Last Question For The Night. Simple Chemistry! Formic Acid In Solution Question

[BigDon]

As some of you are well aware I'm now doing carnivorous plants as a hobby. And besides the sundews I have two kinds of pitcher plants of the genus Sarracenia.

It seems in natural surroundings pitcher plants capture a fair number of ants, with some like S. purpurea having several clever tricks for getting ants from ground level to their traps. (Don't follow the honeydew trail!)

Anywho, it seems that if you eat a lot of ants, as you digest them you release the formic acid contained in their bodies. Great anteaters of South America have gone as far as being the only mammels that don't produce hydrocloric acid. The consumed ants release the formic acid as digestion occurs AND it turns out, formic acid is more efficient than hydrocloric acid is in breaking down insect tissues. Win/win for the ant eater.

Sarracenia purpurea does the same thing. In areas where they capture a lot of ants the pH of the pitcher fluid can be as low as 2.

Now I have two ideas in mind. I just can't tell which one is the "easy" way.

Use an aspirator, go to a park away from the domain of the tiny but ferocious argentine ants to capture the larger native ants and feed them to the pitchers, post drowning OR

Go to the pharmacy and ask the pharmacist for a solution of formic acid I can eyedropper in periodically.

I just need to know what to ask for. Over the decades I've found out most chemical remedies in the aquarium hobby always seem to fall in the 1.5 to 2.5% solution that you add x drops per gallon for a finished product.

So if I were planning to lower the pH of approximately one and a half ounces, (a shot glass full) of deionized water down to the 3 to 4 range, (assuming 2 was an extreme) with one or two drops, what should I ask for?

Thank you.

[profloater]

I wish I could answer your question properly but I am wondering if a variety of acids would do? Vinegar for example (acetic acid) will hit pH 2 as it comes. Vit c (ascorbic) will be similar in saturated solution and even lemon juice can be pH 3 The pH is of course the log of the hydrogen ion concentration which means the number of free electrons so perhaps the plants don't mind where the electrons come from? However that's just a guess I don't want to kill your plants. They must also digest the proteins and fats. Would wasps do? they use formic acid I believe.

[Ivan Viehoff]

pKa of formic acid is 3.77, ie, Ka = 0.000170
so [H+][A-]/[HA] = 0.000170
You want a pH of 3 to 4, ie [H]=0.001 to 0.0001. Ignoring the small contribution of [H] from the water, we have [H]=[A]
So to achieve that, the concentration of [HA] is [H]^2/Ka which is 0.0059 to 0.000059.
We add back in the [H]=[A] to get the total [HA] in the water so that is 0.0069 to 0.00016
The above concentrations are all in molar, ie moles/litre. Formic acid is 46g/mol, so the required concentrations in g/litre are 0.32 to 0.007 g/litre.

1.5 USfloz is 0.045 litres. So you want 0.014g to 0.00033g of pure formic acid in your 1.5 floz of water. I don't know what concentration you can buy, or what size your drops are, they surely depend upon the geometry of the dropper you are using. But maybe you can complete the calculation from there now I've done the difficult bit. You will observe that the amount to achieve a pH of 3 is about 40 times the amount to achieve a pH of 4. That may surprise you; but that is how these things work... It wouldn't work like that for acids like HCl that are expected to be more or less 100% dissociated in solution.

I surprise myself, that is the first time in 30 years I have attempted such a calculation and find myself still able to do it.

[Ivan Viehoff]

I should add that the whilst you may make a solution of a certain pH, what pH it will have when you mix it with Other Stuff cannot be relied upon. That is why chemists who want solutions of reliable pH use special solutions called "buffer" solutions, whose pH is harder to disturb. You would probably make a buffer solution of formic acid by adding some sodium formate or calcium formate, some formate that was reliably soluble and ionic. Whether that was appropriate would depend upon whether your plant could cope with the sodium or calcium. This changes the required quantity of formic acid, etc. Whether I can still calculate the concentrations for a buffer, I am not going to immediately put to the test.

