🧼 Soap pH Testing: How to Test Homemade Soap Safely

Testing the pH of your homemade soap is an important step in making sure your soap is safe, well-made, and gentle on skin.

Soap making is chemistry. When oils and lye combine correctly, they form soap and glycerin through a process called saponification. If the balance is off, the finished soap may be too soft, harsh, or even caustic.

This guide explains what soap pH means, what range is considered safe, and the most reliable ways to test your soap.

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📑 Table of Contents

Click a section below to jump directly to that topic.

• What Is Soap pH?
• Why Testing Soap pH Matters
• What Is a Safe pH Range for Soap?
• How to Test the pH of Soap
• Using pH Strips
• The Zap Test (What It Is & When to Use It)
• The Red Cabbage pH Test
• When to Test Soap pH
• What to Do If Soap Tests Too High
• Frequently Asked Questions

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What Is Soap pH?

pH is a scale that measures how acidic or alkaline a substance is.

• pH 7 = neutral (water)

• pH below 7 = acidic

• pH above 7 = alkaline

Because soap is made using sodium hydroxide (lye), true soap is naturally alkaline. There is no such thing as pH-neutral real soap.

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Why Testing Soap pH Matters

Testing soap pH helps you identify potential problems such as:

• Lye-heavy (caustic) soap

• Incomplete saponification

• Harsh or irritating bars

• Measurement errors in recipes

If too much lye is used, the finished soap may contain free lye, which can irritate or burn skin.
If too little lye is used, soap may remain overly soft or oily.

Accurate measuring and testing are key to safe soap making.

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What Is a Safe pH Range for Soap?

Most properly made cold process soap falls within a pH range of:

👉 8 to 10

• pH below 8 may indicate excess oils or incomplete saponification

• pH above 10 may indicate excess lye and a harsh bar

Lye itself has a pH of around 14, while water has a pH of 7.
Finished soap should never test anywhere near lye levels.

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How to Test the pH of Soap

There are several methods used by soap makers. Some are more reliable than others.

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Using pH Strips

pH strips are inexpensive, easy to use, and widely available.

How to test soap with pH strips:

1. Wet the soap slightly and create a small amount of lather

2. Place the pH strip on the wet surface or bubbles

3. Compare the color change to the chart provided

This method gives a general pH range, which is usually sufficient for handmade soap.

pH strips are best for identifying problem soap, not for exact scientific readings.

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The Zap Test (What It Is & When to Use It)

The zap test is an old-fashioned method passed down through generations of soap makers.

It involves lightly touching the tip of your tongue to a cured bar of soap.
If the soap gives a sharp “zap” sensation (similar to touching a battery), it may be lye-heavy.

⚠️ Important Safety Notes:

• Only perform the zap test on fully cured soap

• Never use this method on fresh soap

• Do not use if you are unsure or uncomfortable

The zap test does not measure pH, but it can indicate the presence of free lye.

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The Red Cabbage pH Test

Red cabbage contains natural pigments that change color based on pH.

How to prepare red cabbage indicator:

1. Chop 3–4 red cabbage leaves

2. Add to 1 cup of distilled water

3. Simmer gently or blend, then strain

4. Use the liquid to test soap lather

Color changes:

• Purple/blue = acceptable soap range

• Green/yellow = highly alkaline (potentially unsafe)

Distilled water is important, as minerals in tap water can affect results.

This method is educational and fun, but less precise than pH strips.

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When to Test Soap pH

Soap should be tested after it has fully cured.

• Fresh soap often tests high

• Cure time allows saponification to complete

• Test after 4–6 weeks for accurate results

Testing too early can give misleading readings.

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What to Do If Soap Tests Too High

If your soap tests above pH 10:

• Do not use it on skin

• Allow additional cure time

• Rebatch with additional oils if necessary

• Review your recipe and measurements

Using a reliable soap lye calculator and weighing ingredients precisely helps prevent this issue.

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Frequently Asked Questions

Is high pH soap always unsafe?
Not always, but very high readings may indicate excess lye and irritation risk.

Can soap be pH neutral?
No. True soap is always alkaline.

Are liquid soaps different?
Yes. Liquid soap has a higher natural pH and requires different testing standards.

Do pH strips expire?
Yes. Old or expired strips can give inaccurate results.

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Important Note

This information is for educational purposes only and does not replace professional testing or medical advice.
Always follow safe soap-making practices and verified recipes.

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