Volumetric Analysis ( Titrimetric analysis) or Titration

Introduction of titration, types, equipment,terminology,Indicators,methods

1. VOLUMETRIC ANALYSIS {TITRIMETRIC ANALYSIS} By Aman Kakne

2. Analytical Chemistry It is the branch of chemistry which deals with the analysis of substances. It is mainly divided into two parts. (1) Qualitative Analysis (2) Quantitative Analysis

3. Qualitative analysis It is primarily concerned with the identification of the constituents present in a chemical substance or mixture of substances. QUANTITATIVE ANALYSIS It is primarily concerned with the exact determination of the quantity of constituents present in a chemical substance or a mixture of substances.

4. Introduction to volumetric analysis Volumetric analysis is also known as titrimetric analysis. Volumetric analysis is a mode of quantitative analysis which is based on the determination of the volume of a solution of known concentration (standard) required to react quantitatively with a solution of the substance to be analysed. The process of finding out the volume of the standard solution required to react completely with a known volume of solution under analysis is known as titration.

5. Equipment The key equipment used in a titration are: Burette Burette Stand Measuring cylinder White tile – used to see colour change in the solution Pipette pH indicator Conical Flask

6. Terms in Volumetric analysis Titrate : The substance to be determined is termed as the titrate. Titrant : The substance of known concentration is termed as titrant. Titration : The process of determining the volume is termed as titration. End point : The point at which the reaction is complete is termed as the end point. Indicator : Some auxiliary substance which is used to detect the end point of the titration is termed as the indicator. Standard solution : The solution of known concentration is termed as the standard solution .

7. PRIMARY STANDARD A standard solution can be obtained by dissolving a definite weight of a substance having purity called a primary standard in a definite volume. SECONDARY STANDARD A secondary standard is a standard that is prepared in the laboratory for a specific analysis . It is usually standardized against a primary standard .

8. Concentrations terms in volumetric analysis In volumetric analysis, the various concentration terms are used. Normality Molarity Molality Weight Percent Equivalent weight Strength

9. Classification of titration Acid- Base Titration : When the neutralisation reaction is involved, then the titration is termed as acid-base or neutralisation titration. NaOH + HCl NaCl + H2O Redox Titration : These are involving oxidation reduction reactions. 10FeSO4 + KMnO4 + H2SO4 5Fe2(SO4)3 + K2SO4 + 2MnSO4 + 8H2O Precipitation Titration : These involve the formation of the precipitate. Precipitation of silver cations with halide is widely used in this analysis. AgNO3 + NaCl AgCl + NaNO3

10. Complexometric titration : When formation of stable complex is the reaction involved, certain organic reagents such as Ethylene diamine tetra acetic acid (EDTA) forms a stable complex with various metal ions and is thus used for analysis of these metal ions. Non-aqueous titration : When the reaction occurs in non-aqueous solvents it means organic solvents, this is mainly used for analysis of very weak acids and bases Diazotisation : These titrations involve the diazotisation reactions.

11. Acid–base titration The reaction between acids and bases are termed as neutralisation reactions. These reactions take place rapidly and it becomes possible to find equivalence points of such titrations. Hence such acid- base titrations have been possible. It has been possible to divide these titrations into: Acidimetry: The titrations of substances which act as bases under the conditions of titrations with standard acid are termed as acidimetry. Alkalimetry: The titrations of substances which act as acids under the conditions of titrations with standard base are termed as alkalimetry. In acidimetry – alkalimetry, the reactions involve neutralisation with the formation of salt and water. NaOH + HCl NaCl + H2O

12. Acidimetry In acidimetry, a known volume of an alkaline substance or a base is put into a conical flask. This solution is then titrated against a standard solution of an acid taken in burette till an equivalent quantity of acid is added to the alkali. The point at which the acid and base are added in equivalent amounts is termed as the equivalent point. If both the acid and alkali are strong electrolytes, the resulting solution would be neutral and is having pH7. On the other hand, if acid or base or both acid and base are weak electrolytes, the salt formed gets hydrolysed to a certain extent and the resulting solution would be slightly acidic or alkaline at the equivalence point and pH will not be seven.

