Holding tank calcium deposits

mcanderson

mcanderson

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My holding tank has lumps of what I can only guess are lumps of calcium in the bottom. Sometimes the smaller of these get sucked into the pump which I use to empty the tank overboard. This results in me having to remove the lumps from the pump - not my favourite job. Any thoughts for how to break these up and then pump them away? I thought draining the tank and then adding 10l of vinegar.
 
The lumps are already free in the tank and so I was looking for something less invasive. Also I don't know French for brick cleaner, I do not the French word for vinegar and its €0.33 a litre.
 
vyv_cox said:
Hydrochloric acid is considerably more effective than vinegar - difficult to hazard a guess but something like 1000 times?
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Hydrochloric more effective – yes, partly because one can readily purchase it (as brick acid) in much stronger solution than acetic acid. But 1000x more effective – I’m not so sure.

(A) Virtually the same amounts of calcium carbonate can ultimately be dissolved by the same volumes of equimolar HCl and acetic acids. See calculations and explanation at https://en.wikipedia.org/wiki/Calcium_carbonate#Solubility_in_a_strong_or_weak_acid_solution - where, for example, it is estimated that 1 litre of IM HCl will dissolve 50.0 g CaCO3, and 1 litre of IM acetic acid will dissolve 49.5 g CaCO3.

(B) Of course, dissolution will be inherently a lot faster with HCl than with acetic acid. Just how much faster I don't know, but I suspect not 1000x faster – though I might be wrong.

(C) Brick acid is about 10x stronger (ca. 10M HCl) than vinegar (ca. 1M acetic acid). Thus, volume for volume, undiluted brick acid will ultimately dissolve ca. 10 times as much CaCO3 as vinegar – but certainly not 1000x more – and will be faster acting (both inherently, and also by virtue of its 10x higher acid concentration).

Descaling solutions based on organic acids like acetic acid (or on complexing agents) are inherently safer than strong mineral acids like brick acid. But if someone can use 10M mineral acid without risk to themselves or to whatever they are cleaning, it will be faster and more efficient.

If they cannot, they should not IMO be deterred from using vinegar by the thought that it will inevitably be 1000x less effective. Provided that they do not need instant dissolution, and can maintain an adequate concentration in contact with the scale over a sufficient period (e.g. by replenishment), it might be only 10x less effective.

And of course applying heat may also speed things up – and a saucepan of vinegar on the galley stove is barely more hazardous than making Bearnaise sauce. :)
 
You seem to be assuming that the acetic acid content of vinegar is considerable. In fact it is somewhere around 2%. My 1000 times was written tongue in cheek but is probably closer to reality than your 10. And that is without considering timescale. I like to minimise contact between acid and hardware, even though there is no known reaction. With HCl it is all over in no more than ten minutes.
 
vyv_cox said:
You seem to be assuming that the acetic acid content of vinegar is considerable. In fact it is somewhere around 2%. My 1000 times was written tongue in cheek but is probably closer to reality than your 10. And that is without considering timescale. I like to minimise contact between acid and hardware, even though there is no known reaction. With HCl it is all over in no more than ten minutes.
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I was using ca. 1M acetic acid for vinegar as figures I have seen indicate about 0.8M – so not an ‘out of thin air’ assumption. Your “… difficult to hazard a guess but something like 1000 times?” question intrigued me, and I dug a bit to try to get a handle on it – with the interesting outcome for me that the inherent capacity of acetic acid to dissolve CaCO3 (leaving aside timescale) appears to be not that different from that of HCl. (I should probably have realised that straight off – but it is dependent on the pKa of acetic acid and the Wiki link was a short cut to an answer.)

It just takes a lot longer, but whether 1000 times longer one just doesn’t know. If HCl takes 10 minutes, 1000 times that would be about 7 days – and if so, keeping the vinegar replenished could indeed be a pain, particularly perhaps in the OP’s case. But in general, depending on what I was trying to descale and the circumstances, I might choose to try vinegar first as a ‘slower but safer’ option - though as I said, I agree that if there is no risk to self or materials HCl would be much more efficient.
 
