Dilution

The addition of caustic soda solution to water will cause a rise in temperature of the solution. If caustic soda solution becomes concentrated in one area, is added too rapidly, or is added to hot or cold water, a rapid temperature increase can result in dangerous mists, boiling and spattering, or even a violent eruption. The figure illustrates temperature rises that occur while diluting 50% caustic soda solution with various initial water temperatures. Remember to always add caustic soda solution to water with constant agitation.

Special Precautions for Diluting Caustic Soda Solution:

1. Always add caustic soda solution to water with constant agitation. Never add water to caustic soda solution.
   
   
2. The water should be lukewarm 80° to 100°F (27° to 38°C). Never start with hot or cold water.
   

Diluting Caustic Soda Solution
The Mixing Chart (22KB PDF) is helpful in estimating the volume of water needed to dilute concentrated caustic solutions to a desired lower concentration, or volumes of caustic soda solutions of different concentrations to mix together to produce solutions of desired intermediate concentrations. The Mixing Chart can be used to:

• Estimate the gallons of water to be mixed with a quantity of caustic soda solution to yield a lower concentration of caustic soda solution (Note: remember to always add caustic to water when mixing)
• Estimate the gallons of a concentrated caustic soda solution that would be required to be added to a weaker caustic soda solution to yield an intermediate caustic soda solution concentration.

Example 1 (Reference the Mixing Chart (22KB PDF))
How many gallons of 50% caustic soda solution must be added to 400 U.S. gal (1.51m³) of a 20% caustic soda solution to give a final caustic soda solution concentration of 25%?

Connect 50% on the left scale (concentrated solution) with 20% on the right scale (dilute solution) with a line. The point where this line intersects the 25% dilute solution scale (right scale) indicates it takes a ratio of 14 volumes of 50% caustic soda solution (volume of concentrated solution, bottom scale) and 86 volumes of 20% caustic soda solution (volume of dilute solution, top scale) to yield a 25% caustic soda solution. Using this volume ratio, set up an equivalence statement as follows:



Example 2 (Reference the Mixing Chart (22KB PDF))
How much water should be mixed with 4.0m³ (1,057 gal) of a 20% caustic soda solution to dilute it to a 15% caustic soda solution?

Connect 20% on the left scale (concentrated solution) with 0% on the right scale (dilute solution) with a line. The point where this line intersects the 15% dilute solution scale (right scale), indicates it takes 72.5 volumes of 20% caustic soda solution (volume of concentrated solution, bottom scale) and 27.5 volumes of water (volume of dilute solution, top scale) to yield a 15% caustic soda solution. Using this volume ratio, set up an equivalence statement as follows:

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