Chemistry Experiment: Volumetric Analysis
Volumetric analysis is a standard method for determining an unknown concentration. With this method, a measured volume of a standard solution is mixed with a measured volume of the solution with an unknown concentration. Another name for volumetric analysis is titration. The element whose concentration needs to be calculated is referred to as titrate. The component whose solution is used to calculate the concentration of an unknown solution is known as the titrant. Volumetric analysis is additionally known as titrimetric analysis.
Table of Content
Aim of the Experiment
Material Required
Theory
Procedure
Precautions
Observation
Result
Lab Manual Questions
Practical based Questions
Viva Questions
Summary
Aim of the Experiment
To determine the concentration of an unknown component in a solution by volumetric analysis technique.
Material Required
A 100 mL conical flask
A Graduated-burette
A Graduated-Pipette
Weighing tube
Watch glass
Measuring flask
Measuring Cylinder
Beaker (250 ml)
Glass rod
Tile
Chemical balance
Wash bottle
Funnel
Funnel stand
Distilled water
Theory
Titration
Titration, also referred to as titrimetry, is a method used to determine the concentration of a specific analyte in a mixture that is used in chemical qualitative analysis. Titration, which is also sometimes referred to as volumetric analysis, is a crucial analytical chemistry technique.
Types of Titration:
Titration can take three different forms:
Simple Titration
When a concentration of one standard solution is known, it is possible to apply simple titration to determine the concentration of an unknown solution.
Back Titration
Back titration is also known as Indirect titration. It uses a known volume of an excess reagent to determine an analyte's concentration, whereas a direct titration looks directly at the concentration of an unknown analyte. When conducting back titrations, more reagents are not introduced.
Double Titration
To compare the concentration of a substance present in a solution with another solution, a double titration is conducted. Double titration is another name for Gilson's titration technique, which determines how much organometallic substance is present in a hydrocarbon solvent.
Principle of Titrations
The following are some of the volumetric analysis's guiding principles:
The solution, which needs to be investigated, must contain an unspecified amount of compounds.
When an indicator is present, the reaction between an unknown chemical and a reagent at an unknown concentration demonstrates the end-point (usually phenolphthalein). The reaction has completed its course at the end-point.
Titration is used to measure the volumes once the reaction between the solution and the reagent is complete.
Titration is used to measure the volumes once the reaction between the solution and the reagent is complete.
The volume and concentration of the reagent used in the titration indicate the amount of reagent and solution.
The amount of an unknown chemical in the specified volume of solution is determined by the mole fraction of the equation.
When the reaction reaches its endpoint, the volume of reactant consumed is measured and used to calculate the analyte's volumetric analysis using the formula below.
$C_{a}=C_{t}\times V_{t}\times \frac{M}{V_{a}}$
Where,
The analyte concentration, usually expressed in molarity, is Ca.
The titrant concentration is usually expressed in molarity Ct.
V is the used titrant's volume, usually expressed in litres.
M is the analyte and reactant's mole ratio according to the balanced equation.
Terms Related To Volumetric Titrations
Standard Solution: Solutions containing a known concentration of a chemical are known as standard solutions. They are used to determine or calculate the amounts of unknown substances. Standard solutions are prepared using primary standards.
Indicator: Indicators are weak acids or bases that change colour as hydrogen ion concentration in a solution or pH varies. Litmus, turmeric and phenolphthalein are examples of indicators.
Endpoint: The point in a titration when an indicator indicates that a solution has obtained the required amount of reactant to complete a reaction.
Molarity: Molarity, or molar concentration, is the ratio of a substance's moles per litre of solution.
Normality: The number of grams of solute present per unit of solution volume is referred to as normality.
Preparation of Standard Solution
Calculating the required amount of the substance is the first step in creating a standard solution.
For example, if we want to make 250.0 mL of a 0.0500 mol/L solution of sodium carbonate:
$\\n = c \times V \\ \\n(Na_2CO_3) = 0.0500 \times 0.250 = 0.0125 \hspace{0.1cm} mol \\ \\m = n \times MM \\ \\m(Na_2CO_3) = 0.0125 \times (2 \times 22.99 + 12.01 + 3 \times 16.00) \\ \\m(Na_2CO_3) = 1.33 \hspace{0.1cm} g \hspace{0.1cm}$
So, to make this solution, 1.33 g of sodium carbonate is needed.