At pH closer to 7, you can change the pH of an non-buffer solution noticeably just by blowing on it, because of teh CO2 in your exhalation. This is why biological samples that require a specific pH are normally held in a buffer solution.

I see this http://en.wikipedia.org/wiki/Buffer_solution gives a link to a buffer calculator.

[trinitree88]

I should add that the whilst you may make a solution of a certain pH, what pH it will have when you mix it with Other Stuff cannot be relied upon. That is why chemists who want solutions of reliable pH use special solutions called "buffer" solutions, whose pH is harder to disturb. You would probably make a buffer solution of formic acid by adding some sodium formate or calcium formate, some formate that was reliably soluble and ionic. Whether that was appropriate would depend upon whether your plant could cope with the sodium or calcium. This changes the required quantity of formic acid, etc. Whether I can still calculate the concentrations for a buffer, I am not going to immediately put to the test.

At pH closer to 7, you can change the pH of an non-buffer solution noticeably just by blowing on it, because of teh CO2 in your exhalation. This is why biological samples that require a specific pH are normally held in a buffer solution.

I see this http://en.wikipedia.org/wiki/Buffer_solution gives a link to a buffer calculator.

ivan. You might want to try potassium formate, as K is an essential nutrient,and the salts are more soluble (though more Moola...$$$

[BigDon]

Thank you very much for the calculations, Ivan. I am familiar with buffering solutions, being a former keeper of corals.

So, in a long round about way you're saying get my old butt out to the park and catch some ants?

Will do.

[Ivan Viehoff]

I think what I am arguing for is a bit of incrementality, given that the liquid will be already acid to be digesting at all. Add a bit, that you have calculated is a conservative amount, and see how it goes. Observing your earlier comment that in the wild they can be at pH 2, and observing the difference in quantity of formic acid between pH 2 and pH 3 is a factor of 90, even larger than the factor 40 for 3 to 4, I think it is going to be very difficult to make it over-acid, even without buffering, if you are being conservative and aiming for solution of pH 3.

Though real ants will have the benefit of being actual food.

[Squink]

pKa of formic acid is 3.75.
That's the pH at which half of it is is dissociated into H+ formate-.
It's also conveniently in the range the OP is looking to adjust his water to.
This allows us to avoid solving the quadratic for equilibrium dissociation of weak bases.

[H+] at pH 3.75 is 10^-3.75 = 0.00018 Molar.
We'll need twice that molarity of formic acid to achieve that pH, so 0.00036 Molar.

Reagent grade formic acid runs about 23.6 Molar, so it'll take 0.00036/23.6 liters of the concentrated acid to take a liter of pure water down to pH 3.75. That's 15.2 microliters acid per liter water. 1 drop is about 50 microliters, so dilute the concentrated acid by 3 to 1, and add 1 drop of that per liter water.

Of course, your pitcher plant doesn't contain pure water, but some sort of mix of acids and bases, a buffer, that will raise your final pH. The best way to determine its effects is with a pH meter, but 0.015 ml/liter formic acid is a decent first approximation to the amout of acid you want to add.

[BigDon]

Ivan, Squink,

I don't even have anything clever to say about this, so I'll just say it.

I forgot what I was dealing with. They've been doing this by themselves since at least the early Miocene.

Five of the twelve carnivorous plants I have were finally showing stress from inadequate light, (the rest are putting on new growth and flowering).

Two of the five were both of my pitcher plants and I moved all my plants outdoors. Adjusting them to full sun over a week. Indian summer is upon us big time.

You know those argentine ants I mentioned as being too small and fast to consider?

I didn't expect them to volunteer in helping the pitcher plants. En masse.

Once they found a way up and into the pot, after the standing water as we're at the end of the dry season. The ants are like kids at the kiddie pool. Sort of. (If you lived in Hell.)

After merely a week in full sun all the missing red pigments in both pitcher plants are filling in dramatically.

So the point seems to rendered moot.

Sorry about that. There's lessons to be learned here.

I do thank the both of you for your efforts at helping me.