Saponification Value of Citric Acid
The saponification value is an important parameter for soap making as it indicates how much Sodium Hydroxide (NaOH) or Potassium Hydroxide (KOH) is needed for a full saponification. When you also want to add Citric Acid to your soap, you also need to add more Sodium Hydroxide. But how much more? Let’s take a closer look at Citric Acid to answer that question.
Different forms of Citric Acid
Citric Acid is a little special. The saponification value of Citric Acid varies depending on its crystal form. That means you have to distinguish between two different kinds of Citric Acid. Sounds complicated first, but you just need to take a look at the Citric Acid packaging to know which kind it is.
For the anhydrous form, Citric Acid Anhydrous, the saponification value is 0,625. This form contains no crystal water and therefore requires more lye per gram. Citric Acid Anhydrous is more often used in industrial processes (e.g., pharmaceutical tablets, technical applications) or where specific moisture requirements are needed.
For Citric Acid Monohydrate, the most common form, the saponification value is 0,571. This means that for each gram of Citric Acid Monohydrate, 0,571 grams of Sodium Hydroxide are needed for neutralization. Citric Acid Monohydrate is the more commonly sold and used form, particularly in food and beverage industry (as an acidifier and preservative), household cleaners (for descaling) and Cosmetic products (as a pH regulator). It is the standard in food additives (E330), where moisture content is less critical. Due to its crystalline water it is more stable in humid environments and therefor easier in storage and handling.
If you can’t find any information on the packaging about which kind it is, chances are high that it is Monohydrate, as this is the most common form that you can buy. You could also check the SDS (safety data sheet) for that product to find out if it is anhydrous or monohydrate.
Simplified Rule of Thumb
For those who want to avoid complicated calculations, there is a useful rule of thumb: For every 10 grams of Citric Acid added to a soap recipe, approximately 6 grams of additional Sodium Hydroxide must be added. This rule of thumb with a factor of 0,6 is close to the more exact values of 0,571 and 0,625.
Application Amount in Soap Making
The calculation fundamentally differs from the saponification calculation for oils, as Citric Acid reacts completely with NaOH. As emphasized in soap calculators: “The NaOH requirement for Citric Acid is calculated separately from the oils. This is because Citric Acid always reacts 100% with the NaOH and should therefore not be included in the ‘superfat’ calculation.” And unlike oils and fats, the reaction between Citric Acid and Sodium Hydroxide is not a saponification reaction, but rather a neutralization reaction, because you add an acid to a base.
The typical amount of Citric Acid in cold process soap making is 1-3% of the oil weight, while for hair soap it can be increased up to 5%. Citric Acid is a white crystalline odourless powder, looking like sugar, which you need to dissolve in water. For this, you use some of the total weight of water which you would use for soap making, you don’t need extra water. Add the Citric Acid to the water and stir until it’s completely dissolved. Then carefully and slowly add the NaOH while stirring. Add it little by little, never all at once. The Citric Acid reacts with the NaOH and the mixture will get very hot. It forms sodium citrate, which acts as a chelating agent and so reduces soap scum (lime soap). If you want to know more about the chemical reaction between Citric Acid and Sodium Hydroxide check out this article about Citric Acid in Cold Process Soap or watch the video on YouTube.
You don’t need Citric Acid to make soap, but it can be a great additive, especially in areas with hard water.
Physical and Chemical Properties
| Property | Citric Acid Monohydrate | Citric Acid Anhydrous |
|---|---|---|
| Physical State, Color, Form and Odour | solid, white, crystalline and odourless | solid, white, crystalline and odourless |
| Molecular Formula | C₆H₈O₇·H₂O | C₆H₈O₇ |
| Saponification Value | 0,571 | 0,625 |
| pH-Value (aqueous solution, 100 g/l, 20 °C) | ~1,7 | ~1,7 |
| Water Solubility (at 20°C) | 590 – 880 g/l | 590 –1330 g/l |
| Melting point | 135-152°C | 153-155°C |
| Density (at 20 °C) | ~1,54 g/cm³ | ~1,67 g/cm³ |
| Bulk density | 0,55 – 1,00 g/cm³ | 0,50 – 0,73 g/cm³ |