Now, 1.33 g of sodium carbonate from step 1 is dissolved in a small amount of distilled water in an appropriate-sized beaker. To assist the solid dissolve fully, use a stirring stick.
Using a funnel, transfer the mixture to a 250.0 mL volumetric flask. Prior to use, make sure the volumetric flask has been washed with distilled water. The volumetric flask should be cleaned with water since distilled water will eventually be added to the flask to create the standard solution.
Make sure all of the sodium carbonate has been transferred completely by thoroughly cleaning the volumetric flask, stirring rod, and beaker with distilled water.
Add distilled water to the volumetric flask once the solution has been placed there until the level is 1 cm below the graduation mark of the flask.
Drop by drop, add distilled water using a plastic pipette until the meniscus's bottom reaches the graduation point.
Invert the flask up to 10 times after stopping it.
Procedure
Beginning with a calibrated burette or pipette containing the titrant, a beaker containing a measured volume of the analyte and a small amount of indicator is placed beneath it.
The amount of substance to be evaluated in the solution must be accurately weighed in the sample to within +/- 0.0001g.
Making the proper material selection for analysis is crucial, as using the incorrect titrant will produce inaccurate results. A chemical is chosen that interacts quickly and thoroughly to create a full solution.
As the indicator's colour changes in response to the titrant saturation threshold, indicating the titration's endpoint, small amounts of titrant are gradually added to the analyte and the indicator.
The titration must be continued until the reaction is finished, and the amount of reactant added must match the amount required to finish the reaction perfectly.
Since molarity is a common metric for calculating the number of moles in a solution, measuring the correct volume of the standard solution is another crucial step.
Single drops or less of the titrant determine whether the indication changes permanently or only temporarily, depending on the desired endpoint.
Weighing and dissolving the reagent into a solution will give it a specific volume within a volumetric flask if the reagent or reactant we employ is to be put into a standard solution.
Observation Table
Result
Molarity of the given KMnO4 solution is _________ moles/litre.
The strength of the given solution is _______ g/L.
Precautions
There should not be any solution hanging at the burette's tip or endpoint.
Check to make sure the stopcock is not leaking.
As soon as the burette is filled, remove the funnel.
Always situate the eye precisely at the level of the meniscus and use an anti-parallax card.
Lab Manual Questions
1. Why should a pipette be never held from its bulbs?
Ans. Holding the pipette close to or below the bulb is not advised. This is because holding it will allow stress to be applied to the bulb, which is the pipette's weakest component. Sharp glass fragments sometimes embed themselves in a student's one or both hands if the pipette breaks while being placed into the filler.
2. Why is the last drop of solution not blown out the pipette?
Ans. This is because volumetric pipets are calibrated to account for the solution that settles at the tip owing to surface tension, they should not be "blown out" to eject all liquid at the tip.
3. Why does a burette with rubber pinch cork should not be in acidic titrations like KMnO4?
Ans. Because permanganate attacks rubber, a burette with a pinch cork regulator is not utilised for the titration of permanganate.
4. Why is the front door of chemical balance closed while measuring the weight of the sample?
Ans. While weighing something, shut the balance door to prevent air currents from affecting the reading. To stop dirt and dust from getting into the balance after finishing, the operator should shut the balance door.
Viva Questions
1. Name the apparatus used in a volumetric titration.
Ans. The pipette, the graduated cylinder, and the burette are the three most often used instruments used in a volumetric analysis.
2. Why shouldn't a titration flask be rinsed before use?
Ans. Since some liquid will stay stuck in the titration flask after rinsing, it is necessary to rinse the flask. This will increase the pipetted volume used in the titration flask.
3. Why are burettes and pipettes required to be rinsed with the solution they are filled with?
Ans. In order to avoid water from clinging to the sides of the burettes and pipettes, which would otherwise lower the volume of the solution taken into it, they are cleaned with the solution with which they are filled.
4. What is the other name for double titration?
Ans. The other name for double titration is Gilson's titration which establishes the concentration of an organometallic compound in a hydrocarbon solvent.
