Caustic soda solutions are described in several ways: percent by weight, grams per litre, normality or molarity, and density. Converting between them is a daily task in laboratories, water treatment plants and process control. This page gives a reference table and the formulas behind it.
Reference table (at about 20 °C)
| % NaOH by weight | Density (g/ml) | g NaOH per litre | Normality (N = M) |
|---|---|---|---|
| 1% | 1.010 | 10.1 | 0.25 |
| 4% | 1.043 | 41.7 | 1.04 |
| 10% | 1.109 | 110.9 | 2.77 |
| 20% | 1.219 | 243.8 | 6.10 |
| 30% | 1.328 | 398.4 | 9.96 |
| 50% | 1.525 | 762.5 | 19.06 |
Values are rounded and for guidance. Density changes with temperature, so for accurate work measure at a controlled temperature or use a density table for your temperature.
The formulas
- g/L = % by weight × density (g/ml) × 10
- Normality = g/L ÷ 40 (for NaOH, normality equals molarity)
- % by weight = g/L ÷ (density × 10)
Diluting a stronger solution
Use C1 × V1 = C2 × V2 with concentrations in g/L or normality. Example: to make 100 litres of 4% (about 41.7 g/L) from 50% lye (762.5 g/L): V1 = 100 × 41.7 ÷ 762.5 = about 5.5 litres of lye, made up to 100 litres. Always add the lye to the water, slowly, while stirring.
Making a solution from flakes
Grams of flakes = target g/L × litres ÷ purity. For 99% flakes, divide by 0.99. Example: 10 litres of a 10% solution needs about 110.9 × 10 ÷ 0.99 = 1,120 g of flakes, made up to 10 litres. Note that 10% by weight is not 100 g per litre; the density correction matters at higher strengths.
Checking strength
Measure density with a hydrometer for a quick check, or titrate against a standard acid for accuracy. Remember that solutions exposed to air absorb carbon dioxide and slowly lose strength.
Sourcing flakes
For users in Oman, caustic soda flakes in Oman are available as 99% flakes in 25 kg bags, with the certificate of analysis and SDS online.