo a selective reaction with the analyte that can be described by a balanced equation

Concept of Titremetric analysis Primary standard are stable chemical compounds that are available in high purity and which can be used to standardize the standard solutions used in titrations. Titrant such as NaOH and HCl cannot be considered as primary standards since their purity is quite variable.

Concept of Titremetric analysis Important requirements for a primary standard are the following: 1. High purity. Established methods for confirming purity should be available. 2. Atmospheric stability. 3. Absence of hydrate water so that the composition of the solid does not change with variations in humidity. 4. Modest cost. 5. Reasonable solubility in the titration medium. 6. Reasonably large molar mass so that the relative error associated with weighing the standard is minimized.

Concept of Titremetric analysis Advantages of titremetric analysis Capable of a higher degree of precision and accuracy The method are generally robust Analyses can be automated Cheap to perform and do not require specialized apparatus Not dependent on the calibration of an instrument

Concept of Titremetric analysis Limitations Non-selective Time consuming if not automated Require large amount of sample and reagents Reactions of standard solutions with the analyte should be rapid and complete

Concept of Titremetric analysis d

Concept of Titremetric analysis Titration curve: are plots of a concentration-related variable as a function of reagent volume. In acid-base titration, it’s a plot between the pH of solution being titrated and the volume of the added titrant. Potential end point

Titration curve for the titration of mL of M AgNO 3 with 0.10 M KSCN.

Concept of Titremetric analysis Titration curve End-point detection is based on large change in relative concentration of the analyte (or the reagent) that occurs at and near the equivalence point for every type of titration. Titration curves for reactions involving complex formation, precipitation, and oxidation/reduction all exhibit the same sharp increase or decrease in p-function in the equivalence- point region.

Concept of Titremetric analysis Back titration Addition of excess of standard solution (titrant) to a weighed amount (or volume) of sample, and determination of excess (un- reacted) titrant using secondary titrant. In general this method is used for: 1) Volatile substances, e.g. ammonia, some would be lost during titration 2) Insoluble substances, e.g. Calcium carbonate, which requires excess volumetric solution 3) Substances for which a quantitative reaction proceeds rapidly only in presence of excess of the reagent, e.g. lactic acid 4) Substances which require heating, e.g. formaldehyde solution

Concept of Titremetric analysis Example of back titration The amount of phosphate in a sample can be determined by adding a measured excess of standard silver nitrate to a solution of the sample, which leads to the formation of insoluble silver phosphate: The excess silver nitrate is then back-titrated with a standard solution of potassium thiocyanate: Here. the amount of silver nitrate is chemically equivalent to the amount of phosphate ion plus the amount of thiocyanate used for the back titration.