Soap pH indications

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pH is a scale for specifying the acidity or alkaline level of an aqueous solution from pH 0 to 14. pH 0 to 6 is acidic, pH 7 is neutral and pH 8 to 14. For handmade soap, pH testing is important to know whether the soap is ready to use on skin. Handmade soap is always alkaline (cleansing ability) with a safe range pH between 10 to 8 to use on skin. Anything above pH 11 is too harsh for skin and will cause irritations. Anything below pH8 is not possible for handmade soap, no more cleansing power.

~ Table 1 (Handmade soap pH)

pH 12	Not advice to use on skin, may result in skin peeling.
pH 11	Not advice to use on skin, too harsh on skin.  Very high cleansing power and lathering soap
pH 10	Safe to use on skin, high cleansing power and lathering soap,  But, little too strong for dry skin
pH 9	Safe to use on skin, slightly lesser cleansing power than pH10 Some moisturizing effect for skin
pH 8	Safe to use on skin, much lesser cleansing power than pH10 moisturizing effect for skin
pH 7	No cleansing power, no lather or bubbles. No longer as soap

pH 0 to 14 [pH0-6 is acidic, pH7 is neutral, pH8-14 is alkaline]

Handmade soap is made for good cleansing. Suitable for facial, body and hair wash. The cleansing power will interchange with the moisturizing effect. So when the cleansing power is high, the moisturizing effect will be low, and vice versa. Usually suggest to go for pH9 handmade soap and use moisturizer after cleansing. 

There are many pH indicators available in the market, such as liquid type, paper type and digital type indicators. pH tests are important for soap making to know when the soap is ready to use. And also important for soap making research, to record and test pH for additives which might or might not influence the pH of soap. I can finish a box of pH paper (100 stripes) within a day. It is expensive to maintain this hobby without proper control of the soap making “wastage”. A proper planning to strategically use the pH indicators to manage between cost and strengths of each pH indicator.

pH paper (100 stripes) used for comparing the colour on strip with the indication on the box to find the same colour. Two ways to use pH papers, (1) commonly soak the pH strip in a solution. (2) pour about a coin size of distilled water on the surface of the soap bar, then smudge with a pH strip till wet. Compare the colour shades on the box, but the results might be a bit difficult to judge sometimes. Read figure 1 (pH paper indications) below for comparison samples

~ Figure 1 (pH paper indications)

This type of pH papers is definitely giving better indication than single colour pH paper.  The actual readings as mentioned in the picture above are only as assumptions. Recommend to use digital pH indicators to get the exact value pH readings with decimal point.

pH papers are expensive for frequent users and disposable because the stripes only can be used once. Not fairly helping in cost saving for the long term. Digital pH indicators is a good product in terms of pH precision readings and giving decimal points value. But require constant maintenance with calibration solutions (pH4 & pH7 buffer solutions) for checking the accuracy and storage solutions to fill some liquid in the cap to keep moisture for the electrode. Digital pH indicators only suitable to check pH in solution, unable to read in thick liquid or dry items. Thus, you need to dissolve the thick liquid or dry items in more distilled water before using it. And at least 30gm of solution in a cup to fit the indicator. Need distilled water to rinse the indicator after testing or for a new test solution.

~ Figure 2 (Digital pH indicator)

Also, best to calibrate with buffer solutions every time before doing any testing in order to receive accurate readings. And refill the storage solution in the black cap for every few days because the solution dries up within a few days. A Lot of work to ensure this indicator functions properly with long life.

pH papers and digital indicators are good pH indicators at certain extent, somehow they are still too costly for heavy users. Red Cabbage Liquid pH indicator (RCI) is a low cost, environmental friendly, natural product, very indicative with changing colours, easily available, and homemade! This indicator is a chemical free pH indicator that gives multi colour indications for different pH readings, much more easy to identify as compared to some pH indicators which are available in the market, such as phenolphthalein (only able to indicate colourless or pink colour). But they are not easily available or produced in the kitchen. RCI is an affordable pH indicator for every household. This indicator is chemical free and can be rinsed away after tested directly on soap bars.

RCI is suitable for checking pH for cold process soap, hot process soap, liquid soap (paste) and liquid soap. It is rather a stable indicator even though it is made of fresh red cabbage. The liquid can be kept in the refrigerator for weeks to reduce the chances of spoilage. The recommended fridge only stores soap making ingredients, and not with daily edible food or meats. 

