Again about formic acid .

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Again about formic acid .

Postby Farshad on February 10th, 2006, 11:56 am 

Is formic acid soluble in oil ??
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Postby Hardstreet on February 10th, 2006, 4:45 pm 

My expertise in chemistry is about zero, but I thought that for an acid to be acidic, the anion had to be pulled apart from the hydrogen ion by a polar solvent, which oil isn't. So maybe acid is miscible in oil, but is that the same thing as being soluble? Like I say, my knowledge of chemistry is next to nothing.
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Postby BioWizard on February 10th, 2006, 7:33 pm 

Even though formic acid is an organic acid, it is the simplest organic acid with a single carbon atom that is bonded to two oxygen atoms, and thus is completely soluble in water. It is also soluble in all proportions in acetone, ethanol, methanol, diethyl ether, glycerol and ethyl acetate.
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Postby BioWizard on February 11th, 2006, 2:38 am 

[EDIT] This Post is made in reply to Silkworm's posts on this thread, which were then removed by him:

Silkworm claimed that:

1- A compound isn't said to dissolve in water unless it dissociates into oppositely charged ions
2- The solubility product constant of salts with low aqueous solubility is also applicable for expressing the solubility of the organic acid formate.
3- Formic acid isnt completely soluble in water.

Well, solubility is solubility, and the definition doesnt change with weak acids, nor with anything else. You mix a weak acid and a strong acid with excess water. The weak acid dissociated partially, the strong acid dissociates completely. Doesnt mean that the strong acid is completely soluble and the weak acid is only partially soluble. It just means that the strong acid is more acidic at the same concentration, and let's see why.

Ok, let's start by going a little into the molecular mechanistics of solvation.

But first, the definition of solubility:

Quoting wikipedia wrote:A substance is soluble in a fluid if it dissolves in that fluid. The dissolved substance is called the solute and the dissolving fluid (usually present in excess) is called the solvent, which together form a solution. The process of dissolving is called solvation, or hydration if the solvent is water.

A solution at equilibrium that cannot hold any more solute is said to be saturated. The equilibrium of a solution is mainly dependent on temperature. The maximum equilibrium amount of solute which can normally dissolve per amount of solvent is the solubility of that solute in that solvent. It is often expressed as a maximum concentration of a saturated solution. The solubility of one substance dissolving in another is determined by the intermolecular forces between the solvent and solute, temperature, the entropy change that accompanies the solvation, the presence and amount of other substances, and sometimes pressure or partial pressure of a solute gas.



Let's consider first the solvation of an ionic compound by water, for example Sodium Chloride, or NaCl. For a salt molecule to become solvated, water molecules arrange themselves around the positive and negative ions constituting a salt molecule, in such a way as to network the charge between the ion and the water molecules around it, and by doing so disperse the change over a larger mass, weakning the ionic pull between the two oppositely charged ions. As this goes on, a hydration shell is formed around each ion, and the charges are disperesed with the solution. The salt is said to have been dissolved. As more and more salt is added, the number of water molecules available to hydrate the ions becomes less and less, up to the point where not enough exist, and whatever salt you add will not experience the solvation effect, and instead remain in the solid unsolvated state and fall to the bottom of the container. Thus, in the case of ionic compounds, charge dissociation is a prerequisite for solvation. The solubility of Sodium Chloride NaCl in water is 35.9 g/100 ml (25 °C).

Now let's consider a polar nonionic compound. For example, formaldehyde. Formaldehyde is a carbonyl with two hydrogen attached to the carbon atom. All of it's bonds are largely covalent, and it does not ionize in aqueous solutions. It is however, a polar molecule, with a partial positive charge on the carbon, and a partial negative charge on the oxygen, since oxygen is more electronegative than carbon. This charge distribution allows the oxygen to act like a negatively charged center, and the carbon to act like a positively charged center, and when placed in water, the water molecules will arrange around these charges and attempt to separate them as they do in the case of ionic compounds. Of course, the separation doesnt take place, due to the strong covalent bonds between the two atoms, however, the hydration shell does form to some extent around the two poles of the molecule, and the molecules does end up to be solvated. The fact that true charge separation doesnt occur means that much fewer water molecules are needed to create the hydration shell, and so more of the polar solute can be hydrated and solvated . The solubility of Formaldehyde in water is greater than 100 g/100 ml (25 °C).

The molar mass of Sodium Chloride is 58.4 g/mol. 35.9 grams of Sodium Chloride thus contain around 0.61 moles.
The molar mass of Formaldehyde is 30.03 g/mol. 100 grams of Formaldehyde this contain 3.33 moles.

