Concentration Testing

Part of Acids and Alkalis

Methods for measuring the concentration of lye (potassium and sodium hydroxide) solutions without laboratory instruments — critical for consistent soapmaking and chemical work.

Why This Matters

A lye solution of unknown concentration is a problem. Too weak and your soap will not saponify properly — you end up with soft, greasy, uncured soap that spoils quickly. Too strong and your soap becomes harsh and caustic, damaging skin. In tanning operations, wrong lye concentration destroys hides. In chemical synthesis, the wrong base concentration alters yields or produces unexpected products.

Modern soapmakers use digital scales and calculated lye concentrations. Pre-industrial soapmakers — who produced reliable, consistent soap for centuries — had no scales capable of measuring lye concentration directly. Instead, they used a series of physical tests based on density, behavior, and organic indicators. These methods are surprisingly accurate and can be taught quickly to anyone with basic chemical sense.

This article documents the full range of concentration testing methods, from the roughest field tests to the most precise pre-industrial techniques. Use them in combination for best results.

Why Concentration Matters: The Chemistry

Lye solutions work by providing hydroxide ions (OH⁻) to react with fats (saponification) or other substrates. The number of hydroxide ions per unit volume is the concentration, typically expressed as:

• % by weight: Grams of NaOH or KOH per 100 grams of solution
• % by volume: Approximate for practical purposes at low concentrations
• Degree Baumé (°Bé): A historical hydrometer scale widely used before mass production of laboratory equipment

Typical target concentrations:

Application	NaOH concentration	KOH concentration
Hard soap (cold process)	28–33%	Not typically used
Soft soap (potassium)	N/A	20–28%
Mild lye for food curing	3–5%	3–5%
Strong caustic wash	10–15%	10–15%
Pulp processing	15–20%	15–20%

Method 1: The Egg Float Test (Traditional Soapmaker’s Test)

This is the oldest and most widely documented method. It uses the relationship between solution density and the flotation depth of an egg.

Procedure:

1. Use a fresh, raw hen’s egg in its shell
2. Gently lower the egg into the lye solution
3. Observe the flotation depth:

Egg Behavior	Approximate Lye Concentration
Sinks completely	Very weak (below 5%)
Floats with a pea-sized spot above surface	~20–25% — too weak for soap
Floats with a coin-sized area above surface	~28–33% — correct for hard soap
Floats nearly vertical, most of egg above	>40% — dangerously strong
Egg floats high and tips sideways	Nearing saturation (50%+)

Calibration tip: Test your egg in plain water first (it should barely float, or sink). If your egg floats noticeably in plain water, it is old and the air cell has grown — use a different egg.

Limitations: The egg test is approximate (±5% concentration range). It is reliable for the typical soapmaking range but cannot distinguish fine differences in concentration.

The correct soapmaking float

Traditional descriptions say the egg should float “the size of a quarter” or “a silver dollar” above the surface. This means a circular area about 2–3 cm diameter visible above the liquid surface — roughly a quarter of the egg’s volume.

Method 2: Hydrometer (Density Measurement)

A hydrometer measures solution density by how deep it sinks. Higher density = floats higher = more concentrated solution. This is the most precise non-titration method.

Making a simple hydrometer:

1. Take a narrow tube of clay, glass, or bamboo — sealed at one end
2. Weight the sealed end with clay, wax, or metal shot so the tube floats upright
3. Float the hydrometer in pure water and mark the water line as a reference
4. Float it in solutions of known concentration (calibrate using the egg test or known salt solutions) and mark the scale
5. Alternatively, use a known weight of dissolved salt in a known volume of water as calibration standards

Baumé scale for lye: The Baumé scale was historically used for lye. For sodium hydroxide:

°Baumé	Approximate NaOH%
10°Bé	~10%
15°Bé	~15%
20°Bé	~20%
24°Bé	~25%
28°Bé	~30%

To make a rough Baumé hydrometer: mark your hydrometer by floating it in salt solutions of known concentration (10% salt, 20% salt) to establish two reference points, then interpolate for lye.

Method 3: Touch Test (Qualitative Only — Caution Required)

Very rough only. Do not rely on this for important work.

Experienced soapmakers traditionally assessed lye strength by touching a drop to the tongue (briefly, immediately rinsing). A strong lye creates an immediate, persistent burning sensation. This is not recommended — it can cause chemical burns to lips, tongue, and throat at high concentrations.

A safer version: touch a single drop to the back of a wet hand for one second, then rinse immediately. Compare the tingling sensation:

• No sensation: very weak (below 5%)
• Mild tingle: moderate (5–15%)
• Definite burning: strong (15–25%)
• Immediate pain: very strong (above 25%)

This method should only be used by experienced practitioners for rough confirmation, never as the primary measurement.

Method 4: Indicator Paper Test

Indicator paper does not tell you concentration directly, but it confirms that your solution is strongly alkaline (above pH 12) — a necessary condition for soap-making concentration.

Procedure:

1. Dip turmeric paper (or red cabbage indicator paper) into the lye
2. A strong lye should immediately and dramatically change the color:
   • Turmeric: turns deep red-brown (characteristic strong alkali response)
   • Red cabbage: turns yellow-green
3. A weak lye gives a slower, less dramatic color change

Interpretation: If turmeric paper turns pale brown or reacts slowly, the lye is too weak. If it turns vividly red-brown within seconds, the lye is in the strong soap-making range.

Method 5: Saponification Test (Practical Verification)

The ultimate test is whether the lye actually saponifies fat correctly:

Procedure:

1. Weigh out 10 grams of known fat (animal tallow preferred — consistent saponification value)
2. Calculate the expected lye needed: for tallow, approximately 1 g NaOH per 9 g fat (≈11% by weight)
3. Add your lye solution drop by drop, stirring constantly, warming gently
4. Observe the trace (when the mixture begins to thicken like mayonnaise)
5. Record how many milliliters of lye solution were needed to reach trace
6. Calculate back to find concentration

This is a slow method but gives you a practical, real-world confirmation of your lye strength. It also tests batch-to-batch consistency in your lye production process.

Concentrating Weak Lye

When your lye tests weak, concentrate it by evaporation:

1. Pour the lye solution into an iron pot (iron is stable with lye)
2. Heat gently over a fire — do not boil vigorously as spattering is dangerous
3. Stir frequently to prevent hot spots
4. As water evaporates, the solution thickens and concentration rises
5. Test periodically with the egg float method
6. Stop when concentration reaches the target range

Do not over-concentrate

Evaporating past the target range produces progressively more dangerous material. Above 50% NaOH, you are approaching the molten solid phase — catastrophically corrosive. Stop and dilute rather than risk this condition.

Record Keeping for Consistency

Once you find a production process that yields consistent-strength lye, record every parameter:

• Volume and type of ash used per batch (hardwood ash produces stronger lye than softwood)
• Volume of water added
• Leaching time and temperature
• Number of passes through the ash bed
• Resulting concentration by egg test

This log allows you to reproduce consistent results without testing every batch from scratch — saving time and reducing risk.

The ability to reliably measure lye concentration is what separates systematic soap and chemical production from hit-or-miss craftsmanship. It is worth spending time establishing a reliable testing protocol early in your chemistry program.