Oxalic acid

[FWB]

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[FWB]

Very timely thread for me. I've never used oxalic acid for cleaning but my deck needs a thorough cleaning and has some rust stains. If I buy it in powder form how much should I put in a bucket of water and what precautions do I need to take to prevent injury to myself.

Use Patio Magic instead. Spray on a solution, leave it. Don’t wash it off.

 

[Refueler]

I intend to use Patio Magic this year - well whatever local version they have !! Basically because I need to kill the 'green' that grows on my boat every year .... but for stain and general cleaning - its Oxalic.

 

[Fr J Hackett]

Very timely thread for me. I've never used oxalic acid for cleaning but my deck needs a thorough cleaning and has some rust stains. If I buy it in powder form how much should I put in a bucket of water and what precautions do I need to take to prevent injury to myself.

Make up a saturated solution in hot water then dilute it twice the volume of cold water. For application wear latex gloves although not strictly necessary and eye protection if you are a messy worker. It's toxic and should not be injected orally and will also be absorbed through the skin so keep it off yourself and if you do get it on you wash it off with copious quantities of cold water. That is the H&S advice in reality apart from drinking it anything else and you won't come to harm. Apply it with a soft brush and leave for an hour or two then wash off with a hose pipe, you may need, depending on the state of your decks to go over it again and or scrub a little. It works in a solution so once it's dried out then it won't be active so do it in the shade and as you are in Türkiye choose a time of day when it's coolest. If you have vertical or difficult areas you can make some of the concentrated solution into a paste with wallpaper paste and apply that with a brush then simply hose off when it's done its work.

 

[Neeves]

Maybe it's not a problem where you sail but in the UK older light coloured GRP boats commonly get brown staining over much of the topsides that oxalic acid removes. It's particularly apparent on boats that sail in murky water like the east coast rivers or the Bristol Channel.

We use oxalic acid for similar reasons, a brown stain above the water line from tannins in the water, a particular problem for yachts that frequent Tasmania. Its a characteristic of yachts having sailed in Tassie. Here most yachts are lighter (supposedly white), whether old or newer . Treatment would be part of the power wash prior to AF usually in the slings. Tannins are not a major issue on deck and for deck cleaning bleach would suffice - and be no more welcomed by the ecosystem (but no-one worries - because the major source of damage to the ecosystem will be the AF which leaches into the water round your hull 24/7/365).

Jonathan

 

[PabloPicasso]

Causes kidney damage.....enters through bare skin.....

Wear rubber gloves and eye protection.

Be careful with it, its a strong acid.

But I state the obvious.

 

[Hydrozoan]

... Just don't be profligate with the oxalic, spot use seems a good option.

Jonathan

Indeed. The mode of action of oxalic acid for rust and brown waterline stains is understood, and that is what I use it for - but locally. That of algal treatments is understood and as they are designed to be toxic, I use them with particular limitations (e.g. local brushing on canvas). I find detergents effective not only on general dirt (e.g. hydrocarbon washout) but also on algae on impervious GRP. So I resort to oxalic acid or algal treatments only locally for their particular benefits.

Whilst neither oxalic acid nor detergents are designed to be toxic, it's my guess that oxalic acid is more harmful - a view I think supported by the greater risk to me personally in using it.

 

[Ian_Rob]

I could see a LOT (several kilos) of oxalic acid doing harm in a locked marina, but most places in the UK, the management changes the water twice a day so unless everybody in the area is going mad with it, it'll be diluted to vanishing point before it can do any damage

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[Irish Rover]

Another numpty question . Will the oxalic acid damage my windows [I'm assuming they're some sort of perspex]. There's one area that needs cleaning which drips onto the salon/cockpit windows.

 

[Fr J Hackett]

Another numpty question . Will the oxalic acid damage my windows [I'm assuming they're some sort of perspex]. There's one area that needs cleaning which drips onto the salon/cockpit windows.

No it won't.

Don't confuse descriptions or people calling it a strong acid and thinking about things like fuming and concentrated Sulphuric acid dissolving b bodies and anything it comes into contact with. Oxalic acid in that sense is mild like citric acid in lemon juice etc. It removes iron stains by reacting with the iron oxide to for iron oxalate a colourless soluble compound which will wash away, it won't stop another iron stain forming later but replication will remove it.. The same applies to woods.

 

[greeny]

Another numpty question . Will the oxalic acid damage my windows [I'm assuming they're some sort of perspex]. There's one area that needs cleaning which drips onto the salon/cockpit windows.

Not the windows but I've had some issues with it causing discolouration on aluminium anodised deck cleats. not a problem if you're careful when you apply it and you don't splash it all over.

 

[Momac]

I sometimes get a little rust stain off my anchor locker latch.
I sprayed some Bar Keepers Friend power spray on the stain. (Oxalic acid is its active ingredient).
Next day stain gone
Amount of product used negligible.

Use the least possible quantity of chemicals and the environment will deal with it.

 

[Ian_Rob]

Another numpty question . Will the oxalic acid damage my windows [I'm assuming they're some sort of perspex]. There's one area that needs cleaning which drips onto the salon/cockpit windows.

As an aside, it can pit aluminium as well as discolouring it….

 

[oldgit]

iron stain forming later but replication will remove it..

A Damascene conversion !
My hull was treated to a mixture of Oxalic and "Fairy"this year , after having been a "paste" man for ages.
Much easier to apply and to wash off afterwards.
Watched the brown scummy stain disappearing revealing the unblemished gelcoat beneath.

Its bit late now but...
Definately a hint of the brown around the water line reappearing, a boater who cared might have applied a decent coat of wax before relaunch.
Lifes to short ?

 

[Ian_Rob]

As a matter of interest and picking up on

I could see a LOT (several kilos) of oxalic acid doing harm in a locked marina, but most places in the UK, the management changes the water twice a day so unless everybody in the area is going mad with it, it'll be diluted to vanishing point before it can do any damage

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.

 

[VicS]

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.

I am not sure if this is right and any expert comment would be appreciated.
PS Other chemical effects/interactions ignored.

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)

 

[Praxinoscope]

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.

 

[Baggywrinkle]

Just as an observation after many years of using an oxalic acid/washing up liquid solution to clean my boat - it works much more effectively and much faster in warm weather (Mediterranean warm) and in my subjective opinion direct sunlight helps too.

 

[Supine Being]

Just as an observation after many years of using an oxalic acid/washing up liquid solution to clean my boat - it works much more effectively and much faster in warm weather (Mediterranean warm) and in my subjective opinion direct sunlight helps too.

Seems logical. Even from my (extremely poor) schoolboy grounding in Chemistry, I remember that heat makes a chemical reaction more vigorous.

 

[FWB]

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