Analytical Chemistry

Transcription

1 Analytical Chemistry Lecture No. 2 Date :2 /12/ 2012 Dr. Mohammed Hamed Analytical Chemistry Definition: Analytical chemistry is the chemistry discipline concerned with the chemical composition of materials. Analytical chemistry also is concerned with developing the tools used to examine chemical compositions. Analytical Chemistry deals with methods for determining the chemical composition of samples of matter. A qualitative method yields information about the identity of atomic or molecular species or the functional groups in the sample; a quantitative method, in contrast, provides numerical information as to the relative amount of one or more of these components. Analytical methods are often classified as being either classical or instrumental. classical methods :- In the early years of chemistry, most analyses were carried out by separating components of interest in a sample by precipitation, extraction, or distillation. For quantitative analyses, the separated components were then treated with reagents that yielded products that could be recognized by their colors, boiling points or melting points, their solubility in a series of solvents, their odors, their optical activities, or their refractive indexes. Qualitative analysis consists of methods for establishing the qualitative chemical composition of a substance that is, the identification of atoms, ions, and molecules that enter into the composition of the substance being analyzed. The most important characteristics of all methods of qualitative analysis are specificity and sensitivity. Specificity characterizes the ability to detect the presence of an unknown element in the presence of other elements for example, iron in the presence of nickel, manganese, chromium, vanadium, or silicon. Sensitivity is defined as the smallest quantity of an element that can be detected by a given method. Sensitivity of modern

2 methods is expressed in magnitudes of the order of 1 mg (one millionth part of a gram). Quantitative analysis consists of methods for determining the quantitative composition of materials that is, the quantitative amounts of the chemical elements or of certain compounds in the analyzed substance. Along with specificity and sensitivity, the most important characteristic of every method of quantitative analysis is accuracy. quantitative analyses, the amount of analyte was determined by gravimetric or by titrimetric measurement. Gravimetric Methods the mass of the analyte or some compound produced from the analyte was determined. Titrimetric Methods the volume or mass of a standard reagent required to react completely with the analyte was measured. Instrumental Methods:- Measurements of physical properties of analytes, such as conductivity, electrode potential, light absorption, or emission, mass to charge ratio, and fluorescence, began to be used for quantitative analysis of a variety of inorganic, organic, and biochemical analyte. Highly efficient chromatographic and electrophoretic techniques began to replace distillation, extraction, and precipitation for the separation of components of complex mixtures prior to their qualitative or quantitative determination. These newer methods for separating and determining chemical species are known collectively as instrumental methods of analysis. Instrumentation can be divided into two categories: detection and quantitation. Measurement of physical properties of analytes - such as conductivity, electrode potential, light absorption or emission, mass-to-charge ratio, and fluorescence-began to be employed for quantitative analysis of inorganic, organic, and biochemical analytes. Efficient chromatographic separation techniques are used for the separation of components of complex mixtures.

3 Solution Concentration Normality:- The normality N of a solution is, given by the number of equivalents of solute per litre of solution No.of equivalents of Solute N= Volume of solution in litre OR W/ EW N= V Where, N = Normality W = Weight of solute EW = Equivalent weight of solute V = Volume of solution in litre Normality varies according to the reaction; as the equivalent weight of a substance may vary according to the reaction, which the solute participates. The equivalent weight is generally calculated after determining the specific changes in chemical reaction of the solute. Percent Concentration:- Concentration is many-a-times expressed in terms of percent (parts per hundered). Percent composition of a solution can be expressed as : mass of solute 1. Per cent w/w = mass of solution

4 volume of solute 2. Per cent v/v = volume of solution mass of solute 3. Per cent w/v = volume of solution Percent w/w is frequently employed to express the concentration of commercial aqueous reagents, per cent v/v is used to specify the concentration of a solution prepared by dilution a pure liquid with another liquid and percent w/v is employed to indicate the composition of dilute queous solutions of solid reagents. Molality :- Number of moles of solute dissolved in 1 kg (1000 g) of solvent moles of solute Molality (or Molal Concentration) = In units of molal = m kg of solvent One of the most commonly used concentration units among chemists is the Molarity concentration term. Molarity is defined as the moles of solute divided by the volume of the solution in liter units. An alternative definition is the number of millimoles of solute divided by the volume of the solution in milliliters. The symbol that we use is the capital letter M.

5 For example, if we have a 3 M HCl solution, then we have 3 moles of HCl in every liter of that solution. Alternatively, we can say that we have 3 millimoles of HCl in every milliliter of solution. A shortened term for this concentration term is the word "molar". Application of the Molarity Term: Example Problem: A sample of NaNO3 weighing 8.5 grams is placed in a 500 ml volumetric flask and distilled water was added to the the mark on the neck of the flask. Calculate the Molarity of the resulting solution. Solution: 1. Convert the given grams of solute to moles of solute by dividing by the molecular weight of NaNO 3 : 2. 1 mole NaNO 3 = Molecular mass of NaNO 3 expressed in grams = (16) = 85grams 8.5 grams NaNO 3 x 1 mole NaNO 3 / 85 grams NaNO 3 = 0.1 mole NaNO 3 3. Convert given ml of solution to liters by dividing by 1000: 1 liter = 1000 ml 500 ml x1 liter / 1000 ml = liters 4. Apply the definition for Molarity: Molarity = moles NaNO 3 / volume of the solution in liters M = 0.1 mole /.500 liters = Molar NaNO 3 Since molarity involves a basis of solution volume, it is apparent that the molarity of a solution will change as volume changes which is associated with changes in temperature. Formal Concentration: Some substances do not exist in molecular form, whether in solid or solution form, they remain in ionic form in solid state as well as in solution. In such cases instead of molecular weight, formula weight is used in preparation of solution and its

6 concentration is expressed in terms of formality, for example Concentration of NaCl and FeCl3 can be defined very simply as number of formula weight of a solute per litre of solution. It is represented by F. Weight of solute in g / Formula weight F = Volume of solution in litre Formal solutions generally show changes in formality where volume changes associated with temperature.