Standardizing Sodium Hydroxide Solution

🧪 Preparation and Standardization of 1 M Sodium Hydroxide Solution

I. Reagents Required

The notes list the following required reagents:

1. Sodium Hydroxide (NaOH), A.R. Grade (Analytical Reagent)
2. Potassium Hydrogen Phthalate (KHP), A.R. Grade (Primary Standard)
3. Phenolphthalein Solution (Indicator)
4. Distilled/Deionized Water

II. Preparation of 1 M Sodium Hydroxide Solution

The preparation process involves dissolving the required amount of solid NaOH in water.

1. Calculation: The note suggests dissolving 42 gm of NaOH to produce 1000 mL (1 L).
   • Sparing Partner Challenge: 1 M NaOH requires 40.00 gm (Molecular Weight of NaOH is ≈40.00 g/mol) per liter. Using 42 gm suggests aiming for a concentration slightly higher than 1 M, perhaps to account for NaOH’s hygroscopic nature or common impurities (Na2​CO3​). For a true 1 M target, 40 gm should be used.
2. Procedure:
   • Carefully weigh out approximately 40 gm to 42 gm of Sodium Hydroxide (NaOH) pellets or flakes.
   • Dissolve the weighed NaOH in a sufficient quantity of CO2​-free distilled water in a clean beaker. NaOH dissolves exothermically, so stir gently until fully dissolved and allow to cool.
   • Transfer the solution quantitatively to a 1000 mL volumetric flask.
   • Add distilled water up to the mark, ensuring the meniscus rests on the line.
   • Mix thoroughly by inverting the stoppered flask multiple times.
   • Label the solution as “Approx. 1 M NaOH” and store it in an airtight plastic bottle (as NaOH can react with glass over time and absorbs CO2​ from the air).

III. Standardization of the NaOH Solution

Since NaOH is a secondary standard (it absorbs moisture and CO2​ from the air, making its exact concentration unknown), its concentration must be accurately determined (standardized) using a primary standard, Potassium Hydrogen Phthalate (KHP).

A. Preparation of Primary Standard (KHP)

1. Drying: Weight accurately about 5 gm of Potassium Hydrogen Phthalate (KHP). The KHP must be previously powdered and dried at 120∘C for two hours to ensure no residual moisture is present, as this would falsely lower the measured weight of the primary standard.
2. Cooling: Allow the dried KHP to cool to room temperature in a dessicator to prevent re-absorption of moisture.

B. Titration Procedure

1. Weighing and Transfer: Accurately weigh three separate portions of the dried KHP (e.g., 0.7 gm to 1.0 gm each). The total weight of KHP mentioned in the note (5 gm) is likely the initial amount prepared, which should be divided for multiple titrations.
2. Dissolution: Transfer each weighed KHP sample carefully to a clean 250 mL conical flask.
3. Dissolve each sample in approximately 75 mL of distilled water.
4. Indicator Addition: Add 2 to 3 drops of Phenolphthalein Solution to the KHP solution in the conical flask. The solution will initially be colorless.
5. Titration: Titrate the solution with the prepared NaOH solution (filled into a clean, rinsed burette).
6. End Point: The titration is complete when the entire solution in the conical flask turns to a permanent pale pink color which persists for at least 30 seconds. This marks the endpoint of the reaction:KHP+NaOH→KNaP+H2​O
7. Recording: Record the volume of NaOH solution used (Titre value, T) for each trial.

C. Calculation of Molarity

The molarity of the NaOH solution is calculated using the weight of KHP used and the volume of NaOH titrant (T) required.

Molarity(M)=Constant×Titre Value(T,mL)Weight of Potassium Hydrogen Phthalate(gm)​

The note provides the constant 0.2042. Let’s verify this constant:

• The equivalent weight of KHP is 204.22 g/mol.
• The general standardization formula is:Molarity of NaOH=Molar Mass of KHP(g/mol)×Volume of NaOH(L)Mass of KHP(gm)​
• To get the constant 0.2042 when volume is in mL (as implied by T being a titre value):M=Molar Mass×1000T​Mass of KHP(gm)​=Molar Mass×TMass of KHP×1000​
• If we use 0.2042, the formula implies:M=0.2042×T(mL)Mass of KHP(gm)​⟹0.2042=1000Molar Mass​
• This confirms that 0.2042 is 204.2 g/mol divided by 1000 mL/L.0.2042=1000 mL/L204.22 g/mol​
• Final Corrected Formula (as per the notes):Molarity of NaOH=0.2042×Titre Value(T)Weight of Potassium Hydrogen Phthalate​

D. Validity

The notes state the validity of the standardized solution is 2 months. This is a reasonable shelf life for a NaOH solution stored properly (airtight and away from light) before re-standardization is required.