• Ácido cítrico monohidratado 99,5 % en envase de 1 kg
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Citric acid 99.5% (1 kg)

Citric acid 99.5% (1 kg)

€10,57 EUR
€10,57 EUR
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Citric acid monohydrate 99.5% — Chelator for controlled aqueous cleaning

A solid organic acid that acts as a chelating agent: a molecule that traps metal ions and forms a stable complex with them, like a pair of tweezers. Once salified with a base, it allows metal salts and oxide deposits to be removed gradually. On its own, citric acid does not have a high complexing power: it is its salts, the citrates, that are used because of their better efficacy, which is why in practice it is always formulated with a base.

It is used in aqueous solution or gelled with a thickener —Klucel G, Nevek, Carbopol— when the substrate is absorbent or porous and penetration needs to be controlled. Against greasy and proteinaceous soiling, it is commonly combined with non-ionic surfactants of the Tween 20 type.

What is it used for?

  • Polychromy and canvases: removal of surface deposits of a proteinaceous nature and of overpaints. The manufacturer warns, in that same technical document, that many pigments may be attacked by these reagents and that they must be used with extreme caution on polychromy: the most exposed layers are the poorly bound ones, with more pigment at the surface.
  • Stone and masonry: rust stains and iron salts. On highly carbonated substrates —marbles, limestones— it is formulated as ammonium citrate, not as the free acid.
  • Buffered aqueous systems and synthetic saliva: citric acid is a tricarboxylic acid, with three acid groups that dissociate at different pH values. That is the reason it is the acid base of buffered cleaning systems: one and the same solution is brought to whatever pH is needed by adding a base, and it holds that pH stable throughout the treatment.

On carbonates —marble, limestone, stucco, fresco and calcareous ceramic bodies— it is not applied as the free acid, but formulated as ammonium citrate at pH 7-8, exactly as the manufacturer's technical document states. The acid reacts with the carbonate and releases carbon dioxide: that effervescence can damage the most superficial layers, especially on low-fired, painted or glazed ceramics. Like every chelating agent, it requires a prior test in an inconspicuous area and rinsing with demineralised water when finished. And the solution is prepared only with demineralised water: mains water contains calcium and magnesium cations that bind to the complexing agent and reduce its action.

Why choose citric acid?

  • It chelates less, and that is why it is easier to control. Compared with EDTA (ethylenediaminetetraacetic acid), the strong chelator of reference, citrate grips the metal far less tightly: its stability constant with iron (III) is Log K 10.9 against 25.1 for EDTA, and with copper (II), 6.1 against 18.8. That «weakness» is precisely the advantage: the literature points out that citrates are preferable because of their lower chelating power, since this means greater control and safety with respect to the original materials. A chelator that grips less is a chelator you can stop in time. (Log K = stability constant of the complex: the higher it is, the more tightly the chelator grips the metal ion.)
  • With chelators, more concentration is not more cleaning. Published studies do not go above 5% w/v (weight in volume: grams of solid per 100 ml of solution), concentrations of 10% or higher are documented as unsuitable because they visibly affect the paint or the varnish, and it has been shown that low concentrations of citrates —even at 0.1%— at an optimum pH can be more effective while reducing the risk to the artwork. A 1 kg pack goes much further than it looks.
  • It lets you work slowly. The most effective cleaning system is not necessarily the most appropriate one: the balance to look for is between efficacy, safety and control, and it is not desirable for the system to be too fast. With this material, pH and concentration are adjusted step by step, and the process stops where the conservator decides.
  • Fewer things in the solution, less possible residue. A surfactant or an additive is only justified if it produces an evident increase in effectiveness; if it does not, there is no point in adding non-volatile chemical agents that may mean a greater contribution of residues to the artwork.

