Encyclopedia · chelator
Citric acid
The most common pH adjuster in shampoos and conditioners and, in a 2025 lab study, a calcium-removing treatment that strengthened chemically treated hair.
What it is
Citric acid is a natural acid found in citrus. In personal care it lowers pH to the hair's natural range and chelates calcium and iron.
How it works
It chelates calcium and other metals, gives an acidic pH and reinforces protein bonds. A 2025 study found citric acid treatment cut calcium in chemically treated hair and improved its mechanical properties, which the authors attributed to chelation, pH and a stronger non-covalent protein network. It was an in vitro tress study. The use of citric acid as a "bond builder" in bleach routines is a separate topic (see bond builders).
What the studies show
- Lab treatment of chemically treated human hair with citric acid cut hair calcium by 21-42%. Keratin crosslink density rose (denaturation temperature up 6.7-15.0%), tensile modulus rose 7.5-23.5% and fatigue life rose 56-124%. The effects were tied to chelation, pH and protein-network reinforcement, and the work was in vitro (Zhang 2025).
- Imaging of coloured hair washed in tap water showed calcium concentrated in the sulfur-rich layers of the cuticle, and copper uptake can add to damage during later colouring (Smart 2009).
- In over 300 women from 9 countries, calcium, magnesium and copper rose from root to tip, and those who rated their hair health as poor had higher metal levels (Godfrey 2013).
The lab evidence on hair is promising. We found no human wear trial or comparison with ordinary shampoo. The abstract does not state the concentration used in the tress treatments.
Side effects and safety
It is very safe on skin at the low concentrations used in shampoos. As a general precaution, acid stings the eyes and broken skin, and concentrated do-it-yourself solutions can irritate.
Bottom line
A sensible cheap tool for hard-water coating: a dilute citric acid rinse or chelating shampoo every 1-4 weeks, followed by conditioner. The right concentration and frequency are not established by trials, and the effect on colour fading is untested here. See shampoo pH and hard water and hair.
Studies 3
Zhang D et al. Reinforcing chemically treated human hair with citric acid. 2025
Tested: citric acid treatment
Result: Citric acid reduced calcium in chemically treated hair by 21-42% and improved mechanical properties.
Read the study ↗Smart KE et al. Copper and calcium uptake in colored hair. 2009
Result: Calcium concentrated in the sulfur-rich layers of the cuticle of coloured hair; copper uptake can add to damage during later colouring.
Read the study ↗Godfrey S et al. Metals in female scalp hair globally and its impact on perceived hair health. 2013
Result: Calcium, magnesium and copper rose from root to tip, and women who rated their hair health as poor had higher metal levels; hardness ions were higher in people who coloured their hair. Unilever-affiliated authors; an association only.
Read the study ↗Related
- Disodium EDTAThe classic cosmetic chelator, which grabs calcium, magnesium, iron and copper so shampoos lather and keep in hard water, with no clinical hair trials of its own.
- Tetrasodium EDTAThe more alkaline, more soluble EDTA salt, with the same chelating job as disodium EDTA and wide use in clarifying and chelating shampoos.
- Sodium phytateA rice- or corn-derived chelator used in natural shampoos as a preservative booster and iron and copper binder, with no hair trials.
- Tetrasodium glutamate diacetateA biodegradable, glutamic-acid-based chelator marketed as a greener EDTA replacement, with essentially no hair-specific data.
- Hard water and hairHard water leaves calcium on hair and can irritate skin, but there is no good human evidence that it causes hair loss, and chelating shampoos are untested in trials.
- Shampoo pHHair and scalp prefer mildly acidic conditions, but most shampoos are less acidic than that and the case for lower pH rests on lab mechanism rather than wear trials.
- Bond builders (Olaplex, K18, maleic acid, "acidic bonding")Bond builder is a marketing label for four unrelated chemistries; all have lab results on cut hair and none has a trial on real heads.