Procedure, Calculations, and Key Precautions Explained
This technique is used to determine the unknown concentration of an acid or a base by neutralizing it with an acid or base of known concentration. The process of determination of the strength of a solution of acid by titration with a standard solution of a base is termed acidimetry, on the other hand when the strength of a solution of an alkali is determined using titration with a standard solution of an acid it is called as alkalimetry. Now we will learn about the titration practical for titration of oxalic acid for the estimation of Sodium Hydroxide.
Aim
To determine the strength of the sodium hydroxide solution by titrating it against a standard solution of oxalic acid.
Theory
Here, the sodium hydroxide solution is not a primary standard and is taken in a burette and a known volume of the oxalic acid (a standard solution) is taken in the titration flask. The titration is carried out using phenolphthalein as an indicator. In an acid-base titration, the amount of acid becomes chemically equivalent to the amount of base present in the end. In the case of strong acid and strong base titration, the solution becomes neutral at the endpoint of the solution.
The reaction involved in the titration:
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Apparatus Required
Pipette
Burette
Burette stand
Conical flask
Stirrer
Measuring flask
Oxalic acid NaOH
Sodium hydroxide solution
Phenolphthalein indicator
Procedure:
Take a burette and wash it thoroughly with clean water and dry it.
Weigh the exact amount of dried watch glass and record its weight.
Using a funnel, shift oxalic acid carefully from the watch glass into a dried measuring flask.
Wash the watch glass with distilled water, the volume of distilled water should not be more than 50 ml.
Wash the funnel carefully using a wash bottle with distilled water, turn the flask of measurement until the oxalic acid dissolves.
Thoroughly add enough distilled water to the measuring flask just below the mark.
Add the last few drops of distilled water into the measuring flask until the reduced meniscus level just touches the mark.
Put the stopper on the mouth of the flask and shake it gently to make the entire solution uniform. Calculate the solution of oxalic acid M/20.
Estimation of Oxalic Acid by Titrating it with NaOH
Rinse the burette with the standard oxalic acid solution and take 10cm3 of oxalic acid solution in a titration flask. Fill the burette with sodium hydroxide solution and also remove the air gap if any.
In a pipette, take out 20ml of NaOH solution in a conical flask. Add two drops of phenolphthalein indicator into it and place it below the nozzle of the burette.
Now run the sodium hydroxide solution slowly and dropwise into the flask until a very faint permanent pink color is just obtained. Read the lower meniscus of the solution again in the burette and record it as the final burette reading. Repeat this procedure two to three times.
Observations
Molarity of oxalic acid solution = \[ \frac{M}{20}\]
Molarity of sodium hydroxide solution = m
The volume of oxalic acid solution = 10cm³
The indicator used = Phenolphthalein
Endpoint indication = Light pink colour
Calculations:
Mass of oxalic acid dissolved (100ml) in standard solution = x g
Estimation of sodium hydroxide
Strength of oxalic acid = X \[\times\] 10 g/L
Normality (N) of standard oxalic acid = Strength/ Equivalent weight = \[X \times \frac{10}{63.04}\]
Normality (N₁) of sodium hydroxide solution
\[N₁ x V₁ = N x V\]
\[N₁ = \frac{V}{V_1} \times N \]
Normality(N₂) of given oxalic acid solution
\[N₂ x V₂ = N₁ x V₁ \]
\[N_2 = \frac{V_1}{V_2} \times N_1 \]
Strength of given oxalic acid = N₂ x 63.04 g/L
Result: The strength of the given solution(NaOH) is _______ g/L.
Precautions
Wash the burette with water before and after titration is over.
Wash the watch glass carefully so that there is no crystal left on the watch glass.
The last few drops should be added using a pipette to avoid any extra addition of distilled water above the mark on the neck.
Properties of Sodium Hydroxide
Sodium Hydroxide is our main component in this titration process against oxalic acid. Let's understand some of the chemical properties of sodium hydroxide to understand the components used in the titration. Sodium hydroxide is an odorless and white crystalline substance that absorbs moisture from the air at the environmental or surrounding temperature. It's a synthetic chemical. When dissolved in water or neutralized with acid, it releases a significant amount of heat, which could ignite combustible objects. Sodium hydroxide is a highly corrosive substance. It's usually applied as a solid or a 50% solution. Caustic soda and lye are two more frequent names for this substance. Products like Dyestuffs, explosives, paper, rayon, Soaps, and petroleum are all made with the help of sodium hydroxide. Sodium hydroxide is also used in the processing of cotton and fabric, metal cleaning and processing, laundry and bleaching, electrolytic extraction, electroplating, and oxide coating, among other things. Commercial drain and oven cleansers frequently contain it.
Properties of Oxalic Acid
Another important component of titration is oxalic acid. Let's understand the properties of oxalic acid. The first member of the di-carboxylic acid group is oxalic acid. One molecule contains two carboxyl groups. Its potassium salt can be found in Rhubarb, Sorrel, and other oxalic group plants. Calcium oxalate is found in several plants and animals. It is created in the lab by oxidizing sugar cane sugar. It's created in a commercial setting by heating Sodium Formate. It comes in two forms: crystal and amorphous. Two molecules of Kelsan water make up one molecule of crystalline oxalic acid. The composition formula for amorphous oxalic acid is H2C2O4 and the molecular formula is H2C2O4.
