Lecture Notes: Volumetric Methods of Analysis PDF

Republic of the Philippines President Ramon Magsaysay State University (Formerly Ramon Magsaysay Technological University) Iba, Zambales, Philippines Tel/ Fax No: (047) 811- 1683...

Republic of the Philippines President Ramon Magsaysay State University (Formerly Ramon Magsaysay Technological University) Iba, Zambales, Philippines Tel/ Fax No: (047) 811- 1683 COLLEGE OF ARTS AND SCIENCES CHEM BIO II: ANALYTICAL METHODS FOR BIOLOGY Chapter 6 VOLUMETRIC METHODS OF ANALYSIS LEARNING OUTCOMES At the end of the lesson, the student must be able to: Describe the different volumetric methods of analysis Perform calculations involving volumetric methods OVERVIEW OF VOLUMETRIC METHODS OF ANALYSIS Volumetric analysis is a widely used quantitative analytical method. It is also called titrimetric analysis. As the name implies, this method involves the measurement of volume of a solution of known concentration which is used to determine the concentration of the analyte. Volumetric analysis is a chemical analytical procedure based on the measurement of volumes of reaction in solutions. It uses titration to determine the concentration of a solution by carefully measuring the volume of one solution needed to react with another. Titration is a category of volumetric analysis. Volumetric analysis methods involve titrations. However, the term titration is used when volumetric analysis is used to determine the concentration of an unknown component in a solution. The first method is exemplified in a procedure devised by a French chemist, Jean-BaptisteAndré Dumas, for determining the proportion of nitrogen combined with other elements in organic compounds. A weighed sample of the compound is burned in a furnace under conditions that ensure the conversion of all the nitrogen to elemental nitrogen gas, N2. The nitrogen is carried from the furnace in a stream of carbon dioxide that is passed into a strong alkali solution, which absorbs the carbon dioxide and allows the nitrogen to accumulate in a graduated tube. The mass of the nitrogen can be calculated from the volume it occupies under known conditions of temperature and pressure, and therefore the proportion of nitrogen in the sample can be determined. BASIC PRINCIPLES OF VOLUMETRIC ANALYSIS The solution to be analyzed contains an unknown amount of chemicals. The reagent of unknown concentration reacts with a chemical of an unknown amount in the presence of an indicator (mostly phenolphthalein) to show the end-point. It’s the point indicating the completion of the reaction. The volumes are measured by titration which completes the reaction between the solution and reagent. The volume and concentration of reagent which are used in the titration show the amount of reagent and solution.  The amount of unknown chemical in the specific volume of solution is determined by the mole fraction of the equation. Important terms of volumetric analysis are: Normality: number of solute gram-equivalents /volume of solution(liters) Molarity: moles of solute/volume of solution Endpoint: It's when the reaction between the two solutions is almost finished. Indicator: A material that shows the achievement of a result. The color of the indicator changes at the termination point. Standard solution: A standard solution is a solution that has a known concentration. PROCEDURE FOR VOLUMETRIC ANALYSIS A typical titration starts with a beaker or flask containing a precise volume of the analyte and a small amount of indicator placed underneath a calibrated burette or pipette containing the titrant. The solution that needs to be analyzed needs to have an accurate weight in the sample of +/- 0.0001g of the material to be analyzed. Choosing the right kind of material to be analyzed is also very important, as choosing the wrong type of titrant will give us the wrong results. A substance that reacts rapidly and completely to produce a complete solution is chosen. Small quantities of titrant are added to the analyte and indicator till the indicator changes color in reaction to the titrant saturation threshold reflecting the arrival at the endpoint of the titration. The titration has to be continued up until the reaction is complete and the amount of reactant added is exactly the amount that is needed to complete the reaction. Another important step is measuring the right volume of the standard solution since molarity is a standard metric to calculate the number of moles present in a solution. Based on the desired endpoint, single drops or less than a drop of the titrant makes a difference between a permanent and temporary change in the indicator. If the reagent or reactant that we use is to be made into a standard solution then we can weigh and dissolve the reagent into a solution so that it is in a definitive volume within a volumetric flask. Apparatus Used for Volumetric Analysis The apparatus used for Volumetric Analysis are: Burette, pipette, measuring flasks, and measuring cylinders. Flasks for general titration, beaker, tile, glass rod, funnel, weighing bottle, and wash bottle. Weighing machine with a chemical balance 1|Page ACID-BASE INDICATORS Acid–base indicators are compounds that change color at a particular pH. They are typically weak acids or bases whose changes in color correspond to deprotonation or protonation of the indicator itself. Acid-base indicators are sensitive to pH change. For most acid-base titrations, it is possible to select indicators that exhibit color change at pH close to the equivalence point. Examples are phenolphthalein and methyl orange. Phenolpthalein is a weak acid, therefore it does not dissociate in the acidic medium and remains in the unionized form, which is colorless. Ionized and unionized forms of phenolphthalein are given below : HPh ⇌ H+ + Ph– Unionized Ionised Colorless Pink (Colourless in acid) (Pink in alkali) For a weak acid vs strong alkali titration, phenolphthalein is the most suitable indicator. This is so because the last drop of added alkali brings the pH of the solution to the range in which phenolphthalein shows sharp color change. When titration between strong base and weak acid is to be performed then phenolphthalein is a good indicator. In this case alkali is dropped from the burette and acid is taken in the tiration flask. Color of the solution taken in the titration flask changes from colorless to pink. This change of color is easily perceptible to the human eye. If we take alkali in the titration flask, the color change will be from pink to colorless and accuracy in noting the color change may be less. In the titration of strong acid versus strong base, anyone of the above indicators can be used. For the titration of weak acid vs weak base, no indicator is available. Methyl orange is a weak base 2|Page and is yellow in color in the unionized form. In the titration of a strong acid and a weak base, methyl orange is chosen as an indicator. OXIDATION-REDUCTION INDICATORS Oxidation-reduction reactions are chemical reactions that involve the transfer of electrons between the reacting species. These electron transfers are accompanied by a change in the oxidation state of the reactants. Redox titration is based on an oxidation-reduction reaction between the titrant and the analyte. It is one of the most common laboratory methods to identify the concentration of unknown analytes. In order to evaluate redox titrations, the shape of the corresponding titration curve must be obtained. In these types of titration, it proves convenient to monitor the reaction potential instead of monitoring the concentration of a reacting species. As discussed earlier, redox reactions involve both oxidation and reduction. The key features of reduction and oxidation are discussed below. Reduction A substance can undergo reduction can occur via: The addition of hydrogen. The removal of oxygen. The acceptance of electrons. A reduction in the overall oxidation state. Oxidation The following points describe a substance that has undergone oxidation. The addition of oxygen. Removal of hydrogen which was attached to the species. The donation/loss of electrons. An increase in the oxidation state exhibited by the substance. 3|Page Thus, it can be understood that redox titrations involve a transfer of electrons between the given analyte and the titrant. An example of redox titration is the treatment of an iodine solution with a reducing agent. The endpoint of this titration is detected with the help of a starch indicator. In the example described above, the diatomic iodine is reduced to iodide ions (I –), and the iodine solution loses its blue color. This titration is commonly referred to as iodometric titration. https://edurev.in/t/94886/Volumetric-Analysis-Redox-Reactions--Physical-Chem https://www.wiredchemist.com/chemistry/instructional/laboratory-tutorials/volumetric-analysis https://www.britannica.com/science/volumetric-analysis https://byjus.com/chemistry/volumetric-analysis/ 4|Page