Oxalic acid

[Praxinoscope]

It doesn’t go in the water. It stays on the teak, ropes, sprayhood etc. Leave it, don’t wash it off.

Tell me that again when it stops raining.

 

[FWB]

Tell me that again when it stops raining.

Don’t apply when rain is expected.

 

[Fr J Hackett]

Don’t apply when rain is expected.

You may have missed Praxinoscope lives in Wales

 

[oldmanofthehills]

Should we be worried about the health of Popeye the Sailorman who eats his spinach, which I believe has an even higher Oxalic Acid content than Rhubarb?
I think I would prefer to use the fairly natural Oxalic Acid rather than Patio Magic which carries the warning that it contains chemicals that have an adverse effect upon marine organisms.

It is normal to blanche spinach before eating it, if it is bitter ie high oxalic acid content. This leaches out the highly soluble oxalic acid, and also some vitamin C

Oxalic acid potentially reduces effective calcium uptake and more seriously can increase risk of kidney stones, however a group of scientists proved the minimal toxicity of rhubarb by eating no other plant matter for six weeks - they got very very bored and probably wont ever want to eat rhubarb crumble ever again but did not suffer any ill health

 

[Fr J Hackett]

It is normal to blanche spinach before eating it, if it is bitter ie high oxalic acid content. This leaches out the highly soluble oxalic acid, and also some vitamin C

Oxalic acid potentially reduces effective calcium uptake and more seriously can increase risk of kidney stones, however a group of scientists proved the minimal toxicity of rhubarb by eating no other plant matter for six weeks - they got very very bored and probably wont ever want to eat rhubarb crumble ever again but did not suffer any ill health

It's in the leaves not the stem

 

[Praxinoscope]

It's in the leaves not the stem

It's actually in both, but the concentration in the leaves is considerably higher.
But I think even in the leaves one would have to eat about 5kg to really be poisoned.

 

[oldmanofthehills]

It's in the leaves not the stem

Not quite true. The rhubarb stems are lower in oxalic acid but sufficient in stems that it was prohibited to dispose rhubarb generally into food waste bins during WW2 as food waste was fed to pigs which were so sensitive to it, that it was believed it could kill them.

I imagine the mad scientist just ate the stems, but I cant be sure

 

[Hydrozoan]

It seems to ignore the fact that the slight alkalinity of seawater is due to the bicarbonate concentration. The calculation should be treated as the partial neutralisation of a weak base by a weak acid.

You also overlook the fact that oxalic acid is usually supplied as the dihydrate. ( ie only about 71.4% oxalic acid)

Indeed. But in any case, a more meaningful endpoint than a decrease of 0.1 pH unit (as given by Ian_Rob to ChatGP) would IMO be one more directly related to toxicity. The calculation is also much simpler (though perhaps less fun for a chemist with the time to spare!).

AFAICS the ErC50 - the concentration resulting in a 50 percent reduction in growth rate within 72hrs exposure - for oxalic acid is about 20 mg/l for the green freshwater alga Pseudokirchneriella subcapitata. (OK, it's a freshwater alga but it's the best data I could find on a quick search, and still I think far more meaningful than an arbitrary pH change.)

If I have not bungled the calculation, 800 kg of oxalic acid would be required to bring 40,000,000 litres of water up to that ErC50.

That is considerably less than the 2053 kg from that attempted calculation of the amount required to reduce the pH by 0.1 units, and it should also be remembered that that represents a potentially much more meaningful endpoint: a 50% reduction in algal growth in receiving water.

Oxalic acid is not AFAICS considered markedly [my word, not theirs] ecotoxic under the chemical classification schemes I have found, but it does have ecotoxic effects and unnecessary, non-specific use as a cleaning agent seems to me best avoided.

As ever, it would be good to have someone check on my reasoning, data searching and arithmetic!

 

[Fr J Hackett]

Indeed. But in any case, a more meaningful endpoint than a decrease of 0.1 pH unit (as given by Ian_Rob to ChatGP) would IMO be one more directly related to toxicity. The calculation is also much simpler (though perhaps less fun for a chemist with the time to spare!).

AFAICS the ErC50 - the concentration resulting in a 50 percent reduction in growth rate within 72hrs exposure - for oxalic acid is about 20 mg/l for the green freshwater alga Pseudokirchneriella subcapitata. (OK, it's a freshwater alga but it's the best data I could find on a quick search, and still I think far more meaningful than an arbitrary pH change.)

If I have not bungled the calculation, 800 kg of oxalic acid would be required to bring 40,000,000 litres of water up to that ErC50.

That is considerably less than the 2053 kg from that attempted calculation of the amount required to reduce the pH by 0.1 units, and it should also be remembered that that represents a potentially much more meaningful endpoint: a 50% reduction in algal growth in receiving water.

Oxalic acid is not AFAICS considered markedly [my word, not theirs] ecotoxic under the chemical classification schemes I have found, but it does have ecotoxic effects and unnecessary, non-specific use as a cleaning agent seems to me best avoided.

As ever, it would be good to have someone check on my reasoning, data searching and arithmetic!

Or more prosaically about 0.0002% by weight and 40 million litres is not really a lot of water when we are talking about the sea with a dynamic mixing capability.