Physical properties of Oxalic Acid
The physical properties of oxalic acid are also important to keep in mind from an exam point of view. The first thing to understand is that oxalic acid is present in a solid form that is colourless in its appearance and odourless in nature, that is there is no particular smell of it. Another form of oxalic acid is crystalline oxalic acid has a melting temperature of around 101.5°C, while anhydrous oxalic acid has a melting point of 189.5°C. Anhydrous oxalic acid is also soluble in alcohol and water and is unbreakable in ether.
FAQs on Titration of Oxalic Acid Against Sodium Hydroxide: Complete Guide
1. What is the fundamental principle behind the titration of oxalic acid with sodium hydroxide?
The fundamental principle is acid-base neutralization. In this experiment, a strong base, sodium hydroxide (NaOH), is used to neutralize a weak diprotic acid, oxalic acid ((COOH)₂). The concentration of the NaOH solution, which is typically unknown, is determined by titrating it against a standard solution of oxalic acid of known concentration using an indicator to signal the completion of the reaction.
2. What is the balanced chemical equation for the reaction between oxalic acid and sodium hydroxide?
The balanced chemical equation for the titration of oxalic acid (a diprotic acid) against sodium hydroxide (a monoacidic base) is:
(COOH)₂ (aq) + 2NaOH (aq) → (COONa)₂ (aq) + 2H₂O (l)
This equation shows that one mole of oxalic acid reacts completely with two moles of sodium hydroxide. This 1:2 stoichiometric ratio is crucial for calculations.
3. Why is phenolphthalein the correct indicator for this titration, and not methyl orange?
Phenolphthalein is the correct indicator because the product of this reaction, sodium oxalate ((COONa)₂), is a salt of a weak acid and a strong base. This salt undergoes hydrolysis, making the solution at the equivalence point slightly alkaline (pH > 7). Phenolphthalein changes colour in the pH range of 8.2-10, which perfectly captures this alkaline end point. Methyl orange, which changes colour in the acidic pH range of 3.1-4.4, would change colour too early and give an inaccurate result.
4. What are the essential steps to perform the titration of oxalic acid against NaOH in a lab?
The key steps for performing this titration as per the CBSE curriculum are:
- Preparation: Rinse the burette with the sodium hydroxide solution and the pipette with the oxalic acid solution.
- Setup: Fill the burette with the NaOH solution and record the initial reading. Pipette a known volume (e.g., 20 mL) of the standard oxalic acid solution into a clean conical flask.
- Indication: Add 2-3 drops of phenolphthalein indicator to the oxalic acid in the flask. The solution will remain colourless.
- Titration: Slowly add NaOH from the burette into the conical flask while continuously swirling.
- End Point: Stop adding NaOH at the first sign of a faint, persistent pink colour. This is the end point. Record the final burette reading.
- Repetition: Repeat the titration until you obtain at least two concordant readings (readings that are very close to each other).
5. In this titration, what is the key difference between the 'equivalence point' and the 'end point'?
While often used interchangeably, these terms have distinct meanings:
- The equivalence point is the theoretical point in the titration where the moles of the acid (oxalic acid) are stoichiometrically equal to the moles of the base (NaOH) according to the balanced equation. It is a calculated, exact point.
- The end point is the practical point observed in the experiment where the indicator changes colour.
In a well-designed experiment with the correct indicator, the end point occurs very close to the equivalence point, allowing for an accurate determination of concentration.
6. Why is oxalic acid considered a primary standard while sodium hydroxide is a secondary standard?
Oxalic acid is an excellent primary standard because it is a stable, non-hygroscopic (doesn't absorb moisture from the air) crystalline solid with a high purity and known molar mass. This allows for the precise weighing of the substance to prepare a standard solution of an accurately known concentration. In contrast, sodium hydroxide is a secondary standard because it is hygroscopic and readily absorbs carbon dioxide from the atmosphere, making it impure. Therefore, a solution of NaOH cannot be prepared by direct weighing and its exact concentration must be determined by standardizing it against a primary standard like oxalic acid.
7. How is the molarity of the sodium hydroxide solution calculated from the titration results?
The molarity of the NaOH solution is calculated using the stoichiometry from the balanced equation in the formula:
(M₁V₁ / n₁) = (M₂V₂ / n₂)
Where:
- M₁ and V₁ are the molarity and volume of oxalic acid.
- M₂ and V₂ are the molarity and volume of sodium hydroxide.
- n₁ (for oxalic acid) = 1.
- n₂ (for sodium hydroxide) = 2.
To find the molarity of NaOH (M₂), the formula is rearranged to: M₂ = (M₁ × V₁ × 2) / V₂.
8. What is the impact of overshooting the end point, and how does it affect the final calculated molarity of NaOH?
Overshooting the end point means adding more NaOH solution than required to neutralize the oxalic acid, resulting in a dark pink or magenta solution instead of a faint pink one. This leads to an erroneously high reading for the volume of NaOH used (V₂). When this larger volume is used in the calculation formula, the calculated molarity of NaOH will be artificially lower than the true value, as you are dividing by a larger number.
