SOP: Preparation of a 0.1 Molar Sodium Hydroxide Solution

Background
This procedure describes the make-up of a sodium hydroxide (NaOH) solution to approximately 0.1 molar concentration (0.1M). The solution is then standardised against the primary standard, potassium hydrogen phthalate (KHP), to accurately determine its concentration.

Method

Preparation of Potassium Hydrogen Phthalate Standard Solution

1. Place 5 ± 0.1 g of KHP (Analytical Reagent Grade) in a weighing bottle and dry in an oven at 110°C for 2 hours. Store the KHP in a desiccator after this time.
   
   
2. Weigh the weighing bottle to 4 decimal places on the analytical balance.
   
   
3. Quantitatively transfer the KHP to a 250 mL volumetric flask.
   
   
4. Reweigh the weighing bottle and find the weight of the KHP to 4 decimal places, m (KHP).
   
   
5. To the volumetric flask, add 100 to 150 mL of distilled water (you may use a wash bottle) and swirl until the KHP dissolves.
   
   
6. Dilute to the mark with distilled water, replace the stopper and mix the contents by inverting and swirling the flask a number of times.
   
   
7. Calculate the molarity of the standard KHP solution, M (KHP), to 4 decimal places:
   
           M (KHP) = m (KHP) x 0.0196

1. Locate the 6M sodium hydroxide stock solution in the laboratory store (CAUTION: sodium hydroxide is very corrosive)
   
   
2. Prepare an approximate 0.1M solution by diluting 20 mL of the 6M stock solution to 1 litre with distilled water. This can be safely done using a 50 mL measuring cylinder and a 1 litre graduated beaker.
   
   
3. Transfer the contents to a plastic bottle for storage and mix the contents well.
   
   

Standardisation of 0.1M Sodium Hydroxide Solution

1. Using a bulb pipette, quantitatively transfer a 25 mL aliquot of the standard KHP solution to a 250 mL Erlenmeyer flask. Wash down the inside of the flask with about 50 mL of distilled water delivered from a wash bottle.
   
   
2. Add 2 drops of phenolphthalein indicator and mix well.
   
   
3. Carefully fill a 50 mL burette with the prepared sodium hydroxide solution using a beaker.
   
   
4. Use a magnetic stirrer to stir the solution during the titration process.
   
   
5. Titrate the KHP solution to the first sign of a permanent pink end point (use a white tile beneath the Erlenmeyer flask during the titration). Record the titre to the nearest 0.01 mL.)
   
   
6. Repeat the titration (steps 1 to 3) until three titres are obtained that agree within 0.10 mL. Average these readings, T (NaOH).
   
   
7. Calculate the molarity of the sodium hydroxide solution M (NaOH) to 4 decimal places:
   
   

   M (NaOH) =	M (KHP) x 25
   	T (NaOH)

   where T (NaOH) is the average of the 3 titres in mL.