Eccrine Sweat and ISO 3160-2: Two Different Fluids, and Why EN 1811 Sweat Will Not Do
Nineteen amino acids are what attack a plated surface. The nickel-release fluid leaves them out on purpose, and the pH is not settled.
1. Why can an EN 1811 sweat not answer a corrosion question?
A laboratory that already buys EN 1811 sweat reasonably assumes it can reuse it on a watch case or a gilded clasp. It cannot: the two fluids answer different questions and are chemically different because of it.
The EN 1811 fluid is deliberately simple — sodium chloride, urea, lactic acid — because the measurement at the end is a trace nickel figure by ICP or AAS, and every additional species is one more thing that can contaminate the blank or chelate the analyte. A corrosion study asks the opposite. What attacks a plated surface over weeks of skin contact is not chloride alone but the amino acids and metabolites, which complex metal ions and keep them in solution instead of letting a passive layer settle. Run a tarnish test in EN 1811 sweat and the surface looks better than it will on a wrist, because the chemistry that does the damage was left out on purpose.
In practice: a nickel-release figure in µg/cm²/week calls for the EN 1811 fluid. Appearance, discolouration, plating durability or metal-ion release into a complexing medium call for the eccrine fluid. Buying one and writing up the other is the mistake this guide exists to prevent.
2. What does a full eccrine sweat actually contain?
Eccrine sweat is the thermoregulatory secretion produced over most of the body surface, and a realistic simulant reproduces its three families of solute rather than a saline approximation of it. The formulation supplied here derives from the NIOSH measurement of real perspiration published by Harvey, LeBouf and Stefaniak in 2010, and carries 16 amino acids at human-sweat concentrations, the electrolytes (sodium, potassium, chloride, bicarbonate, phosphate, sulfate and magnesium) and the metabolites (urea, lactic acid, glucose, uric acid and creatinine), at pH 5.3.
The amino acids are the part that matters, and the part usually missing from a cheaper substitute. Histidine, glycine, serine and their neighbours are metal chelators: they keep copper, nickel and silver ions mobile, produce the dulling a wearer actually sees, and are why the fluid costs more to make than a salt solution.
3. Is the ISO 3160-2 fluid the same bottle under another number?
| Fluid | What is in it | What it answers |
|---|---|---|
| Eccrine sweat | 16 amino acids, electrolytes and metabolites, pH 5.3 | Metal release, contact corrosion and sensor calibration in a realistic medium |
| ISO 3160-2 solution | NaCl 20 g/L, NH4Cl 17.5 g/L, lactic acid 15 g/L, acetic acid 2.5 g/L, urea 5 g/L, pH 4.7, no amino acid | Whether a gold covering survives an accelerated corrosion exposure |
| EN 1811 solution | Sodium chloride, urea, lactic acid, pH 6.5 | Nickel release in µg/cm²/week |
Three chemistries, not one fluid wearing three labels. The ISO 3160-2 bath is roughly thirteen times saltier than human perspiration, carries ammonium chloride and contains no amino acid: it is engineered for severity, to fail a plating fast. The eccrine fluid is engineered for the opposite. Both are legitimate and neither is interchangeable.
A word on DIN 53160, which is often quoted next to ISO 3160-2 in supplier catalogues, including ours until recently: it is a colour-fastness standard for consumer articles, and the sweat simulant it references is the EN 1811 one. It therefore belongs with the nickel-release fluid, and has nothing to do with either product on this page.
4. What pH does each fluid have, and what will we not assert?
The eccrine fluid is supplied at pH 5.3 ± 0.1, the value in the source publication, with the measured figure on the Certificate of Analysis. There is a hard floor underneath it: below about pH 4.8 the salts precipitate rapidly, which is not a slow drift but a lost batch, so the fluid is never corrected downwards with acid. A pH 6.5 variant is documented in the same source and can be supplied for laboratories that want to compare directly against EN 1811 at equal pH.
For the ISO 3160-2 solution we are more careful. Supplier material and secondary sources converge on pH 4.7 and on the composition in section 3, but we have not read that against the normative text, which we have not purchased. We make the fluid to 4.7 and state the measured pH on the certificate.
If you are writing a method rather than following one, take the pH and composition from the standard you cite, not from a catalogue. Ours included.
5. Why is there no preservative, and what does that cost?
An amino-acid fluid with urea, glucose and lactic acid in it is, microbiologically, a growth medium. There are two ways to handle that: add a fungicide and a bactericide and quote a long shelf life, or make the fluid to order, ship it promptly and date it honestly.
We do the second, and not for purity's sake. A biocide is one more ligand in a bath whose entire purpose is to model what a complexing medium does to metal — one more thing that adsorbs, and on a long exposure one more variable the report cannot separate from the result. On a nickel-release test that hardly matters; on a corrosion study it is exactly what makes two laboratories disagree.
The cost falls on both sides: short dated life, made to order, a lead time of a few days. For a study that runs for weeks that trade is usually the right way round. If you genuinely need a stabilised fluid held in stock, say so when you enquire and we will tell you plainly whether we can serve it.
6. How do you order the right sweat in practice?
- Say which question you are answering: coating durability under ISO 3160-2, or realistic metal release. Not the same bottle — see section 3.
- State your pH rather than accepting a default, for the reason in section 4.
- Size the order by exposed surface, not specimen count. A bracelet takes far more than a ring, and its area is easy to underestimate.
- Plan for the lead time. Made to order means a few days.
Need this fluid at a pH your method fixes, in a volume above the bottle sizes, or to a specification the catalogue does not carry? Ask a chemist for a quote.