HA solubility – email chat with Prof Brian Darvell

Brian Darvell is Honorary Professor at the Birmingham University School of Dentistry. He has been involved in research and teaching in Dental Materials Science since 1969.  His work has covered many areas, but his greatest interest (which started as his post-doc study in 1975) is the solubility of calcium phosphates, and especially that of hydroxyapatite. He is also author of the textbook “Materials Science for Dentistry”.


“What do you think might contribute to the hydroxyapatite build-up on nappies?Alkalinity would drive it, if there is both calcium and phosphate present.
It can be made in a myriad of ways If there are Ca and P.  It is almost too easy.

A pH above 7 should drive it more effectively than a lower pH.
Broadly, the higher the pH, the more HA that should form. Solubility (probably) rises again at very high pH, but this is out of normal experience.

In a practical sense there is no minimum pH. It could be as low as pH 3 if the other conditions are right (i.e. enough Ca and P), but that would be unusual I suspect.

Would wash temperature make a difference to removal of HA (ie would washing at 90 degree increase the solubility of HA)?
The converse – solubility falls with increase in temperature.
[but] any mechanical action will remove such material.
The retrograde solubility [temperature dependence] might not extend to such high temperatures – I have no info on this.
Thirdly, dissolution is slow, but would be accelerated at higher temps anyway.

[It seems that attempting to use high temperatures to remove HA is another complicated chemistry issue!]

Does this also mean you believe wash temperature will make no difference to the rate of deposition [creation] on fabric?

It is a common ‘myth’ in nappy circles that nappies lose absorbency over time due to detergent build up, so many people routinely wash their nappies in a half-dose of detergent.  Other experts have said that modern machines do a very good job of washing away detergent. Similarly, we found no evidence of detergent build-up on any of the nappies we had tested.
I think it is a nonsense to suggest it.  The binding is not that strong, and if it were we would all be in trouble!

It is not the machine as such, but allowing enough rinsing water.  I use ‘extra rinse’ because that gives a better outcome.  Higher temperature rinsing would also be beneficial to desorb the detergent from fabrics.

As mentioned, urine could be the source of the phosphate, and if so, could it be forming in the nappy bucket while waiting to be washed?
Good chance of that, yes, if the pH rises on soaking (eg urea decomposition – which seems very likely to be fast enough).

What do you think might remove it?
An overnight soak (warm water would be good but not essential) in any household lime descaler.  Citric acid is probably best (benign) – cheap in some supermarkets and hardware shops, but formic acid (even acetic acid [as found in vinegar]) may be worth a try, although are less pleasant to work with and eye-care is recommended.  (Na-EDTA would be best, but I doubt that this is available to the public, although no hazard.)

Afterwards, simple rinsing with hot water (several times) and drying is enough.
Stress the need for a control whatever is done (i.e. plain water but otherwise identical temperatures, timing and handling)

I would predict that any ‘scale’ formation will be removed and that use of a normal (non-irritant) fabric softener will restore pretty much the expected loft and feel in a normal washing cycle.

Low temperature washing is not a good idea from the microbiological point of view (I am appalled by this).  Cotton may shrink somewhat, but it does not do that cumulatively indefinitely (i.e. pre-shrunk fabric).

An acid wash is the best way other than using a neutral chelator.  Citric acid would be very good, but likely to be slowish, so no quick rinse.

How could we adjust our wash routine to make it less likely to happen?
Normal [wash routine], with a once in a while (i.e if [HA] noticed) citric acid soak.

How will soaking in acid overnight help?
The solubility rises steadily with fall in pH.

You recommend citric acid, but I think vinegar is something people are more likely to have in their cupboards. Is there any reason why citric acid would remove HA better than vinegar?
Yes, because it [citric acid] is a chelator and binds the calcium.

What other experiments do you think we should do?
X-ray powder diffraction (XRD) to fully characterise the deposits.

[Note from Jennifer at Cardiff Catalysis Unit: XRD should be possible, but will require nappy scraping]

Regarding phosphates in laundry detergent: An industry expert tells me that the maximum legally allowed would give a concentration of about 0.01 g of PO4 per litre at the highest. 0.005 g PO4 /l is probably more common.

Even so, at high pH, that might be enough if there is enough calcium.  Calculations are complicated by many factors, so I cannot be definitive in this.  If we had a Ca concentration I might be able to comment better.

Although, of course, this legislation was only brought in in 2012. It’s possible that some of our test nappies had been washed in detergents made before the change.
That stands a chance, if they have never been acid-washed.
One point though – has anybody ever reported loss of that deposit under any circumstances?
[this might be something for us to google – how do people break down HA in other circumstances? Or does no one else try to get rid of HA?)

 Thank you Brian for answering our questions – we may return to you with more!

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