The making of Red Cabbage Liquid pH indicator (RCI)

~ Figure 3 

~ Figure 4

~ Figure 5

~ Figure 6

~ Figure 7

~ Figure 8

Note, after the fresh made RCI liquid, let the liquid cool down to room temperature. Fill the liquid into sanitized bottles. Always keep in airtight bottles and refrigerate in a clean fridge to reduce spoilage. Fill a small bottle for current use, just refill when finished. Prefer to use a dropper bottle, easy to add to small tests.

~ Figure 9 (How much to add RCI liquid into test solution)

~ Figure 10 (Soap has yellowish tone)

Small setbacks of this indicator, if the test item is not colourless. Refer figure 10 above, the soap usually has a yellowish tone which will affect your judgement of reading. The yellowish tone from the soap will add colour to the RCI indication. So any blue indication will become a greenish colour. Even adding a small amount of test item in the distilled water to dissolve into solution, the solution still has a tiny yellow tone. A test solution is a mixture of test subject (ie. soap) dissolve in distilled water, use for testing pH to know whether the test subject is acidic or alkaline.

Figure 11 (RCI colour changes in soap)

Figure 12 (RCI colour changes from pH10 to 9)

Figure 13 (More samples for RCI colour changes in soap)

How much soap should dissolve in the distilled water to make a test solution?

Different percentage soap concentrations will affect the RCI color indications. The results will be different in terms of color intensity. pH papers also showed that different soap concentrations in test solutions give different readings.

Refer to figure 9 below, for samples of testing using different soap concentrations. Total 4 sample tests, they are using the same batch of liquid soap. (A) has the lowest soap concentration 4% in the test solution. The result with a very transparent dark green color RCI indication as compared with other 3 samples and pH9 with lightest shades on pH paper strip. (D) has the highest soap concentration 40%, giving real dark green colour and pH 10 with darkest orange on pH paper strip. (C) appears slightly darker green than (D) with RCI indication and has almost the same shades as (D) with pH paper strip. (B) is very dark as compared with the other 3 samples and pH paper indication already falls in pH9.

The actual reading should be at pH10 according to (D), because (D) has the highest concentration with better reading of pH for the test item. And (A) also proves pH10 with dark green color although its very light tone. but (A) pH paper indicator went wrong to show at pH9 which means the test solution has low soap concentrate to be able to test properly with pH paper. (C) shows darker green than (D), and has the same pH strip shows at pH 10. (B) has double problems, RCI indication has much very dark colour, hard to evaluate whether the indication is black or dark green, and the pH trip shades falls on pH9 which is inaccurate. (D) & (C) will be the best soap concentrations for test solutions. To test liquid soap pH with RCI, suggest to use (C) 30% soap concentrate in the test solution for a darker indication to overwrite most of the yellowish tones of soap colour and sufficient for accurate readings between RCI and pH papers.

Figure 14 (Comparison of test solutions with different liquid soap concentrations)

A special note that the above test is using liquid soap, and liquid soap is much diluted soap as compared with cold process or hot process soap. Soap bars have stronger indications for RCI even with lower soap concentrates in test solutions. Thus, suggest to use 10% soap concentration in the test solution, otherwise the test solution is too thick for pH testing.

Figure 15 (Compare RCI indicator with pH papers)

How to read pH papers? Follow step 1 to step 3 in figure 15 above. Based on the results in figure 15, RCI and pH papers have the same results when the test solution at the right soap concentrations.

Figure 16 (RCI test on other test subjects)

Distilled water is acidic at pH 5.5 to 6. Adding distilled water to dissolve alkaline test subjects will not affect the accuracy of pH reading or making the test solution less alkaline. However, low soap concentration in the test solution, will indicate lower alkaline pH.

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Best ratio for RCI test

30% Liquid soap + 70% Distilled water = 100% test solution + 1gm RCI

10% Cold process soap + 90% Distilled water = 100% test solution + 1gm RCI

10% Hot process soap + 90% Distilled water = 100% test solution + 1gm RCI

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This is the RCI color chart for all time reference

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My way of using pH indicators for each type of soaps (cold process, hot process and liquid soap)

RCI can test directly on dry test subjects. But the indication will be slightly different with the test solution.

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Cold process soap 

Step 1 – use RCI indicator to check whether the soap is ready to use. Test directly on the soap bar.

Just a few drops of RCI liquid on the soap bar to check colour changes. Gently shake the liquid on the soap surface to allow the liquid to blend with the soap. If colour turns to dark grey/blackish green, the soap is fully saponified and pH below 10. If the colour turns bright green, meaning the soap pH is still above pH11. Allow the soap to continue cure, usually curing period takes about 4 to 6 weeks.

Step 2 – If you want to check pH value indication with color, test RCI in a solution. 