5. What various kinds of volumetric methods are there?
Ans. There are three different categories for volumetric analysis:
Simple titration, Back titration, and double titrations.
6. What is the purpose of preparing a standard solution for volumetric analysis?
Ans. To calculate the analyte concentration during titration, a standard solution is employed. The number of moles in the reaction's solution is determined using this method. For this purpose, the volumetric flask is provided with the scale marks.
7. What impact does temperature show in volumetric titrations?
Ans. When the temperature changes, the equilibrium constant also changes, which can have an impact on titrations by moving equilibria in one way or another and adding sources of error to the titration process.
Practical Based Questions
A strong acid and a weak base are titrated using which of the following as an indicator?
Methyl orange
Phenolphthalein
Thymol blue
Fluorescein
Answer: a)
2) The ideal indicator should fall between the pH range of _____ when used to titrate strong acids and weak bases.
a) 3-5
b) 4-6
c) 6-8
d) 7-9
Answer: b)
3. Which titration will have the equivalence point at a pH greater than 8?
a) NaOH and CH3COOH
b) NaOH and HCl
c) NH3 and CH3COOH
d) NH3 and HCl
Answer: c)
4. If 25 ml of the barium hydroxide solution is titrated with 35 ml of 0.1 M HCl, what is the molarity of the barium hydroxide solution?
a) 0.28
b) 0.21
c) 0.14
d) 0.07
Answer: d)
5. When titrating a weak acid and a strong base, which of the following is employed as an indicator?
a) Methyl orange (3 to 4.6)
b) Bromothymol blue (6 to 7.5)
c) Phenolphthalein (8 to 9.6)
d) Methyl red (5 to 6.9)
Answer : c)
6. If 50 ml of 0.5 M Ba(OH)2 solution is used to neutralise 100 ml of sulphuric acid solution, what is the concentration of the resulting solution?
a) 0.25 M
b) 0.5 M
c) 25 M
d) 50 m
Answer : a)
7. A process called a titration is used to
determine the identity of an unknown substance
determine the volume of an unknown substance
determine the molarity of an unknown substance
determine if an unknown substance is a solid, liquid, or gas
Answer: c)
8. Phenolphthalein changes its colour in
Acidic medium
Basic medium
Neutral medium
All of the above
Answer: b)
Summary
Volumetric analysis is a method of quantitative analysis. It is a technique where the volume of a substance is measured along with the volume of another substance that mixes with the first in a specific ratio to determine how much of the substance is present. This is sometimes referred to as titration or titrimetric analysis.
FAQs on Volumetric Analysis
1. What are primary and secondary standard solutions? Why are they important in volumetric analysis?
In volumetric analysis, the accuracy of results depends on using a solution of a precisely known concentration. These are categorised as:
- Primary Standard Solution: A solution prepared by dissolving an accurately weighed amount of a highly pure, stable substance in a known volume of solvent. It does not decompose or react with the atmosphere. Example: Anhydrous sodium carbonate (Na₂CO₃) or hydrated oxalic acid (H₂C₂O₄·2H₂O).
- Secondary Standard Solution: A solution whose concentration is determined by titrating it against a primary standard solution. These substances are often less pure or less stable. Example: Sodium hydroxide (NaOH), which is hygroscopic, or potassium permanganate (KMnO₄).
They are important because a primary standard acts as the ultimate reference for all volumetric calculations.
2. How is the correct indicator selected for an acid-base titration? Provide examples for different types of titrations.
The correct indicator is selected based on its pH range of colour change. This range must coincide with the steep pH jump that occurs at the equivalence point of the titration. A poor choice of indicator will result in an incorrect end point.
- Strong Acid vs. Strong Base (e.g., HCl vs. NaOH): The equivalence point is at pH 7. Both Phenolphthalein (pH range 8.2-10) and Methyl Orange (pH range 3.1-4.4) can be used because the pH jump is very large (approx. 3.5 to 10.5).
- Weak Acid vs. Strong Base (e.g., CH₃COOH vs. NaOH): The equivalence point is in the alkaline range (pH > 7). Therefore, Phenolphthalein is the suitable indicator.
- Strong Acid vs. Weak Base (e.g., HCl vs. NH₄OH): The equivalence point is in the acidic range (pH < 7). Therefore, Methyl Orange is the suitable indicator.