So as we can clearly see, the same volume of water can dissolve MORE formaldehyde, the nonionic nondissociating compound, than it can NaCl, the ionic dissociating compound requiring extra hydration for charge stabilization.
Now let's consider a nonpolar molecule, say octane. Octane is an 8 carbon aliphatic alkane, a nonpolar molecule that has neutral net charge distribution due to the absence of significant differences in the electrogenativities of it's atoms on one hand, and to it's symmetric geometry on the other. When placed in water, the molecules of water cannot interact electrostatically with any part of the octane molecule, and so tend to form a cage-like arrangement around the nonpolar molecule, which is thermodynamically unfavorable (decreased entropy), and so the water molecules try to find a more energetically favorable state by interacting more with each other, bringing all octane molecules in the solution together and out of the body of water. Since Octane has a lower density than water, it will rise to the top. The two liquids are thus said to be immiscible, and Octane is insoluble in water. The same is the case for a nonpolar solid.

Some ionic compounds are also insoluble in water, such as Barium carbonate, possibly because the energy released in the solvation process is not sufficient to overcome the interaction between the two ions.

Molecules that are a combination of polar and nonpolar groups tend to have the characteristics of both, and their solubility in water depends on the relative activity of either group, which is often dependent on the size and the charge density. The larger the nonpolar group, the lesser the solubility in water. The bigger the charge, the more the solubility in water. The more the hydrophilic groups, the more the solubility in water, etc...

Finally, we arrive at polar solvents, which contain protonatable nucleophilic centers, and thus are weak acids or bases, for example, methanol. Methanol is a polar molecule, and as such, when present in an excess of water can form a hydration shell around it, enough to solubilize it in water. Inversly, when a molecule of water is present in a volume of methanol, the methanol molecules will arrange themselves around the water molecule in such a way as to orient their partial negative charge on the oxygen towards the partial positive charge on the water's protons, and away from the partial negative charge on the water's oxygen. It's as if that the water is now dissolves in methanol. This ability to solvate one another removes the limit on how much of either liquid can be added to the other while still producing a true solution. As such, the two liquids are said to be miscible, and can be mixed to gether in any proportion to form a true solution. Ethanol and Formic acid are two more examples of liquids that are freely soluble in water in any proportion, and thus are COMPLETELY soluble in water.

The fact that formic acid can dissociate in water to give you a proton and a formate anion does not tell you much about the ability of water to dissolve a limitless amount of formic acid, and vice versa.

Again Quoting Wikipedia wrote:Solubility constants are used to describe saturated solutions of ionic compounds of relatively low solubility. For salts, solubility in aqueous solutions or the maximum amount of salt that can be dissolved is the solubility constant. The solubility constant is a special case of an equilibrium constant. It describes the balance between disolved salt and undisolved salt. The solubility constant is also "applicable" (i. e. useful) to precipitation, the reverse of the dissolving reaction. As with other equilibrium constants, temperature can affect the numerical value of solubility constant.


In other words, solubility constants serve as a measure of solubility when the undissociated species of the ionic compound are not soluble in water. As such, the dissociated species are the exclusive soluble form, and so the Kdiss (dissociation constant) becomes directly correlated with solubility, hence Ksp (solubility product constant).

This is clearly not the case for formic acid, which is COMPLETELY solube in water, in ANY proportion.
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Postby Hardstreet on February 11th, 2006, 6:54 pm 

But, to repeat Farshad's original question, "Is formic acid soluble in oil?"
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Postby jc__denton on February 11th, 2006, 7:10 pm 

Since formic acid is polar and oil is not, expect these two liquids to be non-miscible.
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Postby BioWizard on February 11th, 2006, 7:45 pm 

Good point hardstreet, this discussion has gone way off topic. The short answer to that question is: partially soluble in oil, to a much lesser extent that its solubility in water, probably for the reason that denton mentioned.

Coefficient of Oil (octanol)/Water Distribution (Partition Coefficient): Log P(oct) = -0.54 (0.28840315)
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Thanx Biowizard

Postby Farshad on February 12th, 2006, 3:28 am 

I would like to thank Biowizard on his replies.
But would you also please tell me which of these acids are stronger , Acetic acid or Formic acid ?
and about the solubility of acetic acid in oil or water.
Thanx again
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Postby BioWizard on February 12th, 2006, 6:39 pm 

Forhad,

You should be able to figure out the relative acidity of the two acids by looking at their pKa or Ka. Do you know what a bigger Ka would mean?

As for the properties of Acetic Acid, check out this page:

http://www.cdc.gov/niosh/ipcsneng/neng0363.html
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