 

[Hydrozoan]

Or more prosaically about 2% by weight

Er ... of what in what, Fr H?

 

[Fr J Hackett]

Er ... of what in what, Fr H?

See amended post got me sums wrong

 

[VicS]

All you need to know about rhubarb
The Rhubarb Compendium

 

[Hydrozoan]

See amended post got me sums wrong

Oh dear - easily done.

Or more prosaically about 0.0002% by weight and 40 million litres is not really a lot of water when we are talking about the sea with a dynamic mixing capability.

I was just using Ian_Rob's selected volume for comparison. What would also be interesting would be to look at equivalent ErC50 values for detergents, for comparison with oxalic acid. I've no time or that just at the moment ...

 

[jac]

As a matter of interest and picking up on

Just for fun, I asked ChatGP calculate the amount of 99.8% pure oxalic acid needed to decrease the pH of 40,000,000 litre of seawater from 8.1 to 8.0.

Notes:

- By my rough estimate, 40,000,000 litres is approximately the amount of seawater in a marina the size of The Tide Mill at Woodbridge. Tide charges ignored.

- 8.1 is the average pH of U.K. seawater.

The result was as follows:

Using the Henderson-Hasselbalch equation:

pH = pKa + log([A-]/[HA])

where pH is the target pH (8.0), pKa is the dissociation constant of oxalic acid (1.23), and [A-]/[HA] is the ratio of the concentrations of the conjugate base (oxalate) to the acid (oxalic acid) at equilibrium.

Rearranging the equation, we get:

[A-]/[HA] = 10^(pH - pKa)

Substituting the values, we get:

[A-]/[HA] = 10^(8.0 - 1.23) = 7.94

This means that the ratio of oxalate to oxalic acid in the solution needs to be 7.94.

Assuming that the seawater has a density of 1.025 g/mL, the total mass of seawater is:

mass = volume x density = 40,000,000 L x 1.025 g/mL = 41,000,000 g

To calculate the mass of oxalic acid needed, we can use the following equation:

mass of oxalic acid = mass of seawater x ([A-]/[HA]) / (purity x molecular weight)

where purity is the purity of the oxalic acid (99.8%), and molecular weight is the molecular weight of oxalic acid (126.07 g/mol).

Substituting the values, we get:

mass of oxalic acid = 41,000,000 g x 7.94 / (0.998 x 126.07 g/mol) = 2,053,000 g

Therefore, approximately 2,053,000 grams (or 2,053 kilograms) of 99.8% pure oxalic acid would be needed to decrease the pH level of 40,000,000 litres of seawater from 8.1 to 8.0.

I am not sure if this is right and any expert comment would be appreciated.


PS Other chemical effects/interactions ignored.

It does put the whole debate about runoff into the water into context. Dump 2 tonnes of pure Oxalic acid into a medium sized closed marina and it has a marginal effect and people are worrying about the equivalent of a couple of tubs of Y10 !!

 

[Refueler]

"40,000,000 litres" is actually quite a small amount

To give an idea ... the size of tankers that brought Crude to Fawley were averaging 60,000 to 100,000 MT .... multiply those MT by approx 1250 to give you an idea of the volume in litres ... my company used to look after loading of some of those vessels ...

 

[Fr J Hackett]

"40,000,000 litres" is actually quite a small amount

To give an idea ... the size of tankers that brought Crude to Fawley were averaging 60,000 to 100,000 MT .... multiply those MT by approx 1250 to give you an idea of the volume in litres ... my company used to look after loading of some of those vessels ...

Yes quite a small amount

 

[Ian_Rob]

It does put the whole debate about runoff into the water into context. Dump 2 tonnes of pure Oxalic acid into a medium sized closed marina and it has a marginal effect and people are worrying about the equivalent of a couple of tubs of Y10 !!

Hydrozoan’s figure of 800kg based on toxicity rather than pH is a more relevant figure. That being said it is difficult to see how even that quantity could end up in the water of a marina.

 

[Hydrozoan]

Hydrozoan’s figure of 800kg based on toxicity rather than pH is a more relevant figure. That being said it is difficult to see how even that quantity could end up in the water of a marina.

Thank you, Ian, I think it is - but I agree with you, and I am conscious that you did say your queston to ChatGP was 'just for fun'. One ideally would like figures to compare the toxicity of oxalic acid with that of detergents under real conditions, but that seemingly simple issue is not so easy to answer.

 

[Irish Rover]

I used the oxalic acid to clean my boat this morning. I mixed 900g in 7 litres of hottish water on a pressure sprayer. I'm very pleased with the result. I had rust stains in a few areas of the deck- one area in particular which I think was the result of some idiot drilling something metal close to my boat quite recently. All rust stains gone. There was heavy rust staining on and around the base of my passerelle both on the metal and the deck. I had tried previously with metal restorer polish with very limited affect. All gone now. Thanks to all who offered advice. For those concerned about the environmental affect please be assured I'm going to say "mea culpa" and 3 hail marys as I'm passing the mosque on my way home.

 

[Irish Rover]

I asked this on another thread without response so trying here. Would it be ok to use oxalic acid on the tubes of my 20 year old rib tender or would it cause damage?