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Alternative pH buffers
Can gluconic acid/sodium gluconate be used as an alternative to lactic acid/sodium lactate or citric acid/sodium citrate?
Looking for a final pH of 4.5 and like that it has both mild humectant and strong chelating properties. Wanting to keep the LOI as short as possible by using multi-functional ingredients.
Thanks in advance for any advice.
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6 Replies
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yes - in fact, any acid/conjugate base combination included at a sufficiently high level can act as a pH buffer
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Thanks @Bill_Toge
On a similar note… below is the LOI for a wildly popular hydrating toner. Do you think the formulator included the succinic acid/disodium succinate combo as a pH buffer?
Water, Butylene Glycol, Glycerin, PPG-10 Methyl Glucose Ether, Methyl Paraben, Disodium Succinate, Sodium Hyaluronate, Hydroxyethylcellulose, Succinic Acid,Hydrolyzed Hyaluronic Acid, Sodium Acetylated Hyaluronate.
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Whilst gluconic acid/sodium gluconate would make a good buffer, it must be formulated with great care.
Gluconic acid as normally purchased is a nominal 50% acid. What has to be taken account of is that this comprises an equilibrium mixture of gluconic acid and glucono-delta-lactone equivalent to 50% gluconic acid. It is only after considerable dilution that the lactone hydrolyses to the acid and this takes time.
What it means in practice is that after making a buffer mixture of a certain pH, you will find after a time (hours or days, depending on the concentration) that the pH drifts downwards as more of the lactone is hydrolysed.
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@johnb Thanks for that. Yes, this does add an extra step to the mix and some uncertainty on my part.
Dry Glucono-delta-lactone is all I could get my hands on. But I did know that it hydrolyzes to gluconic acid.
The product below is sold as a 50% gluconic acid solution in water where it states 5% GDL remains.
http://www.jungbunzlauer.com/en/products/gluconates/gluconic-acid.html
So I was assuming (perhaps incorrectly) that if I made a 55% solution of my GDL and waited 48 hrs, I could use it as the equivalent to the Jungbunzlauer product.
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More or less.
I have worked with the Jungbunzlauer gluconic acid and know that the true ratio of acid to lactone varies with temperature so working with the preparation as supplied can present some problems in certain circumstances. Gluconic acid is a very weak acid and, from distant memory, you may not require much gluconate salt to give the pH you require.
I think with these materials and the delay in the hydrolysis that it may be best to prepare test buffer solutions on an empirical basis, carefully noting the quantities of each component, giving time for equilibration and proceeding from there.
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Good food for thought @johnb
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