A 1 N (one normal) sodium hydroxide solution is one of the most common reagents in laboratories, water treatment plants and quality control rooms. You can buy it ready-made, but it is easy and much cheaper to prepare from caustic soda flakes or pellets. This guide shows how much NaOH to weigh for each strength, how to make the solution safely, and how to check its exact normality.
What 1 N means for NaOH
Normality is the number of gram-equivalents of a substance per litre of solution. Sodium hydroxide has one hydroxide ion per molecule, so for NaOH normality equals molarity. With a molecular weight of 40 g/mol, a 1 N solution contains 40 g of NaOH in each litre of solution.
How much to weigh
Flakes are not 100% NaOH, so correct for purity. For 99% flakes, divide the pure amount by 0.99:
| Strength | Pure NaOH per litre | 99% flakes per litre |
|---|---|---|
| 0.1 N (N/10) | 4.0 g | 4.04 g |
| 0.5 N | 20.0 g | 20.2 g |
| 1 N | 40.0 g | 40.4 g |
| 5 N | 200 g | 202 g |
| 10 N | 400 g | 404 g |
The general formula is: grams of flakes = normality × 40 × volume in litres / purity (purity as a decimal, for example 0.99).
Step-by-step preparation of 1 litre of 1 N NaOH
- Put on splash goggles, chemical-resistant gloves and a lab coat or apron.
- Weigh 40.4 g of 99% caustic soda flakes quickly, because they absorb moisture from the air while on the balance.
- Pour about 700 ml of distilled or deionised water into a 1 litre beaker.
- Add the flakes slowly to the water while stirring. Never add water to flakes. The solution will become hot.
- Let the solution cool to room temperature, because volume changes with temperature.
- Transfer to a 1 litre volumetric flask, rinse the beaker into the flask, and make up to the mark with water.
- Mix well and transfer to a labelled polyethylene bottle with a tight cap.
Why you should standardise
Flakes pick up some moisture and carbon dioxide, and weighing is never perfect, so the real strength can be a little off. For accurate work, standardise the solution against a primary standard such as potassium hydrogen phthalate (KHP, 204.22 g/mol):
- Dry KHP and weigh it accurately into a flask: about 4 g for a 1 N solution, or about 0.4 g for 0.1 N, so the titration uses roughly 20 ml.
- Dissolve in about 50 ml of water and add two drops of phenolphthalein.
- Titrate with your NaOH solution until a faint pink colour lasts 30 seconds.
- Calculate: N = mass of KHP in g / (0.20422 × volume of NaOH in ml).
Repeat three times and use the average. Write the exact normality and the date on the bottle label.
Storage tips
- Store in polyethylene, not glass. Strong NaOH slowly attacks glass and can seize glass stoppers.
- Keep the bottle tightly closed. The solution absorbs carbon dioxide from air and slowly loses strength.
- Re-standardise dilute solutions (0.1 N and below) regularly, ideally weekly for accurate work.
Safety
Sodium hydroxide causes severe burns to skin and eyes, even in dilute solutions. If it splashes into the eyes, rinse with plenty of water for at least 15 minutes and get medical attention. Keep it away from aluminium, zinc and tin, which it attacks with release of hydrogen.
Where to buy caustic soda flakes in Oman
For laboratories and plants in Oman, you can buy caustic soda flakes 99% from ready stock in Barka, in 25 kg bags, with the SDS and certificate of analysis available to download and next-day delivery across Oman.