Glucono-δ-lactone as chelating agent

I would like to know if GDL is a strong enough chelator to prevent salicylic acid from forming complexes with iron?


  • I also would like to know the answer. I use it as PHA in a toner but it would be great if I could use it as a chelator, as I have to get EDTA from the US.
  • PharmaPharma Member, Pharmacist
    Depends on pH and other factors. GDL is not an iron ligand, gluconic acid is. Hence, it might be advisable to add the latter instead of the former if you have severe issues with salicylic acid-iron complexes.
    I wonder why you have an issue with that anyway???
  • @Pharma, I bought it as a PHA and can't find information about its chelating properties. Do you know the average usage (Disodium EDTA is 0.1-0.2%) by any chance?
  • DoreenDoreen Member
    Pharma said:
    I wonder why you have an issue with that anyway???
    I don't have issues with it. I just wanted to know, to prevent useless SA-iron complexes. If iron complexes are present, would that be visibly noticable? Can I detect it without performing special tests?
    I haven't worked with GDL before, so I know nothing about it.
    I can get disodium EDTA for example, but it's much more expensive for me to get it here (shipping costs mainly). The only other options I have regarding chelating agents is phytic acid and its sodium salt. :/ 
  • PharmaPharma Member, Pharmacist
    @ngarayeva001 Unfortunately, I don't. The recommended usage level is "quantum satis" = as much as needed. Gluconolactone as well as gluconic acid and it's salts have GRAS status, you can literally eat it by the spoonful.
    Being used as sequestrant, I'd go with what's used for others like EDTA or phytic acid.
    Gluconolactone slowly releases gluconic acid and thereby lowers pH if no buffer is used. This is not just bad because adding too much might drop your pH considerably but also because gluconic acid works best at a high pH (and I mean really high, like laundry detergent high).

    @Doreen Ah, okay. Well, I assume that you don't have THAT MUCH iron in your product as to cause noticeable salicylic acid loss or a colour change. The job of a common sequestrant/chelate is to catch trace amounts of iron in order to slow down fatty acid peroxidation and microbial growth. We're talking minute quantities of iron here, that is, unless you were to use iron oxide pigments... though, I don't see (prolly I'm blind?) why one would add salicylic acid to makeup and then add gluconolactone to dissolve said pigments :smiley: .
    What's wrong with phytic acid? Seriously asking because I have it here, bought out of curiosity and waiting to be used the first time in my life.
    Citric acid works too but the complexes are fairly weak.

    Something to consider with sequestrants/chelates is a point usually ignored completely: Once antioxidants (especially ascorbic acid) are added, iron(III) aka ferric iron, is reduced to iron(II) aka ferrous iron. The former is what builds all sorts of usually fairly stable complexes and what oxidises fatty acids whereas the latter is the form assimilated by microbes (and humans too) because it's a rather poor complex builder. Although adding antioxidants prevents rancidity, it annihilates most of the action of sequestrants and even enhances bioavailability of iron. Not adding antioxidants but good sequestrants instead can theoretically reduce free iron down to the point where fatty acid peroxidation and microbial growth are both inhibit nearly 100%. Iron is one of the limiting elements for microbial growth (except lactobacilli, which do not require iron but are killed by it) and depriving the environment/product is an efficient "preservation hurdle" and a strategy used by plants and microbes alike.
  • DoreenDoreen Member
    Thanks for your extensive answer! Extremely interesting topic.
    No I don't use the GDL to disperse pigments in colour cosmetics. :joy:
    It's for a simple salicylic acid 2% exfoliating toner that I have been making for years with disodium EDTA and without much else in it.
    I'm not even adding the EDTA from a microbial point of view, but mainly to keep the salicylic acid free from complexes. (Suffice it to say that the toner is well preserved of course.)

    Re: phytic acid. I've read an article about several bacterial species that are able to produce the enzyme phytase.
    I've read an answer of a manufacturer of phytic acid containing products somewhere on this forum which was hardly satisfactory, so I'm sceptic.
  • PharmaPharma Member, Pharmacist
    I'm only familiar with phytic acid in plant physiology and gardening. There are really just a few soil microbes I'm aware of which can use phytic acid. Sure, if you happen to have exactly those in your product...
    Problem is that (reminds me that I need to answer another thread regarding lecithin which has the same issue), besides iron, phosphate is THE limiting nutrient/mineral for microbial growth. Only a very few ingredients contain phosphorous; Lecithin and phytic acid as organophosphate and some phosphonate detergents (IIRC usually high pH laundry detergents) but these are even harder to degrade. Apart from that, it's only present in some plant extracts and claim ingredients such as ATP and DNA. The extra amount of available phosphate is probably the reason why lecithin containing formulations are a PITA to preserve. Theoretically, phytic acid with its 6 phosphate groups could be a real superfood for microbes but as I said, it's not readily degraded (in soil, that is).
    Back to your salicylic acid: How much iron do you guess are in your toner? 10 ppm as contaminants? 2% minus 10ppm = 2%. Question: what's the difference between 10 ppm iron-salicylic acid complexes and 10 ppm iron-gluconic acid complexes?
    Out of curiosity, what's in your formulation that requires addition of a chelate?
  • DoreenDoreen Member
    edited June 2019
    I have no idea what the level of iron is in my toner, if it would be as low as 10 ppm I would be very glad.
    I use 5 ltr jerrycans of water that, according to the label, should be distilled ánd deionized, but the levels of trace metals aren't mentioned. It's the cheapest choice for me (the prices of distilled water in DIY shops here are outrageous).

    And then the salicylic acid. I don't know the precise equivalent of the chemical purity grade that's used in the US, but I think the salicylic acid that I used to buy was comparable to 'lab' ("A chemical grade of relatively high quality with exact levels of impurities unknown; usually pure enough for educational applications. Not pure enough to be offered for food, drug, or medicinal use of any kind." Source).
    Since a week I use Curcylic 40 by Vantage Specialty Ingredients, which also contains cocamidopropyl dimethylamine.

    These are the two main reasons that I use a chelating agent in it. 

    Edit: I think the grade of the SA is 'purified', a level below 'lab', but I'm not sure.
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