I’m not looking for another scale on which to put acids and bases, but something analogous. Some other type of reaction that is put on a scale. The acid/base relationship seems to be the most important such scale, since, loosely speaking, it deals with hydrogen ions in aqueous solutions, but it’s the only one I know of. If there are lots of relationships like this in chemistry, what is the second most important?
In other words: does the pH concept have any siblings?
Thanks,
jepflast
elements and molecules have lots of properties. pH is one of dozens of properties.
the strength of holding electrons is a value of which there is a similar scale.
lots of properties and lots of scales. billions of pages of data.
I have always wondered something possibly related to the question with solvents. I have never seen any kind of a scale used with them besides this can disolve this etc.
I know there’s a pH/acidity for oils (like vegetable oils) that is different from aqueous pH but loosely related
Just to be clear (since the above two comments make it look like there’s some misunderstanding here), pH measures an equilibrium between two ions common in aqueous solutions. ALL solutions or pairs of solutions together reach some equilibrium, and the total free dissolved ions changes based on how much of the solutes you put in. Part of this is that, depending on the chemical, a certain amount of the molecules do NOT dissociate into their ions. At the extreme, certain chemicals will just precipitate out.
I know that was awkwardly worded but I’m tired and I’m sleepy and this ticking clock is bugging me, but I’ll try a little more here
That is to say, let’s say you have a chemical made up of A and B. It’s chemical AB. In water, it dissociates into A and B ions in solution, but some of it just stays as AB molecules dissolved in water. Now, depending on the chemical the extent to which the chemical dissociates can be different. Salts (ionic compounds of oxidized metals and reduced non-metallic elements, or certain strong organic ions) have to dissociate into their ions to dissolve, so to any extent they’re dissolved, they’re dissociated. A lot of organic compounds, on the other hand, have a large percentage of their molecules remain as AB and only some separate into AB. In the latter situation, where you get SOME dissociation, adding more of the solute means a higher percentage of the molecules are in AB form, as in not dissociated. Adding more of something else that has B in it, like let’s say another compound CB, would “give” lots of B ions to the any A ions that are in solution, changing more of the AB to be in AB form and not dissociated.
pH encapsulates this relationship for one very important and common pair of ions in water, the H+ and OH- ions. Because the concentrations of both are directly related, and change perfectly in sync with each other (as any such solutions do), the whole deal can be encapsulated with one number based off the concentration of one of the ions (and the other, in this case pOH, can be easily calculated from the pH, based on the consistent relationship. You could also go the other way around, getting a pH from a given pOH)
sorry for the weird wording. like I said, sleepy, clock is bugging me
Polarity is a property of solvents that’s directly linked to what can they dissolve. Polar solvents (water, ammonia, small alcohols) can dissolve more-polar things better; apolar solvents (such as oils) dissolve apolar things. This is also described as a property of the solute (or even, for very large molecules such as proteins, for parts thereof) as being “hydrophilic” (polar, will have strong interactions with water including dissolving) or “hydrophobic” (apolar, will not have strong interactions with water).
The pH of oils is exactly the same as “aqueous pH”, it’s not even a different concept (no need to invoke a definition involving electrons instead of protons, for example). It’s exactly the same as the pH concept for acetic acid or hydrocloric acid, in all cases having to do with the separation of a neutral molecule into a proton and an anion.
There’s the Hydrophilic-lipophilic balance (HLB Scale) for surfactants. Basically a hydrophilic vs. hydrophobic scale for fatty molecules and whether they’re soluble it water vs. oil, or somewhere in between.
Redox potential would fit the bill, I think, and the electronegativity/positivity of elements. Both, like pH, are graduated scales on which substances can be rated or ordered, and, in fact, they both have a wider range of application than pH, so they are probably more important in the wider chemical scheme of things.
Harking back to my very crusty chemistry, I would have though redox potential would meet the OP’s request.
Solubility product is something that is also possibly in the right vein. This is essentially what EdwinAmi is describing, where pH is a single case.
Chemistry is all about electrons, and any way you can share them makes for some sort of balance that you can measure.
Superb answers! Thank you all! The explanation of pH by EdwinAmi was especially enlightening, and now I can perhaps ask a more intelligent and specific question.
If pH deals with the ions H+ and OH-, is there another pair of ions that are particularly important? Will we have to trade water for another solvent?
Thanks again.
Your explanation is an oversimplification and doesn’t cover several key concepts regarding the dissociation of molecules into ions even for inorganic compounds dissolved in water.