13. Alkalimetry In alkalimetry , a known volume of an acidic substance is taken into a conical flask. This solution is then titrated against a standard solution of an alkali or base taken in a burette till an equivalent quantity of alkali or base is added to the acid. The point at which the acid and base are added in equivalent amounts is termed as the acid equivalence point . The pH of the resulting solution will be 7 if both acid and base are strong electrolytes. The pH of the resulting solution will be less or greater than 7 if acid or base both acid and base are weak electrolytes .

14. indicators Acid-base indicators : These are the indicators which are used in acid-base titration to locate their equivalence point. These indicators have one colour solution while an altogether different colour in alkaline solution . The colour change of an acid-base indicator is not sudden but takes place within a small range of pH value. This range is called the pH range of an indicator.

15. Indicator Colour on acidic side Range of colour change Colour on basic side Methyl violet Yellow 0.0-1.6 Violet Bromophenol blue Yellow 3.0-4.6 Blue Methyl orange Red 3.1–4.4 Yellow Methyl red Red 4.4–6.3 Yellow Litmus Red 5.0–8.0 Blue Bromothymol blue Yellow 6.0–7.6 Blue Phenolphthalein Colourless 8.3–10.0 Pink Alizarin yellow Yellow 10.1–12.0 Red

16. Acid–base titration is performed with a  bromothymol blue  indicator, when it is a strong acid – strong base titration , a  phenolphthalein  indicator in weak acid – strong base reactions, and a  methyl orange  indicator for strong acid – weak base reactions . If the base is off the scale, i.e. a pH of >13.5, and the acid has a pH > 5.5, then an  Alizarine yellow  indicator may be used. On the other hand, if the acid is off the scale, i.e. a pH of <0.5 , and the base has a pH < 8.5, then a  Thymol Blue  indicator may be used . Indicators used

17. An acid-base titration using phenolphthalein as the indicator. The conical flask contained solution that just reached the endpoint. Before starting the titration a suitable  pH indicator  must be chosen. The  equivalence point  of the reaction, the point at which equivalent amounts of the reactants have reacted, will have a pH dependent on the relative strengths of the acid and base used. The pH of the equivalence point can be  estimated  using the following rules : A strong acid will react with a strong base to form a neutral (pH = 7) solution. A strong acid will react with a weak base to form an acidic ( pH < 7) solution. A weak acid will react with a strong base to form a basic (pH > 7) solution . Method

18. When a weak acid reacts with a weak base, the equivalence point solution will be basic if the base is stronger and acidic if the acid is stronger. If both are of equal strength, then the equivalence pH will be neutral. However, weak acids are not often titrated against weak bases because the colour change shown with the indicator is often quick, and therefore very difficult for the observer to see the change of colour . The point at which the indicator changes colour is called the end point. A suitable indicator should be chosen, preferably one that will experience a change in colour (an end point) close to the equivalence point of the reaction. method

19. First , the burette should be rinsed with the standard solution, the pipette with the unknown solution, and the conical flask with distilled water. Secondly , a known volume of the unknown concentration solution should be taken with the pipette and placed into the conical flask, along with a small amount of the indicator chosen. The known solution should then be allowed out of the burette, into the conical flask. At this stage we want a rough estimate of the amount of this solution it took to neutralize the unknown solution. The solution should be let out of the burette until the indicator changes colour and the value on the burette should be recorded. This is the first (or rough) titration volume and should be excluded from any calculation . method

20. method At least three more titrations should be performed, this time more accurately, taking into account roughly where the end point will occur. The initial and final readings on the burette (prior to starting the titration and at the end point, respectively) should be recorded. Subtracting the initial volume from the final volume will yield the amount of titrant used to reach the end point. The end point is reached when the indicator just changes colour permanently.

21. An acid-base titration using phenolphthalein as the indicator. The conical flask contained solution that just reached the endpoint

22. Titration of weak acid The pH of a  weak acid  solution being titrated with a strong base solution can be found at different points along the way. These points fall into one of four categories : initial pH pH before the equivalence point pH at the equivalence point pH after the equivalence point

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