Unit 4: Classical Methods of Analysis PDF

B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis Table of Content 4.1 Standard solutions: primary and secondary standards 4.2 Titrimetric analysis: Types of tit...

B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis Table of Content 4.1 Standard solutions: primary and secondary standards 4.2 Titrimetric analysis: Types of titrations 4.3 Equivalence point and End point 4.4 End point detections 4.5 Indicators, theory of indicators and their selection for volumetric analysis 4.6 Theory of volumetric and gravimetric analysis 4.7 Important organic precipitants Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis INTRODUCTION The presence of toxic substances in the environment is a threat to the nation and must be assessed by various methods used in chemical analysis. The chemical analysis is divided into two branches: qualitative analysis and quantitative analysis. The branch, which deals with the detection and identification of substances, is known as qualitative analysis and the amount of substances is known as quantitative analysis. Mostly a quantitative analysis is proceeded by the qualitative analysis whereas a qualitative analysis may give a rough idea of the relative amount of the constituents present in the sample. After preliminary identification an analytical chemist is mainly interested in quantitative analysis i.e. the measurement of mass or relative mass of a particular constituent of the sample.  The measurement of mass can be made in two different ways: i) measuring mass directly ii) measuring the magnitude of one of the constituent properties depending on its mass or relative mass For first of the two ways i.e. for measuring mass directly the methods used are known as classical methods of analysis. These methods involve simple procedures of equipment. In these methods the determination of mass of the constituent substance is done by one of the two procedures: gravimetry and volumetry. In gravimetry the substance is directly weighed in the suitable form after precipitation of a compound of known composition. In volumetry (Titrimetry) the mass is calculated by knowing the volume of reagent of known concentration used to titrate the sample solution. Both these procedures grouped together are also known as chemical methods of analysis, Analysis because the basis of these determinations is a chemical reaction. In a general way a moles of the constituent C react stoichiometrically with b moles of a reagent R to form a compound CaRb. aC + bR CaRb Both, gravimetric analysis and titrimetric analysis are especially suited for major constituents. For quantitative determination of some elements gravimetric methods are preferred to titrimetric method. On the other hand, for some elements Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis titrimetry is preferred to gravimetry. A gravimetric method generally takes much time and is suitable when only a single or a few determinations are to be made. Titrimetric determinations usually take little time and are especially advantageous when analysis of large number of samples is carried out. 4.1 standard solutions: primary and secondary standards A few volumetric reagents are obtainable in such a high state of purity that they can be used for the direct preparation of standard solutions by the use of accurately weighed quantities. Many others, including some of the most useful, cannot be so used, either because they are hydrated and subject to small variations in content of water of crystallization, or because for some other reason they cannot be guaranteed to have the degree of purity necessary. A reagent of this kind is made upto approximately the correct concentration, the solution so obtained is standardized against a pure compound, and a factor is calculated. Volumetric solution also known as Standard Solution are solutions of reagents of known concentration intended primarily for use in quantitative determinations. Primary standards Primary standards are defined as the substances of known high purity (99.95 – 100.05%) with respect to the active component which should be stable at over temperatures for drying used to standardize the volumetric reagents, or Primary standards are the substances which are available in pure form with definite chemical composition are called as primary standards. Several primary standards are available from the National Bureau of Standards and other chemical suppliers. Several substances of analytical reagent grade are sufficiently pure to serve as primary standards for ordinary work, or they may be made so by recrystallization. A Primary Standard should satisfy the following requirements; 1. It must be easy to obtain, to purify, to dry (preferably at 110 – 120˚c), and to preserve in a pure state. (This requirement is not usually met by hydrated substances, since it is difficult to remove surface moisture completely without effecting partial decomposition). 2. The Substance should be unaltered in air during weighing: this condition implies that it should not be hygroscopic, oxidised by air, or affected by carbon dioxide. The standard should maintain an unchanged composition during storage. 3. The substance should be capable of being tested for impurities by qualitative and other tests of known sensitivity. (The total amount of impurities should not, in general exceed 0.01-0.02 per cent). Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis 4. It should have a high relative molecular mass so that the weighing errors may be negligible. (The precision in weighing is ordinarily 0.1-0.2 mg; for an accuracy of 1 part in 1000. it is necessary to employ samples weighing at least about 0.2gm). 5. The substance should be readily soluble under the conditions in which it is employed. 