Standardization of Sodium Hydroxide solution with standard Oxalic Acid solution

Experiment Name: Standardization of Sodium Hydroxide (NaOH) solution with standard Oxalic Acid solution.

Theory: In this experiment we shall determine the strength of NaOH solution by a standard solution of Oxalic Acid. This is done by means of Titration. The important matters that are related with the experiment are stated below:

Standardization of Sodium Hydroxide solution with standard Oxalic Acid solution

Titration:— In presence of a suitable indicator, the volumetric analysis in which a standard solution is added to another solution (whose strength is not known) to reach its end point to determine the strength of that solution is called titration.

Standard Solution: — A solution of known concentration is called a standard solution.

Indicator: — In acid-base titration there is an important use of indicator. An indicator is a chemical substance that detects the equivalent point (i.e. the end point) of reaction by changing its color.

Equivalent Point: — The equivalent point is the point in a titration when a stoichiometric amount of reactant has been added.

Molarity: -In current practice, concentration is most often expressed as molarity.

Molarity (symbol M) is defined as the number of moles of solute per liter of solution. If n is the number of moles of solute and V liters the volume of solution,

Molarity= Moles of solute/ Volume in liters Or, M= n/V (in liters)

Here, n= x (in gm)/ Molecular weight

Normality: -The number of gram equivalent weight of a solute per liter of solution is called normality.

Normality (N)= gm equivalent of solute /liters per solution.

In this experiment the reaction we shall use is as follows:

HOOC-COOH + 2NaOH ——>NaOOC-COONa + 2H2O

The formula required to determine the strength of NaOH solution is: -

V base × S base = V acid × S acid

or, V b × S b = V a × Sa

Where, V represents volume and S represents strength.

The volume of Oxalic acid is measured by watching the Equivalent point. The point at which acid- base neutralizes each other is called Equilibrium point. This point is determined with the help of an indicator.

Why Phenolphthalein is used as indicator: In this experiment, Phenolphthalein is selected as indicator whose working pH range is 8.3-10.0 i.e. it is works when the environment is acid. This indicator gives pink color in basic solution and becomes colorless when the base is neutralized.

Why Phenolphthalein is used as indicator?
In this experiment we are using NaOH and Oxalic acid. NaOH is a strong base but Oxalic acid is a weak acid. So, the solution at equilibrium point consists of a salt whose basic part is strong. As a result, there will be more OH- in the solution than H+ as the salt will be dissociated in the aqueous solution. So, the solution would be basic which provides phenolphthalein to work properly. So, Phenolphthalein becomes the perfect indicator to determine the end point of this reaction.

Data and calculation: -

Table: Data for standardization of NaOH solution with standard ……… M Oxalic Acid Solution

No. of Obs.

Volume of NaOH

(mL)

Burette Reading

(mL)

Volume of Oxalic-Acid

(mL)

Average Reading

(mL)

Initial Reading

(mL)

Final Reading

(mL)

1

10

0

5

5

 

5.3


2

10

0

5.2

5.2

3

10

0

5.5

5.5


Calculation:
Preparation of Oxalic acid solution in 100ml volumetric flask: -
Molecular weight of oxalic acid 126
Molarity M= no. of moles/ Volume in L
No of moles = amount/ Molecular weight
From
2V base × S base = V acid × S acid
we get
V Oxalic-acid × S Oxalic-acid = V NaOH× S NaOH
S NaOH = (2V Oxalic-acid × S Oxalic-acid)/ V NaOH 
S NaOH = (2 × 5.3 × 0.1) / 10
S NaOH =  0.106 M
Here,
V Oxalic-acid = 5.3 ml (average)
S Oxalic-acid = 0.1 M
VNaOH = 10 ml
S NaOH= 0.106 M

Result:
Determined strength of NaOH solution is: S NaOH= 0.106 M

Discussion: -
1. Phenolphthalein is a good indicator for the first reaction because it responds to the pH change caused by the formation of sodium hydrogencarbonate. It is pink in basic solutions and turns colorless as soon as the solution becomes acidic.
2. An acid-base conjugate pair has different colors. Any indicator changes color when the pH of its solution is 7.
3. The higher the concentration of hydrogen ions in a solution, the more acidic the solution is. These alternating double/single bonds can absorb wavelengths from visible light, making them appear coloured. That’s why why we see different color of different solutions.

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