pH Testing

Part of Soap Making

Measuring safe alkalinity levels in finished soap using natural indicators you can make yourself.

Why This Matters

All soap is alkaline. This is not a defect — it is the chemical mechanism by which soap cleans. The saponification reaction converts acidic fats and strongly alkaline lye into soap salts (sodium or potassium fatty acid salts) that sit at a mildly alkaline pH. Understanding where on the pH scale your finished soap lands tells you whether it is safe, too harsh, or possibly not fully reacted.

Human skin has a natural pH of approximately 4.5-5.5 — slightly acidic. Soap sits at pH 8-10, which is why prolonged exposure dries out the skin (it temporarily disrupts the skin’s acid mantle). This is normal and unavoidable with real soap. What is not normal — and what is dangerous — is soap above pH 11. At that level, the bar contains significant free sodium hydroxide (lye), which will chemically burn skin with repeated use. A pH of 12-14 means the soap is essentially undiluted lye. Burns from high-pH soap accumulate over days of use before becoming visible, which makes them easy to miss and difficult to trace back to the soap.

In a world without litmus paper or electronic pH meters, you need practical methods for testing soap alkalinity using materials you can produce yourself. Natural pH indicators from plants are reliable, reproducible, and sensitive enough to distinguish safe soap (pH 8-10) from dangerous soap (pH 11+). Every soap maker should know how to make and use at least one of these.

The pH Scale

The pH scale runs from 0 to 14. It measures the concentration of hydrogen ions in a solution:

• pH 0-3: Strongly acidic (battery acid, stomach acid)
• pH 4-6: Weakly acidic (vinegar pH 2.5, skin pH 4.5-5.5, rain pH 5.6)
• pH 7: Neutral (pure water)
• pH 8-10: Weakly alkaline — target range for finished soap
• pH 11: Moderately alkaline (fresh lye solution before use, ammonia)
• pH 12-13: Strongly alkaline (freshly made lye water, bleach)
• pH 14: Maximum alkalinity (concentrated NaOH solution)

The scale is logarithmic: each unit represents a 10x difference in hydrogen ion concentration. pH 11 is 10 times more alkaline than pH 10. pH 13 is 1,000 times more alkaline than pH 10. This means small differences in the upper range represent large differences in chemical aggressiveness.

Fresh cold-process soap straight from the mold typically sits at pH 12-13 — the saponification reaction is still incomplete and free lye is present. After full cure (4-6 weeks), the pH should have dropped to 8-10 as remaining lye reacts with fats and excess water evaporates. Hot-process soap finishes saponification during cooking and exits the process at pH 9-10 immediately.

Red Cabbage Juice Indicator

Red cabbage contains anthocyanins — pigments that change color predictably across the pH scale. This makes red cabbage juice the most reliable and colorimetrically rich natural pH indicator available.

Making the indicator solution:

1. Chop half a head of red (purple) cabbage into rough pieces. Exact amount is not critical — more cabbage produces more indicator solution.
2. Place in a pot with just enough water to cover (approximately equal volume of water to cabbage by weight).
3. Bring to a boil, then reduce to a simmer for 15-20 minutes. The water will turn deep purple-blue.
4. Strain out the cabbage and allow the liquid to cool completely.
5. The resulting liquid is your indicator solution. It keeps for 1-2 weeks at room temperature, or several months if refrigerated or dried.

Color reference for cabbage juice indicator:

pH Range	Color
pH 1-3	Bright red / pink
pH 4-6	Purple (near original color)
pH 7	Violet-blue
pH 8-9	Blue-green
pH 10-11	Green
pH 12-13	Yellow-green
pH 14	Yellow

Using the indicator on soap:

Method 1 (solution test): Dissolve a small shaving of your soap in a few teaspoons of distilled or clean water. Add 5-10 drops of cabbage juice indicator. Observe color immediately.

Method 2 (direct surface test): Wet the surface of the soap bar. Apply 2-3 drops of cabbage juice directly to the wet surface. Let sit 30 seconds. Observe color.

Reading the result:

• Blue-green color → pH 8-9 → safe, well-cured soap
• Green color → pH 10-11 → borderline; retest after more cure time, or do the zap test
• Yellow-green or yellow → pH 12+ → dangerous free lye present; do not use; rebatch

The transition from safe (blue-green) to dangerous (green toward yellow) is visually distinct and easy to learn after one or two calibration tests with known references.

Turmeric Indicator

Turmeric contains curcumin, which turns from yellow to red-orange in alkaline conditions above approximately pH 8.6. It is less precise than cabbage juice but useful as a secondary check, especially if cabbage is not available.

Making turmeric indicator:

1. Mix 1 teaspoon of turmeric powder with 1 cup of rubbing alcohol or high-proof grain alcohol (vodka works). If no alcohol, use water, though the color is less vivid.
2. Stir thoroughly. Allow to settle. The liquid will be deep yellow-orange.
3. Filter through cloth to remove undissolved particles.
4. Use the clear yellow liquid as your indicator.

How to use it:

Apply a few drops to a wet soap surface or to a soap-water solution. Observe after 30 seconds:

• Yellow: pH below ~8.6 — would indicate soap that is neutral or below normal range (unusual for soap; may indicate fully spent or rancid batch)
• Orange: pH 8-10 — target range for cured soap, safe
• Red-orange to red: pH 10+ — elevated alkalinity; check further with cabbage juice or zap test
• Bright red: pH 12+ — strong alkalinity; likely excess lye

Turmeric is less useful for distinguishing between pH 10 and pH 12 — for that precision you need cabbage juice. Use turmeric as a quick first pass or when cabbage is unavailable.