Many salts of strong acids and weak bases or weak acids and strong bases do not dissociate completely. Lead acetate, for example, when dissolved in water will form some lead and acetate ions but also form some undissociated acetic acid molecules. Also the degree of dissociation for any salt (or any compound dissolved in water) is also a function of concentration, in dilute solutions there will be still be some undissociated molecules as a result of considering activity coefficients.
With regard to pH, the equilibrium relationship between the sum of the negative logarithms of the hydrogen and hydroxide ion concentrations in a water solution will always equal 14. From this relationship the concept of pH and pOH can be easily derived. And yes, pH and pOH are easily converted from one to the other.
Except that pH is a fixed scale that is based on water and the sum of the negative logarithms of the hydrogen and hydroxide ion concentrations will always be equal to 14. Redox potential and electronegativity are different measurements that do not rely on the equilibrium of the component ion pair for water (hydrogen and hydroxide) and therefore do not have a similar fixed value.
Probably more detail than requested but …
The degree of dissociation of a salt (or more specifically an ionically bonded compound) in any given solvent system is a measure (of specific solvent related properties) of the dielectric constant of the solvent (see below). For example, sodium chloride dissolved in formamide will dissociate to a higher extent than sodium chloride dissolved in water because formamide has a higher dielectric constant.
The same concept of pH in water can be established for any solvent system that exhibits any degree of polarity. Liquid ammonia can also be used as a solvent and the equilibrium relationship for the solvent can be defined between undissociated ammonia and its ionic constituents of ammonia ions and amide ions. A pAmmonia equilibrium relationship can be set for that particular solvent. These relationships are solvent dependent not solute dependent.
In dilute solutions (less than 0.01 M) for example, the relationship between dissociation of an ionically bonded molecule (solute) in a polar solvent is defined by the Debye-Huckel Limiting Law, and its variants, and one of the parameters for determining these relationships is the aforementioned dielectric constant of the solvent.
These are not topics easily covered in a few sentences so pardon my brevity.
True, but solvent polarity (and solubility of one solvent in another solvent) has always seemed to me to be very much empirical. My favorite reference source for work has a map of solvent polarities and it’s very much a two-dimensional mapping, even if there’s a general relationship for most solvents. Similarly, it is possible to have to worry about partial solubility of water in your other solvent (or the other way around, depending on your phase of interest). And then there’s the weirdness with azeotropes. How many people would guess that toluene forms an azeotrope with water but methanol doesn’t?
At 25°C. If you increase the temperature, more dissociation occurs and the balance changes. At 100°C the neutral pH is 6.14 (so the sum of the negative logs would be 12.28, not 14).
/nitpick
pKa, the acid dissociation constant, is the more fundamental scale for acids and bases, as it is an intrinsic property that enables comparison - e.g. Phenol has a lower pKa than ethanol, so it is a stronger acid.
pH is a measurement of H+ in solution, as discussed - its value is governed by the acid dissociation constant. You can’t say a substance has a pH of x like it is a constant, as it will obv vary with concentration.
One can still give a pretty good, clear ordinal scale for them, though, and it will even be quite accurately quantitative for the most part. I still think redox potential and electronegativity are concepts that fit what the OP was asking for very nicely. He wasn’t asking for things that are exactly like pH, just " Some other type of reaction that is put on a scale." There are scales for redox and electronegativity.
Not to nitpick but the component in a mixture present in the greatest amount is defined as the solvent and the other components are defined as the solute(s). Also, unless I am mistaken, the term for two components that form a homogeneous mixture is that they are miscible.
Azeotropes, as you point out, are an interesting topic, though most solvents do form azeotropes. In fact, the azeotrope between water and ethanol, which consists of 95.5 % by weight of ethanol, defines the maximum concentration of alcohol that can be obtained in conventional distillation. To achieve a 100% by weight fraction of ethanol from a water-ethanol mixture requires the formation of a ternary azeotrope between ethanol-water-benzene which can be ultimately separated to yield 100% ethanol.
pH deals with the H nucleus, a single proton. Chemically, the smallest possible positive object. Chemically, there is no other “smallest possible positive object”. It’s simplicity makes it a useful object for catagorisation and description.
I think that’s what I already had said… At least I tried to.
Really? I thought my simplistic wording was confusing and repetitive. It really was/is hard for me to describe. It’s better explained person-to-person. But the main thing to remember is different substances have different tendencies to dissociate to different extents.
Isn’t there a coefficient that describes this, for each chemical? Like, if the negative log of [H+]/[OH-] is always 14, isn’t there a similar number for each compound to describe it’s prevalence compared to its dissociated ion? I remember something like that from chemistry, I think
What are the ions?
From what I understand, while there is some dissociation, it’s much, much less than with water, right?, since oil is non-polar?
Is it the fatty acid with a O=C=O- end and the glycerin side with a C+ ?
