Disodium EDTA

Disodium EDTA sits in a great many cosmetics at a tenth of a per cent or less, and does nothing a consumer can see. It is there to deal with metal.

Trace iron, copper, calcium and magnesium arrive in a formulation uninvited, in the water, in plant extracts, in pigments, and from the equipment. Iron and copper catalyse the oxidation of oils and the degradation of colour and fragrance. Calcium and magnesium interfere with surfactants. EDTA takes them out of circulation.

The parent molecule, ethylenediaminetetraacetic acid, is a two-carbon bridge carrying a nitrogen atom at each end, and each nitrogen bears two acetic acid arms. Six of those points, four oxygens and two nitrogens, can bind a single metal ion at once, wrapping around it. The resulting complex is soluble, stable and unreactive, so the metal is removed from the chemistry without being removed from the bottle.

The INCI name DISODIUM EDTA denotes one particular salt of that acid, registered as CAS 139-33-3 and EC 205-358-3 and named in full as disodium dihydrogen 2,2',2'',2'''-(ethane-1,2-diyldinitrilo)tetraacetate.^[1] The free acid is barely soluble in water and would drag the pH down; neutralising two of the four acid groups with sodium makes the material soluble and manageable near neutral pH, which is why a formulator working at skin pH reaches for a salt rather than for the acid.

EDTA was made by Ferdinand Münz. The patent that records it gives his address as Frankfort-on-the-Main, Germany, and assigns the invention to General Aniline Works, Inc., of New York.^[2] The problem it sets out to solve is neither cosmetic nor medical. The specification describes the amino polycarboxylic acids as a means of avoiding and rendering harmless the precipitates of water-insoluble metal salts, particularly those formed owing to the hardness of water.^[2] Hard-water scale, in other words, rather than anything to do with a face cream.

The dates need separating, because the year and the document do not coincide. Münz filed first in Germany, on 30 October 1935. A United States application followed on 22 October 1936, Serial No. 107,020. The patent usually cited for EDTA, US 2,130,505, is a division of that earlier case: it was filed on 3 April 1937 as Serial No. 134,737 and granted on 20 September 1938, and all of those dates are printed on its own front page.^[2] A reader who follows the reference expecting a 1936 filing will find a 1937 one, which is why this entry dates the substance from the German application rather than from either American one.

At the levels normally used, often between 0.05 and 0.2 per cent, disodium EDTA does three things at once. It slows oxidative rancidity by taking iron and copper out of play, complexing them into a soluble and unreactive form. It stabilises colour and fragrance, which degrade by the same metal-catalysed routes. And it makes the actual preservative work better than it otherwise would.

That last point is the one most often garbled. EDTA is not a preservative and has little antimicrobial effect of its own. What it does is strip divalent metal ions out of the outer membrane of Gram-negative bacteria, destabilising the membrane and letting the preservative through. It is a potentiator. A formula relying on EDTA in place of a preservative is not protected.

Disodium EDTA is not listed among the substances prohibited or restricted by the annexes to Regulation (EC) No 1223/2009, and is in ordinary cosmetic use in the European Union.^[3]

Two things are documented against it, neither of them about skin. The first is environmental. EDTA degrades poorly in surface waters, particularly once complexed with heavy metals, and the European Union risk assessment of edetic acid treats it as persistent on that basis; photolysis of the iron complex is the main route by which it breaks down at all.^[4]

The second is a naming problem with fatal consequences, and it belongs to intravenous medicine rather than to cosmetics. Edetate disodium and edetate calcium disodium are different drugs. The calcium salt is used to treat lead poisoning; the plain disodium salt strips calcium out of the blood. Between 2003 and 2005 three deaths from hypocalcaemia during chelation therapy were reported in the United States: a two-year-old girl in Texas and a five-year-old boy in Pennsylvania, both given the disodium salt, and a 53-year-old woman in Oregon. The report attributes the error to the two products' brand names, Endrate and Calcium Disodium Versenate, being used interchangeably.^[5] A tenth of a per cent in a face cream is not administering a drug, and the two situations should not be run together, but the ingredient's reputation has been shaped by the confusion.