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Problem:

On my boat I have a marine toilet. It is operated with salt water.

The problem is, on the interior of the outflow tubing it accumulates some sort of hard grime. I am looking for a good way to dissolve this grime as it is impossible to replace the tubing.

From experience it is known that citric acid work - but not very well. Also it leaves a "sandy" residue that is difficult to flush.

Does anybody have an idea of what this "grime" consists of - and thus how best to dissolve it?

Materials:

On hand I have:

  • citric acid
  • 56% phosphoric acid
  • 23% HCl

Materials:

  • tubing: PVC
  • toilet itself: porcelain
  • pump: ABS or polypropylene with selected parts in acetal resin, with 316 passivated stainless steel fastenings, brass weights and neoprene seals and gaskets
  • valve: grey, harder plastic

I know this quite vague but am hoping somebody can help anyway.

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To my knowledge, marine water contains great amount of $\ce{Ca}$ and $\ce{Mg}$ ions, that gives insoluble carbonates and phosphates. So, most probably, you have mix of carbonates, phosphates and, possibly but unlikely, sulfates of this metals and possibly some organic salts, like oxalates. Carbonates are readily dissolved by $\ce{HCl}$ wich in weak solutions will not dissolve copper, but will slowly dissolve steel. It is more interesting to use $\ce{Na2H2edta}$ . It dissolves $\ce{CaCO_3}$ easily and, to my knowledge, in hight concentration dissolves $\ce{Ca_3(PO_4)_2}$ as well, but not sure in the latter.

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  • $\begingroup$ Thanks for the answer. You say "insoluble carbonates". That means insoluble in water, right? What is the name of Na2EDTA if I want to buy it? Is it better than HCl or did you recommend it only because it affects metals less than HCl? $\endgroup$ – ARF Aug 13 '12 at 19:19
  • $\begingroup$ Ok, I found EDTA on Wikipedia. However I did not find Na2EDTA. I did find Na2H2EDTA, Na4EDTA and CaNa2EDTA. Did you mean any of those? How would I use them? If I understand correctly those are salts, right? I assume I would dissolve them in water and let is sit in the tube? $\endgroup$ – ARF Aug 13 '12 at 20:20
  • $\begingroup$ I meant the first one. One should made reasonably concentrated solution and submerge surface to be cleaned under it for several hours or even several days. EDTA derivatives under different names may occur in everyday chemistry, like some machine washing products, so may be you can find prepared solution on shelfs with products to threaten hard water for wash machines. Citric acid is way to go, according to this article ncbi.nlm.nih.gov/pmc/articles/PMC1173122 but I never saw it used this way, unlike EDTA derivatives, so I cannot say for sure. $\endgroup$ – permeakra Aug 14 '12 at 4:39
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A cheaper and easier to find solution than EDTA (permeakra's answer) is citric acid. Citric acid reacts with metal carbonates (bicarbonates, phosphates, chlorides, etc.) to produce the soluble citrates. The citrate ion keeps the metals in solution by chelation, the same process that EDTA (ethylene diamine tetra acetate) does.

Citric acid can be purchased in the grocery store or convenience store as one of the principle components for any sour powdered drink mix (like Tang).

You could also use any substance or cleaner designed to remove hard water deposits or lime scale, such as CLR, which contains lactic and gluconic acids as chelating agents.

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