Specifications

  • Manufacturer's designation: citric acid monohydrate
  • Purity according to the manufacturer: 99.5%
  • CAS No. declared in the safety data sheet: 77-92-9 (the CAS number, from the Chemical Abstracts Service, is the international code that identifies each chemical substance)
  • Appearance: solid, white, odourless
  • Melting point: 153 °C
  • pH declared by the manufacturer: 1.7 — this is a property of the solid substance, not the pH at which it is used
  • Solubility in water: 590 g/L. Also soluble in alcohols and ethyl acetate, according to the manufacturer's technical document
  • 1 kg pack

Presentation and storage

  • Presentation: 1 kg pack
  • Store between 5 and 25 °C, in a dry, well-ventilated place, away from heat sources and direct sunlight
  • Once opened, close the container carefully and keep it upright. Store away from oxidising agents and from strongly acidic or alkaline materials
  • Closing the container properly matters more than it seems: citric acid is sensitive to humidity in both directions —it takes up water in a humid environment and loses it in a dry one— and in both cases what the balance reads stops being what you think it is

Article manufactured by C.T.S., reference 01201010.

Safety

Manufacturer's classification (CTS safety data sheet, version 2 of 12-05-2023, in accordance with Regulation (EU) 2020/878). Signal word: Warning.

Hazard class and category: Serious eye damage/eye irritation, Hazard Category 2.

Pictogram: GHS07 (exclamation mark).

  • H319 — Causes serious eye irritation.

Precautionary statements: P264 Wash hands thoroughly after handling · P280 Wear protective gloves, protective clothing, eye protection and face protection · P305+P351+P338 IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing · P337+P313 If eye irritation persists: Get medical advice/attention.

Full safety data sheet available on request at info@innovart.art.

Frequently asked questions

At what pH is it used?
The working pH is not the pH of the acid in the jar: citric acid is brought to its working pH by salifying it with a base. On paint structures with oil binders, the accepted recommendation is to stay within the range of pH 5.5-8.5; above that, the risk of pigment loss and of erosion of the paint layer increases. And it is worth having the other half of the picture: aqueous solutions are normally used at pH 8-9 because they clean better, especially against greasy materials. You clean better higher up and you risk more higher up — that tension is the conservator's decision in front of the artwork, not something a product page can settle.
What about the conductivity of the solution?
Wolbers places the aqueous solutions used in cleaning processes within the range of 4-8 mS/cm (millisiemens per centimetre, the unit of electrical conductivity). The reason matters more than the figure: if the ion concentration is very high, the ions diffuse into the interior of the paint structure and exert a pressure on the surface that can end up breaking up the paint.
What water should it be prepared with?
Only with demineralised water, and the manufacturer is categorical about it. Mains water contains, to a greater or lesser degree, calcium and magnesium cations that bind to complexing agents and reduce their action: preparing the solution with tap water takes efficacy away from it before you even start.
Does it need rinsing afterwards?
Yes, and it is not an optional step. The manufacturer's technical document states that, once the compress has been removed or the object taken out of the solution, a wash must be carried out to eliminate the excess reagent. Rinse with demineralised water, straight away. The correct word is rinsing, not neutralisation: neutralising would mean adding a base onto the artwork, which is exactly what is not done.
Does it leave residues?
The literature indicates that, in low-concentration solutions and after rinsing, no residues of the chelator are detected, and that rinsing with water is usually enough to remove them. That is why the subsequent wash is not optional: it is part of the treatment, and the manufacturer's technical document says so too.
Can I use it on marble, limestone or stucco?
Not as the free acid. On carbonate stone, the manufacturer's own technical document says to formulate it as ammonium citrate at pH 7-8, precisely for removing iron salts from carbonate stones. The reason is the chelator's mechanism: once the iron and the copper are exhausted, it starts attacking the calcium that makes up the binder of the stone or of the fresco being worked on, with corrosion similar to acid attack.
Does it weigh the same anhydrous as monohydrate?
Citric acid exists in two forms: anhydrous and monohydrate, the latter with 8.6% water of crystallisation. For the same amount of actual acid, you have to weigh out around 9% more of the monohydrate. In practice this does not affect most treatments, because the formulations published in the literature are gravimetric —of the 0.5 g per 50 ml of distilled water type— and do not depend on the form. If you need to prepare a solution of exact molarity, write to us first at info@innovart.art and we will confirm it with you.

Any questions? Write to us on WhatsApp, visit innovart.art or email us at info@innovart.art and we will advise you.

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