There are four basic ways to test your soap for an excess of caustic.
If the soap has been cold processed (CP) the soap will test positive for caustic during the curing phase.
Hot Processed (HP) soap is cooked until neutral and should test negative for caustic as that is the indication that you
can stop the cooking.
Neutral soap should have a pH of 9.0 - 10.5.
Battery Test -
This test is done by simply touching the soap to your tongue.
If you get a bite similar to the bite you receive from a 9 volt battery then the soap is caustic hot.
This is a good quick test and actually fairly accurate.
Phenolphthalein Test -
This test is done by putting a few drops of phenolphthalein onto the soap.
If the color is not clear then there is caustic left in the soap.
The darker the color the "hotter" the soap is.
Note: Phenolphthalein will work on soaps with a pH of 9.0 - 9.5. With soaps of a higher pH,
9.6 - 10.5 can also be tested this way but the phenolphthalein will have a very light appearance.
Litmus Paper or pH strips -
This test is done by making a slurry of soap on a bar or diluting a liquid sample with some distilled water.
You then rub it in the bar slurry or into the diluted liquid.
The strips can be somewhat difficult to read but are a great quick test during hot processing and cure time.
pH Meter -
A pH meter test is the most accurate.
The pH meter should be calibrated prior to each testing timeframe.
Meaning it will be fine for a series of testing on the same day.
With this test you dilute a sample of bar or liquid soap with distilled water.
You then put the probe of the pH meter into the sample. The display will indicate the pH of the soap.