4 Volumetric Analysis

Dr. Meenu

Learning Objectives : To study the basics of volumetric analysis and know the following about self generated questions.

1. What is Volumetric Analysis?
2. What are the glassware requirements for volumetric analysis?
3. What are the standards and their salient features?
4. What are the basic requirements for volumetric analysis?
5. What are the different ways in which concentrations of analyte be reported?
6. What is a response curve and its characteristics?
7. What are the advantages of titrations?

1 . Introduction

The volumetric analysis involves the determination of volume of solution of known concentration (titrant) reacted with analyte. The strength of analyte is determined with titrant. The titrant is taken in burette and analyte which is also known as titre a known volume of which is taken in Erlenmeyer flask. The volumetric analysis is based on the principle that equivalent weight of titrant react with titre at equivalent point. The combination is detected with the help of some color change with indicator or by electrochemical means. A well-known example is the titration of acetic acid (CH3COOH) in vinegar with sodium hydroxide NaOH,

2 Use of Reagents

To keep laboratory neat and clean tidy habits contribute a lot. Following are certain set rules which are to be observed while maintaining a laboratory.

• For analytical work select best grade chemicals.
• Always use the smallest bottle sufficient to do the job so that the wastage of chemical can be minimized.
• Returning of excess reagent back to bottle should be avoided as it may lead to contamination.
• Avoid inserting spatulas, spoons, or knives into a bottle that contains a solid chemical. Shake the capped bottle vigorously and tap it gently against a wooden table to break up an encrustation. Then pour out the desired quantity. If it does not work use a clean porcelain spoon.
• Keep the reagent shelf and the laboratory balance neat and clean.
• Always refer to Material Safety Datasheet (MSDS) of reagents for personal safety and accidental exposures.
• Clean up any spills immediately.
• Follow the lab instructions for disposal of surplus reagents and solutions.

3 Cleaning of Laboratory Ware

The glassware should be properly cleaned before its use in laboratory. Cleaning solution can be made by dissolving 25 g of sodium dichromate in 15 ml of water. To this solution add 450 ml of sulphuric acid carefully with stirring. Store the solution after cooling in stoppered bottle. The solution can be used effectively for cleaning until the appearance of green color in solution, which indicates the complete reduction of dichromate ions.

4 Heating Apparatus and Devices

4.1 Bunsen burners are generally used where flame of medium size with temperature (600 °C) is required.

4.2 Hot plate of various shapes and size with temperature monitoring are available. Wiring in hot plates is protected from chemical spillage.

4.3 Heating mantle consists of knitted fiber glass sheath which fit around a flask. The mantle is held in aluminum case and is commonly used for refluxing and distillation.

4.4 Electric oven is used for drying of precipitates at temperature 110°C – 120 °C.

4.5 Microwave oven used in the laboratory is slightly different from a home microwave. They are used for determining moisture content of materials and syntheses of materials through green route.

4.6 Muffle furnace are used to ignite samples where higher temperature up to 1200 °C is required.

5 Apparatus for Measuring Volumes

For volumetric analysis measurement is done with pipette, burette and volumetric flask. Volume measuring glassware could be classified in to two categories

a. TD (To deliver) glassware: TD glassware includes graduated cylinders, burettes, pipettes (Mohr and Serological)

b.  TC (To contain) glassware: TC glassware is specially calibrated with mark on the neck. TC glassware is supposed to contain a specific volume but may not deliver the same volume. A 250 ml volumetric flask will contain 250 ml solution if filled up to graduated mark but may not be able to deliver the same if it is inverted and drained.

5.1   Pipette: Pipette is used to transfer exact amount of liquid. Pipettes are calibrated for transfer of

0.1 ml to 25 ml of liquid. A small amount of liquid tends to remain in the tip after the pipette is emptied. This residual liquid is never blown out of a volumetric pipette or from some measuring pipettes, but it is blown out of other types of pipettes. Never pipette by mouth because there is risk of accidentally ingesting the liquid being pipetted. Instead, use a rubber suction bulb or one of a number of similar, commercially available devices.

There are three types of pipettes

5.1.1 Transfer Pipette: These pipettes deliver constant volume and have one mark. Transfer pipettes deliver an aliquot of a solution [Figure 2 (a)].

5.1.2 Graduated Pipette: These pipettes have graduated stems used to deliver small volumes according to requirement. Mohr pipette and serological pipettes fall in this category [Figure 2 (b) and (c)].

5.1.3 Syringe Pipette: These pipettes are used for rapid delivery and dispensing solutions in large numbers for multiple analyses [Figure 2 (d)].

