Phenolphthalein
Phenolphthalein is an acid-base indicator, sold as a solution in alcohol, that turns pink from about pH 8.2 upwards. Soapmakers put a drop on a sample to look for free alkali; it is classed as a carcinogen in the EU, so it is handled as a laboratory reagent and kept out of the product.
- H341 Suspected of causing genetic defects
- H350 May cause cancer
- H361f Suspected of damaging fertility
Harmonised classification in Annex VI of Regulation (EC) No 1272/2008 (CLP), index number 604-076-00-1: Muta. 2, Carc. 1B (specific concentration limit 1 %) and Repr. 2. Indicator solutions in ethanol are also Flam. Liq. 2, H225, by their suppliers' safety data sheets.
- What it shows
- How liquid and transparent soap makers use it
- What it cannot tell you
- Handling it safely
Identity
- Formula
- C20H14O4
- CAS
- 77-09-8
- EC
- 201-004-7
- Also called
- phenolphthalein indicator solution
Guides
What it shows
Phenolphthalein is an acid-base indicator: colourless in acid and neutral solutions, turning pink as the pH rises from about 8.2 to 9.8. It hardly dissolves in water, 0.4 g in a litre by the US National Toxicology Program's figure, so it is sold dissolved in alcohol, usually at 1 %.
Every true soap is alkaline. A solution of well-made soap sits at about pH 9 to 10, above the point where the indicator starts to turn, so a drop on a moist bar often goes faintly pink even when there is no free lye, especially on young soap that still holds water. What should worry you is an instant, vivid magenta, above all in spots or streaks. In strongly alkaline conditions the pink also fades with time, as the indicator turns into a colourless form, so read the colour straight away.
How liquid and transparent soap makers use it
Transparent soap makers test the gelled base before the alcohol goes in. Catherine Failor puts a drop on a small sample and looks for the faintest pink: deep pink means spare lye, no colour at all means the soap is short of it. Once the base is dissolved in alcohol the test gets sharper, because in alcohol soap barely splits back into fatty acid and hydroxide and the indicator answers to free alkali alone.
Liquid soap is harder to judge in water. Thomssen warned in 1922 that a soap holding much water can turn phenolphthalein pink "in spite of the fact the mass is on the acid side", and he dissolved his samples in alcohol. The standard laboratory method, ISO 456, does the same, dissolving the soap in neutralised ethanol and titrating any free caustic alkali with acid. A pink drop on a watery paste proves little either way; the calculator's range check and a small superfat do more to keep a liquid soap free of spare lye.
What it cannot tell you
A drop on a bar cannot say how much free alkali there is, cannot tell hydroxide from the carbonate of soda ash, and says nothing about a pocket of lye the knife missed. Strong colours in the soap can hide it. A calibrated pH meter on a 1 % solution, or a laboratory titration, tells you more; the soap pH guide explains what each can and cannot show.
Handling it safely
Phenolphthalein is classed as a carcinogen in the EU. Its harmonised classification (CLP Annex VI, index 604-076-00-1) is Carc. 1B, "may cause cancer", with Muta. 2, suspected of causing genetic defects, and Repr. 2, suspected of damaging fertility. The carcinogen class applies to mixtures from 1 % upwards, which takes in the usual 1 % indicator solution: Carl Roth labels its 1 % solution in ethanol for professional users only, advises against household use, and classes it as highly flammable too. Under REACH, a carcinogen of that category at or above its limit may not be sold to the general public. Weaker solutions fall below the limit.
The evidence behind the classification comes from animals. The US National Toxicology Program has listed phenolphthalein since 2000 as reasonably anticipated to be a human carcinogen, on tumours in mice and rats fed it, and in 1999 the US Food and Drug Administration removed it from the ingredients recognised as safe and effective in over-the-counter laxatives. That is reason enough to handle it as a laboratory reagent.
In practice, wear gloves, keep it in a labelled dropper bottle away from food, children and flames, test a sliver cut from the batch rather than a bar someone will use, and throw the tested piece away. Phenolphthalein is on the EU list of substances prohibited in cosmetic products (Regulation (EC) No 1223/2009, Annex II, entry 417), so it must never go into the soap itself. A calibrated pH meter needs no indicator at all.
Sources
- Regulation (EC) No 1272/2008 on classification, labelling and packaging of substances and mixtures (CLP) , European Union (2008) . Annex VI, index 604-076-00-1, phenolphthalein: Muta. 2, H341; Carc. 1B, H350; Repr. 2, H361f.
- Phenolphthalein (CID 4764) , PubChem, US National Library of Medicine . The harmonised classification and the ECHA C&L notifications.
- Safety data sheet: Phenolphthalein (C.I. 764) 97 %, p.a., ACS, article T126, version 7.1 , Carl Roth GmbH + Co. KG (2024) . Index number 604-076-00-1; specific concentration limit Carc. 1B, H350: C ≥ 1 %.
- Safety data sheet: Phenolphthalein indicator solution 1 % in ethanol 90 %, denatured, article K034, version 7.0 , Carl Roth GmbH + Co. KG (2024) . Flam. Liq. 2, H225; Eye Irrit. 2, H319; Muta. 2, H341; Carc. 1B, H350; for professional users only; private household use advised against.
- Regulation (EC) No 1907/2006 (REACH), Annex XVII, entry 28 and Appendix 2 , European Union . Category 1B carcinogens listed in Appendix 2, phenolphthalein among them, may not be supplied to the general public at or above their specific or generic concentration limit.
- Regulation (EC) No 1223/2009 on cosmetic products, Annex II, entry 417 (CosIng) , European Commission . Phenolphthalein listed among the substances prohibited in cosmetic products.
- Report on Carcinogens, Fifteenth Edition: Phenolphthalein , US National Toxicology Program (2021) . Reasonably anticipated to be a human carcinogen on evidence from experimental animals; first listed in the Ninth Report (2000); water solubility 0.4 g/L; used as a 1 % alcoholic solution; removed from the FDA's list of safe and effective over-the-counter laxative ingredients in 1999.
- Little White Lyes (Handcrafted Soap and Cosmetic Guild conference) , Kevin M. Dunn (2015) . Phenolphthalein changes colour between pH 8.2 and 9.8.
- Evaluation of pH of Bathing Soaps and Shampoos for Skin and Hair Care , Tarun J. et al., Indian Journal of Dermatology 59(5):442–444 (2014) . Most bar soaps between pH 9 and 10 at 1 %.
- Kinetics of the fading of phenolphthalein in alkaline solution , Nicholson L., Journal of Chemical Education 66(9):725 (1989)
- ISO 456:1973 Surface active agents: Analysis of soaps, Determination of free caustic alkali , International Organization for Standardization . Soap dissolved in neutralised ethanol and titrated with acid.
- The Soap-Maker's Manual , E. G. Thomssen, D. Van Nostrand (Project Gutenberg edition) (1922) . Watery soap can turn phenolphthalein pink although on the acid side; samples tested in alcohol.
- Making Transparent Soap , Catherine Failor, Storey Publishing (2000) . Phenolphthalein on a sample of the gelled paste: the faintest pink as the target, deep pink for spare lye, no colour for too little.