Beet Juice Indicator

Beet juice is less reliable than cabbage juice as a pH indicator because its color change is more limited, but it can detect strongly alkaline conditions.

Making beet indicator:

Boil chopped beets in water for 20-30 minutes. Strain and cool. The deep red liquid is your indicator.

Reading beet juice:

• Deep red/purple: Acidic to neutral (pH below 7)
• Red: Near neutral to mildly alkaline (pH 7-9)
• Pale pink or fading: Moderately alkaline (pH 9-11)
• Yellow or colorless: Strongly alkaline (pH 11+)

Beet is most useful as a “pass/fail” check: if beet juice applied to your soap fades to pink or yellow, something is wrong. The color shift at pH 11+ is dramatic and easy to spot even without calibration.

Making Indicator Test Strips

Liquid indicator applied directly to soap surfaces is convenient for testing individual bars. But for batch-testing many bars quickly — or for giving community members a way to test soap they receive — paper indicator strips are more practical.

How to make cabbage juice test strips:

1. Use any absorbent paper: coffee filter paper, plain paper (not glossy), or absorbent cloth strips.
2. Cut into strips approximately 1 cm wide, 5-10 cm long.
3. Soak strips in cabbage juice indicator solution for 2-3 minutes.
4. Remove and lay flat on a non-reactive surface (wood, ceramic, glass) to dry.
5. Do not use metal surfaces — iron and copper react with the indicator and ruin it.
6. Allow to dry completely in a shaded area. Direct sunlight degrades anthocyanins.
7. Once dry, store in a sealed container away from light, moisture, and acidic fumes (vinegar vapors will turn all your strips red).

Properly made and stored strips last several months.

Using the strips:

1. Wet the surface of the soap bar with a few drops of clean water.
2. Press the indicator strip against the wet surface for 5-10 seconds.
3. Remove and observe color immediately. The color will continue to shift as it dries — read it within 20 seconds.
4. Compare to your color reference table.

Making a color reference card:

When you first make a batch of strips, also make reference samples. Dip strips into solutions of known pH:

• Vinegar (pH ~2.5): bright red
• Clean water (pH 7): violet-blue
• Baking soda solution (pH ~8.3): blue-green
• Washing soda solution (pH ~11): green
• Fresh lye water (pH ~13): yellow-green

Dry these reference strips alongside your test strips and glue to a card with labels. This gives you a visual comparison key that anyone can use without knowing the chemistry.

What To Do If pH Is Too High

If your soap tests at pH 11 or above, the batch has excess free lye. You have three options:

Option 1: Extended cure If pH is borderline (pH 10.5-11) and the bar is less than 3 weeks into cure, continue curing and retest weekly. Saponification can continue slowly for up to 6 weeks. This is only appropriate if the lye excess is marginal.

Option 2: Rebatch with additional fat This is the correct fix for genuinely lye-heavy soap (pH above 11 after full cure).

Calculate the extra fat needed. If you know the original batch weight and the recipe, work backward from your SAP value (the amount of lye needed per gram of a specific fat). A simpler approximation: for every 500g of original batch, try adding 30-50g of melted soft fat (olive oil, lard) during the rebatch.

Rebatch procedure: grate soap into a pot, add 15% of batch weight in water, heat gently until melted to applesauce consistency, add calculated extra fat, stir well, continue cooking at low heat for 45-60 minutes, test again. The pH should drop to 9-10 within that cooking time if the fat addition was sufficient.

Option 3: Industrial/cleaning uses Soap at pH 11-12 is safe for laundry, floor scrubbing, and tool cleaning. Reserve it for these uses, clearly label it as industrial soap, and do not allow use on skin.

Adding vinegar wash (surface neutralization): Some sources suggest washing finished bars with diluted vinegar to lower surface pH. This does not fix the underlying problem — it only neutralizes the surface while the interior remains lye-heavy. Do not rely on vinegar washing to make lye-heavy soap safe for skin use.

Common Mistakes

• Assuming that soap which passes the zap test also has an acceptable pH — the zap test detects free lye, but pH testing gives a more complete picture of overall alkalinity
• Testing immediately after unmold and concluding the batch is bad — all cold-process soap will test high at unmold; test again after full cure
• Storing cabbage juice indicator in metal containers — the iron reacts with anthocyanins and ruins the indicator
• Reading turmeric indicator after it has dried — turmeric color shifts continue as the paper dries; always read immediately while wet
• Confusing beet juice color shift with other causes — beet juice also fades with heat and age; make fresh indicator for each testing session
• Not making a reference color card and then being unable to interpret results consistently

Key Takeaways

• Finished cold-process soap should test pH 8-10 after a full 4-6 week cure; above pH 11 indicates dangerous free lye
• Red cabbage juice is the best natural pH indicator — blue-green means safe, green-to-yellow means excess lye
• Turmeric and beet juice provide secondary confirmation, especially when cabbage is unavailable
• Paper indicator strips soaked in cabbage juice can be made in bulk, dried, and stored for months of testing
• A color reference card with known-pH calibration samples allows anyone to interpret results reliably
• Lye-heavy soap can be rescued through rebatching with additional fat — it should not be used on skin or discarded without first attempting a rescue