3. What is the fundamental difference between the 'equivalence point' and the 'end point' in a titration?
This is a frequently asked and important concept in volumetric analysis. While often used interchangeably, they are different:
- The Equivalence Point is a theoretical point in a titration where the amount of titrant added is chemically equivalent to the amount of analyte in the sample, according to the stoichiometry of the reaction. It cannot be observed directly.
- The End Point is the experimental point in a titration where a physical change, such as a colour change from an indicator, signals that the reaction is complete.
An ideal titration ensures the end point is as close as possible to the equivalence point to minimise titration error.
4. Why is KMnO₄ known as a self-indicator, and why must titrations involving it be performed in the presence of dilute H₂SO₄?
This is a Higher Order Thinking Skill (HOTS) question often asked in board exams.
- Self-Indicator: Potassium permanganate (KMnO₄) acts as its own indicator because the permanganate ion (MnO₄⁻) has an intense purple colour, while the product it forms upon reduction, the manganese(II) ion (Mn²⁺), is colourless. The first drop of excess KMnO₄ after the equivalence point imparts a permanent pink colour to the solution, signalling the end point.
- Use of Dilute H₂SO₄: The titration requires an acidic medium. Dilute H₂SO₄ is used because it is a strong acid that does not react with KMnO₄. HCl is not used because KMnO₄ is a strong enough oxidising agent to oxidise chloride ions (Cl⁻) to chlorine gas (Cl₂), which would lead to an inaccurate reading. HNO₃ is not used because it is an oxidising agent itself and would interfere with the reaction.
5. What are four essential precautions an examiner expects you to follow during a volumetric analysis practical exam?
To ensure accuracy and obtain reliable results, the following precautions are considered important:
- Rinse the burette and the pipette with the respective solutions they are to be filled with, not with water.
- Ensure there are no air bubbles in the burette tip before starting the titration.
- Always read the lower meniscus for colourless solutions and the upper meniscus for coloured solutions like KMnO₄ to avoid parallax error.
- Add the titrant drop by drop, especially near the end point, and swirl the conical flask continuously to ensure proper mixing.
6. If a student forgets to rinse the pipette with the solution to be measured (analyte) and instead uses a water-rinsed pipette, how will this mistake affect the final calculated concentration? Explain your reasoning.
This is an important question on experimental error. If a water-rinsed pipette is used, some water droplets will remain inside. When the analyte solution is drawn into this pipette, it will be slightly diluted. This means the actual amount (moles) of analyte transferred to the conical flask will be less than what is assumed from the pipetted volume. To neutralise this smaller amount of analyte, less volume of the titrant from the burette will be required. As a result, the calculated concentration of the analyte will be incorrectly reported as lower than its actual value.
7. What is 'back titration'? In what situations is this method considered more important than a direct titration?
Back titration, or indirect titration, is a technique where a known excess amount of a standard reagent is added to the analyte. The unreacted excess reagent is then titrated with another standard solution. It is considered an important method in the following situations:
- When the reaction between the analyte and titrant is very slow.
- When the analyte is a volatile substance.
- When the analyte is an insoluble solid (e.g., determining the purity of calcium carbonate).
- When the end point of the direct titration is difficult to detect or unstable.
8. A student prepares a standard solution for titration by dissolving 1.575 g of hydrated oxalic acid (H₂C₂O₄·2H₂O) in water to make a 250 mL solution. Calculate the molarity of this solution, a typical question for your exam. (Molar mass of H₂C₂O₄·2H₂O = 126 g/mol)
This is a standard numerical problem expected in the CBSE Class 12 Chemistry exam. Here is the step-by-step calculation:
- Step 1: Calculate the moles of oxalic acid.
Moles (n) = Given mass / Molar mass
n = 1.575 g / 126 g/mol = 0.0125 mol - Step 2: Convert the volume of the solution to Litres.
Volume (V) = 250 mL = 250 / 1000 L = 0.250 L - Step 3: Calculate the Molarity.
Molarity (M) = Moles of solute / Volume of solution in Litres
M = 0.0125 mol / 0.250 L = 0.05 M
The molarity of the prepared oxalic acid solution is 0.05 mol/L.