Dissolve 1gm cold process soap in 9gm distilled water to make 10gm test solution. Then add 1gm RCI liquid into the test solution. At least 10% to 15% soap concentration (cold process soap) in solution.

Optional Step 3 –  use pH paper to double confirm the pH indication is the same as RCI.

This step is optional for better precision reading. If pH reading is lower with pH strip which means the test solution is error, the soap concentration is insufficient to give accurate reading. 

Scrap some pieces from the soap bar, dilute in the distilled water. 1gm soap scraps and 9gm distilled water. Allow the soap to dissolve for about 10 to 15minutes. Then dip 1 unit of the pH paper strip into the solution to see colour shades, compare the strip with the colour on the box to find the colour matching.

Optional Step 4 – optional to use digital pH indicator to check the exact figure of pH. Require to make a larger size solution to fit the digital pH indicator. Suggest 3gm soap and 27gm distilled water. Make sure the soap is fully dissolved and mix well before checking pH.

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Hot process soap 

Step 1 – use RCI indicator to check whether the soap is fully saponified after the soap becomes “gel” and pH is below 10. Test directly on the soap bar.

If RCI shows vibrant green colour (meaning pH indicates above 11), continue to cook until pH drops to 10 or below. When RCI indicates dark grey blue colour meaning the soap is now fully saponified, no more floating lye in the soap. Always remember to stir or mix the soap batter for less than a minute before checking the pH. If pH is 10 or less, the soap is ready to mix with additives then pour to soap mold.

Step 2 – If you want to check pH value indication with color, test RCI in a solution. 

Dissolve 1gm cold process soap in 9gm distilled water to make 10gm test solution. Then add 1gm RCI liquid into the test solution. At least 10% to 15% soap concentration (hot process soap) in solution.

Optional Step 3 – Final check pH of the soap by using pH paper to know the actual pH of the soap. Hot process soap usually cures for less than 7days depending on the liquid in the recipe. Soap at least 10% to 15% concentration in solution.

Scrap some pieces from the soap bar, dilute in the distilled water. 1gm soap scraps and 9gm distilled water. Then dip 1 unit of the pH paper strip into the solution to see colour indication, compare the colour indication on strip with the colour on the box to find the colour matching. 

Optional Step 4 – using a digital pH indicator to check the exact figure of pH. Make a larger size solution to fit the pH indicator. Suggest 3gm soap and 27gm distilled water. Make sure the soap is fully dissolved and mix well before checking pH.

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Liquid process soap 

Step 1 – use RCI indicator to check whether the soap is fully saponified after the soap becomes “paste” and pH is below 10. Test directly on the liquid soap paste

If RCI shows vibrant green colour (meaning pH indicates above 11), continue to cook until pH drops to 10 or below. When RCI indicates dark grey blue colour meaning the soap is now fully saponified, no more floating lye in the soap. Always remember to stir or mix the soap batter for less than a minute before checking the pH.

Step 2 – If you want to check pH value indication with color, test RCI in a solution. 

Artiz Soap liquid soap making method, the soap paste is ready to dilute with distilled water anytime when paste is done.

Note that handmade liquid soap has colour, from yellowish, orangy to brownish tone liquid soap. Therefore, diluting the liquid soap with more distilled water will lighten the original colour of liquid soap and be able to give a more accurate reading of RCI when the solution is almost colourless. Add approximate 1gm of RC1 into a 10gm test solution. The test solution will change colour according to the pH colour indication of RCI. 

Optional Step 3 – optional to check again with pH paper to confirm the value of pH.

Dilute 3gm soap paste with 7gm distilled water (total 10gm solution). Then dip 1 unit of the pH paper strip into the solution to see colour indication, compare the colour indication on strip with the colour on the box to find the colour matching. Step 3 is just for confirmation. Liquid soap at least 20% to 30% concentration in solution.

Optional Step 4 – optional to check with digital pH indicator, unless you want to know the exact value of pH.

Digital pH indicator able to give high accuracy readings for liquid soap as compared with RCI and paper, especially when trying to get pH8 reading. However, digital pH indicators only can read highly fluid solutions, unable to give good readings in thick liquid soap. I did tested in a thick shower gel liquid soap, the digital pH indicator no longer response properly on other test.

Thus, thick liquid soap must dilute more with distilled water before using a digital pH indicator. For example, 9gm thick liquid soap dilute with 21gm distilled water. Liquid soap at least 20% to 30% concentration in solution.

Special Note 1

Special Note 2

Special Note 3

Special Note 4

more depiction for making RCI

We will update this page from time to time for adding new information.

[Updated 4th November 2020] by Artiz Soap [Esther Cheng]