Figure 2: Pipettes- (a) Transfer pipette, (b) Mohr pipette, (c) Serological pipette, (d) Syringe pipette

5.2 Burette: Burette is another calibrated tube to deliver titrant. The tube is fitted with a valve arrangement by which the flow of titrant is controlled. This valve is the principal source of difference among burettes. The top surface of a liquid confined in burette exhibits a marked curvature known as meniscus. For colorless solutions lower meniscus is to be observed to avoid parallax. For colored solutions upper meniscus is observed. The stopcock of the burette should be greased at regular interval avoiding the area adjacent to holes.

5.3 Volumetric flask: Volumetric flasks are used to make standard solution. Flasks are available from 5 ml to 5 L capacity. Insert a funnel into the neck of the volumetric flask, and use a stirring rod to direct the flow of liquid from the beaker into the funnel. With the stirring rod, tip off the last drop of liquid on the spout of the beaker. Rinse both the stirring rod and the interior of the beaker with distilled water and transfer the washings to the volumetric flask as before. The volumetric flask is never to be dried, particularly in oven.

6   Various requirements for volumetric analysis are

There should be single reaction occurring between desired constituent and titrant so that calculation could be carried out based upon the chemical change which occurs. There should be no side reaction.

• The reaction must be complete when equivalent amount of reactants have been brought together in a reaction vessel. Only on this basis we can make the calculation from volumetric data.
• The reaction must have well defined stoichiometry. It means that it should proceed according to molar relation between reactants to form definite product under given conditions.
• The reaction must be rapid so that little time is lost in obtaining the equilibrium during performance of the titration.
• A suitable standard solution must be available as the titrant for carrying out reaction. The suitable indicator must be available to mark the end point of titration.
• The equivalence and end point must almost coincide with each other.
• A solution of accurately known concentration is known as standard solution. It can be prepared directly or by standardization with a primary standard.

7    Standard Solution

To the solution whose concentration is known with great precision and which reacts stoichiometrically with the analyte. Standard solutions are referred to either as primary standards and secondary standards.

7.1 Primary standard: is a reagent that is extremely pure, stable, it should not be hygroscopic and has medium to high molecular weight. Ex- Sodium chloride, Sodium oxalate, Sodium bicarbonate.

7.2 Secondary standard: is a standard that is prepared in the laboratory for a specific analysis. It is usually standardized against a primary standard. Ex-Sodium hydroxide, Hydrochloric acid, Sulphuric acid.

7.3         Essential requirements for primary standard:

• It must be 100% pure or of known purity. The impurities if present must be inert w.r.t. the substance with which it is brought to react.
• The standard should be crystalline in nature which reduces the impurity.
• It should be stable at oven temperature used for drying. The hydrated substances are seldom used for drying.
• The standard solution should be unaltered by contact with air during weighing. It must not be hygroscopic or should not react with O2 or CO2 in air at room temperature.
• A  primary  standard  should  be  readily  available  at  reasonable  cost.  It  should  be preferably of high equivalent weight so that error associated with weighing is small.

8  Methods for expression of concentration

In an aqueous solution, the weight of solute in a given quantity of water is known as concentration. The concentration of solution is expressed by two methods a) the weight of solute dissolved in given volume of solution, b) the weight of solute dissolved in a given weight of water or solvent.

8.1 Weight of solute in given volume of solution:

8.1.1 Molarity: It expresses the number of moles of solute dissolved in sufficient water to produce one liter of solution. The molecular weight is the sum of atomic weights of elements indicated in formula of substances.

8.1.1.1 Analytical Molarity: Analytical molarity provides total number of moles dissolved in one liter. One molar solution of H2SO4 is prepared by dissolving 98.1 g of H2SO4 in water.

8.1.1.2 Equilibrium Molarity: Equilibrium molarity represents molar concentration of particular species at equilibrium. The equilibrium molarity of one molar H2SO4 is zero as the acid is completely dissociated.

8.1.2 Normality: The normality is expressed as the number of equivalent of dissolved solute contained in one litre of solution. A gram equivalent weight of an acid or base is that weight (in grams) which furnishes or reacts with one gram formula weight of H+ ions. The value of normality of a solution or that of equivalent weight of a solute depends upon the reaction which the solute undergoes. Consider orthophosphoric acid H3PO4, the normality of which varies depending upon the number of proton which it loses. It may lose one, two or three protons depending on the reaction conditions.

H3PO4  + OH-    → H2PO4-    +   H2O

H3PO4  + 2OH-   → HPO4-2    +    2H2O

H3PO4  + 3OH-   → PO4-3    +   3H2O

For the calculation of normality the exact nature of the intended chemical reaction must be clearly understood. The following mathematical relationship that serves as basis for computation in volumetric analysis can be derived from the definition of normal solution.

The product of normality by volume in liters furnishes the number of gram equivalent weight of the solute in the given volume. Thus 2 L of 1.5 N solution of reagent contain a total of 2 x 1.5 = 3 gram equivalent of the solute.

