Separation Science

1. Separation Science • -Separation is the hypothetical conditions where there is complete isolation of "m" chemical components, which comprise a mixture, in their pure forms, into "m" separate vessels.(Ref.6,page: 4) • - The word "hypothetical" is used for two reasons: First: it is theoretically impossible to accomplish the complete separation of the components of a mixture (Why?). Second: separated compounds are not actually isolated, but rather a record (e.g on chart paper) contains the information that indicates how well the components were separated.

2. Separation in simple word involves the removing of the effect of interferences and increases the selectivity of the method toward the analyte with necessary accuracy and precision. Interferences • The analytical methods were affected by the presence of the other component which called interferences. Therefore, its important to remove the effect of the interferences in quantitative analytical methods, thus its selectivity is increased. • Specific method; that is to say, it should be capable of measuring the amount of desired substance accurately, no matter what other substances may be present. • Selective method; can be used to determine any of a small group of ions in the presence of certain specified ions. • The selection of methods for separating the interferences from the substance to be determined are as important as the choice of the method of determination.

3. Typical separation procedures include the following: I- Chemical Tretment -The following procedures will help to increase the selectivity a) Masking and De-masking agent:Masking may be defined as the process in which a substance, without physical separation of it or its reaction products, is so transformed that it does not enter into a particular reaction. De-masking is the process in which the masked substance regains its ability to enter into a particular reaction. Example: An effective masking agent is the CN-; this forms stable CN-complexes with the cations of Cd, Zn, Hg(II), Cu, Co, Ni, Ag, and Pt, but not with the alkaline earths, Mn, and Pb: It is therefore possible to determine cations such as Ca2+, Mg2+, Pb2+, and Mn2+ in the presence of the above-mentioned metals by masking with an excess of CN-.

4. Example: The CN-complexes of Zn and Cd may be demasked with formaldehyde-acetic acid solution: The use of masking and selective demasking agents permits the successive titration methods of many metals.

5. b) Selective oxidation (or reduction): The sample is treated with a selective oxidizing or reducing agent which will react with some of the ions present the resultant change in oxidation state will often facilitate separation. • Example Thus, Cu(II) may be reduced in acid solution by hydroxylamine or ascorbic acid. After rendering ammoniacal, Ni or Co can be titrated using, for example, murexide as indicator without interference from the Cu, which is now present as Cu(I).

6. c) Suitable control of the pH of the solution: • Example This, of course, makes use of the different stabilities of metal-EDTA complexes. -Thus Bi and Th can be titrated in an acidic solution (pH = 2) with xylenol orange or methylthymol blue as indicator and most divalent cations (M2+) do not interfere. -A mixture of Bi and Pb ions can be successfully titrated by first titrating the Bi at pH=2 with xylenol orange as indicator, and then adding hexamine to raise the pH to about 5, and titrating the Pb.

7. d) Formation of Complex ion • Example

8. II- Physical methods • -The general separation steps involve: 1- Formation of a system consists of the two phase, one of them contains the component under study. 2- Mechanical separation of the two phases. 3- Quantitative collection of the desired phase which contains the component

9. Classifying Separation Techniques An analyte and an interferent can be separated if there is a significant difference in at least one of their chemical or physical properties. Table bellow provides a partial list of several separation techniques, classified by the chemical or physical property that is exploited. (Ref. 1 , page: 205)

10. -Formation of a system consists of the two phase can usually proceed by: I- Separation based on the Kinetic properties (Physical properties) a. Decantation b. Filtration c. Sieving d. Centrifugate II- Separation based on Thermodynamic 1- Direct methods (or Phase-formation methods) Involves creation of a second phase from the original phase

11. 2- Indirect Methods (or Phase- transformation methods) Involves an addition of a second phase to the original phase, when the separation includes transferring of the analyte in to this new added phase.

12. Separation by Phase-Formation methods I-Volatilization Methods (conversion of Solid to Gas) Volatilization is the conversion of all or part of a solid or a liquid to a gas. This evolved gas may be recovered and measured or the residue remaining may be measured. • The gas may be produced by several methods: 1) Direct heating

13. 2) Applying the principle that strong acids displace weaker acids and strong bases displace weaker bases; for example: 3) Oxidation, such as burning a sulfide in air to produce SO2. 4) Reduction or converting the elements to hydrides such as AsH3 or H2Se by NaBH4.

14. In all cases a gaseous substance is formed. If this is to be collected, then it is passed through an absorption train to remove unwanted gaseous impurities. The desired gas then is either absorbed on a solid absorber and weighed, or is passed into a solution where it reacts quantitatively with another chemical and the excess chemical is determined.