6. The reaction with the standard solution should be stoichiometric and practically instantaneous. The titration error should be negligible, or easy to determine accurately by experiment. Examples of Primary Standards: Secondary standard: - “A secondary standard is a solution which contain exactly known amount of the substance in unit volume of the solution and which is expressed as normality or molarity and can be determined by titrating against a primary standard”. It follows that a secondary standard solution is a solution in which the concentration of dissolved solute has not been determined from the weight of the compound dissolved but by reaction (titration) of a volume of a primary standard solution. Thus, a solution of sodium hydroxide may be standardised by titrating against a standard solution of Potassium Hydrogen Phthalate or against a standard solution of Hydrochloric Acid (Secondary standard). A secondary standard is a substance which for one or more of the reasons cannot be used as a primary standard. e.g., sodium hydroxide cannot be used as a primary standard for the reason that it absorbs water and carbon dioxide from Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis the atmosphere and the composition of its solution is subject to wide variations at different periods. Similarly, Sodium thiosulphate absorbs CO2 from the atmosphere and gets decomposed. A deposit of sulphur settles at the bottom. Similarly, the compositions of solution of various other substances like mineral acids, Potassium permanganate, Iodine etc, are also variable at different times. There-fore, these cannot be used as primary standards. The normality or molarity of solution of such a substance can be found by titrating against a standard solution of a primary standard or in other words the solution may be standardized by titrating against the standard solution of a primary standard. The mole concept: Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis  Concentration units:- There are various methods of expressing the concentration of a solution such as: formality (F), molarity (M), normality (N), percentage (%),parts per million (ppm), mole fraction etc. Those, which are in common use, will be discussed here. Normality: This is the most commonly used system of representing the concentration of a solution and is defined as: the number of gew (gram equivalent weight) of that substance present in one litre of solution. Molarity: It is defined as the number of moles of the solute dissolved in one litre of the solvent. Molality: It is defined as the number of moles of the solute dissolved in one kilogram of the solvent. Mole Fraction: It is the ratio of number of moles of that component to the total number of moles of all the components present in that solution. Percentage Concentration: Percentage (or parts per hundred) is often used to express the approximate concentration. The various ways to express the percentage composition of a solution are; i) Volume percentage is generally used for liquid mixtures to indicate the concentration of a liquid in the solution. For example, 5% alcohol means by volume 5 parts of alcohol in 100 parts of solution. ii) Weight-volume percentage usually indicates the concentration of dilute aqueous solutions of solid substances. iii) The percentage of commercial aqueous reagents is given in terms of weight percentage frequently. For example, 36% of HCl in water. The weight percent is independent of temperature. Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis 4.2 Titrimetric analysis: Types of titrations 4.3 Equivalence point and End point  Classification of titrimetric analysis: -It is classified into four groups, (i) Neutralization (or acid-base) titrations. (ii) Oxidation - reduction (or Redox) titrations. (iii) Complexometric titrations (or EDTA Titration). (iv) Precipitation titration. Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis 1.Neutralization Titrations Neutralization or acid-base titrations basically involve the reaction between H+ and OH- to give H2O) as: H+ + OH- H 2O These titrations may be classified in a number of ways, such as: (a) Titration of a strong acid by a strong base (b) Titration of a weak acid by a strong base (c) Titration of a strong base by a strong acid (d) Titration of a weak base by a strong acid (e) Salts of very weak acids can be titrated with strong acids, and salts of very weak bases can be titrated with strong bases. (Sometimes, the terms acidimetry and alkalimetry are also used respectively to these types of titrations. Nevertheless, such a classification is meaningless, if one has to use the Bronsted, concept. Since, the salt of a weak acid is a weak base and salt of a weak base is (weak) acid according to the Bronsted concept.) (f) Stage-wise titration of a polyprotic acid or that of a poly-acidic base. Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis  Neutralization titrations can be best studied by considering their titration curves where the appropriate concentration variable pH is plotted against either the volume or the amount of titrant added.  In a useful titration curve the solution pH shows a great change in the immediate vicinity of equivalence point.  The end point is frequently detected with the help of an indicator showing colour change at the pH of equivalence point.  The choice of the indicator depends upon the relative strength of the acid and the base and the pH range of the indicator.  For an indicator of choice its pH range should lie on the steep portion of the titration curve in the vicinity of equivalence point.  We shall now give consideration to certain neutralization titration systems with the help of their titration curves. Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis 1. Titration of a Strong Acid with a Strong Base: The terms "strong acid" or "strong base" are used for substances such as HCl, HN03, NaOH and KOH, which are practically 100 percent, dissociated in aqueous solution.  Ex: Strong Acid Let us consider the titration of 25 ml of 0.1N HCl with 0.1N NaOH.  2. Titration of a weak base versus a strong acid: To discuss such titrations, the titration of ammonium-hydroxide versus hydrochloric acid will be considered. The type of the curve obtained for this titration is given in Fig. It is evident from the titration curve that a narrow steep vertical portion is obtained and the equivalence point will be Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis located at a pH value significantly lower than 7. Hence, methyl-orange with pH range 3.0 – 4.4 will be suitable indicator for this titration. Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis Ques on Bank Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar  B. Sc.- B. Ed. Semester- II, General Chemistry- II Unit 4: Classical methods of analysis 2 Marks 1. Write the Formula for Molarity. What is the number of moles of HCl in 250 ml of its 0.1 M solution? 2. Classify Chemical analysis. 3. Define: Quantitative Analysis & Qualitative Analysis. 4. Define: Volumetric Analysis & Titrimetric Analysis. 5. Fill in the blanks; i) The number of gram equivalent weight of a substance present in one litre of a solution is known as______. ii) The number of moles of solute dissolved in one litre of the solvent is known as______. iii) The numbers of moles of a solute dissolved in one kilogram of the solvent is known as______. 6. bnbn Dr. Darshana Pandya, Assistant Professor, IITE- Gandhinagar