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I plan on making a very simple cleaning solution to clean metal contacts on PCB boards. It cannot contain anything that is flammable or hazardous so I've come up with the solution below.

  • Distilled Water ~92%
  • Ethoxylated Alcohol (CAS 68439‐46‐3) ~5% "surfactant"
  • Tetrasodium Glutamate Diacetate (CAS 51981-21-6) ~1% "chelating agent"

I am not making this solution in a perfectly sterile environment so I would like to add a preservative to increase shelf life. I've been researching for weeks and I cannot find anything definitive. I just want something that will easily dissolve into the solution above and not be harmful to metal. The solution will be applied to the metal via a cotton swab. Even if I find a suitable preservative how would I go about calculating how much to use?

My ideas for preservatives: I think Citric Acid (CAS 77-92-9) would be okay. I think a combination of Methylchloroisothiazolinone (CAS 26172-55-4) and Methylisothiazolinone (CAS 2682-20-4) would be okay as well.

Other concerns are: Do I need an emulsifier? Is there an additional non-hazardous chemical I can add to decrease drying times?

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Most producers/suppliers have recommendations for concentration ranges of their products. If you can´t get it directly, it is often possible to find the information on their websites (e.g. Schülke or BASF). The ranges are typically around 0,1% - 0,5%, but different products are often mixtures of different preservatives and with different concentrations, so it can be difficult to compare products directly.

Citric acid has some biocidal effect but is not very efficient, so I would not recommend it.

Why do you need a chealant? You use demineralized water and a nonionic surfactant, so ions in the product should not be a big issue.

The CAS-number you refer for the surfactant covers a variety of chemicals, with different C-chains and degrees of ethoxylation. They are generally classified as mostly soluble in water, but it may be necessary to add a hydrotrope (e.g. Cumenesulfonate) or co-solubilizer (e.g. Butylglycol).

It is difficult to suggest the appropriate preservative, as it depends on your exact formulation. I will rather refer to the suppliers/producers. If you want to look through possible chemicals, a place to start could be ECHAs list of biocidal products: https://echa.europa.eu/information-on-chemicals/biocidal-active-substances (see under “In-can preservatives” –PT6). Things to consider is: solubility (and stability) in your formulation, at what pH is it effective and off course any regulatory issues.

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  • $\begingroup$ I know, or at least think, that the main use for chelants in cleaning solutions is to deal with the metal ions in the tap water they use to make them. I understand my solution uses distilled water but the metal contacts I'm trying to clean come into contact with other metal contacts and they leave fine metal debris on each other over time. Not sure if ultra fine metal debris classifies as metal ions or not but I read surfactants are rendered useless against metal ions without a chelant. Surfactant is Neodol-91-6. Not sure if it's any good. Only thing I could find. $\endgroup$ – Mickey Jan 6 at 8:48
  • $\begingroup$ Handling Ca/Mg in tap water is one purpose of chealants. Another could be to bind catalytic ions (eg. Fe, Mg, Cu) which can cause stability problems. Neither seems to be a problem in your case. But a chealant could very well be helpful in dissolving the dirt. Be aware that pH has a great effect on the efficiency of a chealant. Your type of surfactant is used in many different applications, whether it is the best choice for this purpose can only be determined in some realistic tests. $\endgroup$ – FrankS Jan 6 at 9:44
  • $\begingroup$ Thank you for the advice. I will have to look into that. My main concern at the moment is determining if I need a preservative, what kind, and how much. My cleaning solution will be worthless if bacteria/mold forms in it. From what I've read, even in a perfect sterile environment it's still recommend to have some, even if it's a minute amount. $\endgroup$ – Mickey Jan 6 at 19:14
  • $\begingroup$ You will probably need a preservative. Only solutions with either very low water activity, pH<2 or pH >10.5 are generally considered self-preserving. Production under strictly aseptic conditions is a possibility, but it is not easy to do. And if the product is not distributed in single-dose units, there is a great risk of contamination afterwards. $\endgroup$ – FrankS Jan 8 at 7:30
  • $\begingroup$ That's smart. I wasn't event thinking about contamination afterwards. Are there any preservatives you can think off hand that do well with solutions like mine? $\endgroup$ – Mickey Jan 8 at 22:47

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