The product of millilitres (V) and normality represents not only the number of gram weight of solute contained in the given solution but also the number of gram weights of another substance that will react with first or that is chemically equivalent to the first. The relation may be written as

nA  x vA  (in L) = number of gram weights of A

nB  x vB (in L) = number of gram weights of B

where the subscript A and B indicate the different substances. In the same type of reactions, the number of A gram equivalent of A is consumed by the same number of gram equivalent of B. Therefore

nA  x vA  =   nB  x vB

when any of three of these terms are known, the fourth term can be calculated.

8.2 Weight of solute in a given weight of solvent: The most commonly employed methods are

8.2.1 Molality: A molal solution contains one mole of solute dissolved in 1000 g of solvent. This differs from molar solution in that the ratio of solute to solvent is determined by weight.

8.2.2 Percentage Composition: The % concentration of solution is the weight in grams of the solute contained in 100 g of the solution. It may also indicate the volume of concentrated liquid solute in given volume of solvent for example 20 ml of glacial acetic acid diluted in water to 100 ml is known as 20 % of acetic acid.

Percentage composition may indicate weight of solute in 100 ml of solution. Other concentration units such as mole ratio, mole fraction and mole percentage are employed in some cases.

8.2.3 Parts per million: concentration of very dilute solutions are conveniently expressed in ppm

ppm =    weight of solute/weight of solvent       x  10⁶

The distinction between weight and volume of solution is not normally made in very dilute aqueous solution.

9   Types of reactions in volumetric analysis

The reactions are categorized in four different classes.

9.1 Neutralization reactions: These reactions are characteristics of acid-base reactions resulting into a reaction between H+ and OH- resulting in water in the product in aqueous solution.

HA + OH^–   →      A^–   +  H2O

B + H3O⁺ → BH⁺   +  H2O

9.2 Redox reactions: Reactions where one of the reactant accepts e- and simultaneously the other reactant looses e-. The reactant which shows decrease in oxidation number of the central atom is oxidizing agent while other reactant which shows increase in oxidation number of central atom is reducing agent.

A_ox  +  B_red       →       A_red       + B_ox

9.3 Precipitation reactions: These reactions involve combination of ions to form undissolved molecule or complex which precipitates as a solid.

M(aq) + nL (aq) → MLn (s)

9.4 Complexation reactions: These reactions involve combination of metal ions with ligand to form complex.

M (aq) + nL → MLn (aq)

10   Titration Curves

Two types of curves are generally observed in volumetric titrations a) sigmoid curve, b) linear segment curve.

10.1 Sigmoid curve: the p-function (negative logarithm) of analyte is plotted as function of reagent volume. The important observations are made in 0.1 ml to 0.5 ml of equivalent point.

Thus in case of acid-base titrations pH, in case of complexometric and precipitations –log[metal] or –log[anion] is taken into consideration whereas in case of redox titrations [oxidant] or [reductant] and its p-function is taken into consideration.

10.2 Linear segment curve: in this curve before and after equivalence point important observations are made. The instrument readings are plotted against vertical axis w. r. t. analyte concentration along horizontal axis.

11    Advantages of Titration

There are several reasons for the wide acceptance of titration in laboratories worldwide:

• Titration is fast.
• Titration is an established analytical technique.
• It is a very accurate and precise technique.
• A high degree of automation can be implemented.
• Titration offers a good price/performance ratio compared to more sophisticated techniques.
• It can be used by low-skilled and low-trained operators.

12 Tips for Volumetric Analysis:

• For all titrations distilled water must be used as solvent.
• Stoppered bottles or tubes should be used for weighing.
• Only analytical grade weights should be used and they should be handled with forceps.
• Solutions should be homogeneous.
• The solution which needs longer time for dissolution should be prepared 12 hrs before the titration.
• The burette and pipette should be rinsed twice before titration.
• For each titration burette should be filled to zero mark so as to avoid inaccurate readings.
• The concordant set of readings should be obtained for each set of titration.
• Two to three drops of indicators should be used for titration. The concentration of indicator should be same for every titration.
• Base solution should be stored in polythene bottles.

Bibliography

1. G. Marr and B.W. Rocket, ‘Practical Inorganic Chemistry’, University Science Books, 1999.
2. G. Pass and H. Sutcliffe, ‘Practical Inorganic Chemistry’, Chapman and Hall, London, 1968.
3. Vogel’s Textbook of Quantitative Chemical Analysis, Arthur Israel Vogel, Prentice Hall, 2000.
4. J. Mendham, R. C. Denney, J. D. Barnes, M. Thomas, ‘Vogel’s Textbook of Quantitative Analysis’, Pearson Education, 2006.
5. A Douglas,   Skoog   and   Donald   M.   West,   F.J.   Holler,   ‘Fundamentals   of   Analytical Chemistry’, 8thEdn., Harcourt College Publishers, 2004.