I find the most effective way to clean my plastic holding tank is by blasting the deposits with Greek mains water pressure while banging the tank with my hand. Considerably more effective than trying to dissolve the deposits with whichever acid one prefers. Last season I managed to get the internals back to pretty much their as-new condition in about half an hour.
 
vyv_cox said:
You seem to be assuming that the acetic acid content of vinegar is considerable. In fact it is somewhere around 2%.
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IIRC typically 4 to 5% ( determining which were the "best" brands of vinegar was one of the school chemistry experiments). What's in my kitchen cupboard is stated to be "about 5%". Sarsons pickling vinegar is 6%

Hydrozoan said:
I was using ca. 1M acetic acid for vinegar as figures I have seen indicate about 0.8M – so not an ‘out of thin air’ assumption. Your “… difficult to hazard a guess but something like 1000 times?” question intrigued me, and I dug a bit to try to get a handle on it – with the interesting outcome for me that the inherent capacity of acetic acid to dissolve CaCO3 (leaving aside timescale) appears to be not that different from that of HCl. (I should probably have realised that straight off – but it is dependent on the pKa of acetic acid and the Wiki link was a short cut to an answer.)
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The mass of CaCO3 dissolved is not dependent the value of pKa A given volume of dilute acetic acid will dissolve the same mass as the same volume of hydrochloric acid of the same molar concentration.
The value of pKa indicates the degree of dissociation of the acid in dilute solution.
 
VicS said:
IIRC typically 4 to 5% ( determining which were the "best" brands of vinegar was one of the school chemistry experiments). What's in my kitchen cupboard is stated to be "about 5%". Sarsons pickling vinegar is 6% ...

The mass of CaCO3 dissolved is not dependent the value of pKa A given volume of dilute acetic acid will dissolve the same mass as the same volume of hydrochloric acid of the same molar concentration.
The value of pKa indicates the degree of dissociation of the acid in dilute solution.
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Yes I think it's more than 2% for strong vinegars - I haven't time to check at the moment, but I'm guessing that the higher figures (4-6%) tie in with a figure of about 0.8M acetic acid.

The Wiki page I cited (https://en.wikipedia.org/wiki/Calcium_carbonate#Solubility_in_a_strong_or_weak_acid_solution) says 'For the same total acid concentration, the initial pH of the weak acid is less acid than the one of the strong acid; however, the maximum amount of CaCO3 which can be dissolved is approximately the same. This is because in the final state, the pH is larger than the pKA, so that the weak acid is almost completely dissociated, yielding in the end as many H+ ions as the strong acid to "dissolve" the calcium carbonate.'. Perhaps that's not how you would express it (nor perhaps how I would if I went through it in detail!) but the calculations there do indicate that, as you say, equivalent concentrations of HCl and acetic acid will ultimately dissolve essentially the same amounts of CaCO3 - which was the point I was making.
 
Hydrozoan said:
, equivalent concentrations of HCl and acetic acid will ultimately dissolve essentially the same amounts of CaCO3 .
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Exactly the same amounts. Just a big difference ( I am tempted to suggest even more than the 1000 fold difference Vyv suggests) in the rate of the reaction.
 
MC thanks for the link, I will wander up tomorrow and have a look for some acid. The calcium deposits are small lumps which break down into smaller lumps which then do not allow the duck bill valves to operate properly and form a seal hence me having to clean out the pump.
 
VicS said:
Exactly the same amounts. Just a big difference ( I am tempted to suggest even more than the 1000 fold difference Vyv suggests) in the rate of the reaction.
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(i) Well not according to that Wiki page which shows some very slightly smaller ultimate solubilities of CaCO3 in the acetic acid - presumably because its equilibrium calculations do involve the acetic acid pKa (rightly or wrongly, you may say - but I cannot at present see any other explanation for those small differences).

(ii) Yes, you and Vyv might be right that acetic acid needs a week to do what HCl achieves in 10 minutes - but I 'd still like to see some rate data, and will try to find some when time permits.

(BTW I make 5% acetic acid to be ca. 0.8M and 6% is 1M, so at least my use of 1M was reasonable for strong, pickling vinegar - which Wiki says can run from 5% up to 8% or 1.3M.)
 
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mcanderson said:
MC thanks for the link, I will wander up tomorrow and have a look for some acid. The calcium deposits are small lumps which break down into smaller lumps which then do not allow the duck bill valves to operate properly and form a seal hence me having to clean out the pump.
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Slight correction. The deposits are carbonates, mostly magnesium in seawater with lesser amounts of calcium carbonate. 'Calcium' is a metal that reacts with water.
 
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