pH and oxidation

[chemnovice]

Hi, as a physicist, I'm learning chemistry from scratch!!

 

What exactly is pH in molecular term?

(I remember it was something like logarithm of number of hydrogen ions?)

 

And how does the pH value of an organic mixture affects the oxidation of a moiety in the mixture, say, silver nitrate --> silver oxide?

 

Thanks,

Jack.

[budullewraagh]

pH is the negative logarithm for the concentration of hydronium (H+) ions.

 

i've never heard of the term "moiety".

[Gilded]

Hmm... So a one number change in pH is a tenfold decreasement or increasement in H+? So pH 1 (stomach) would be a million times more acidic than blood (about pH 7)? Even though I like acids very much, I know very little of pH and other fun acid related stuff.

[chemnovice]

I just meant part (moiety) of the mixture.

 

I have a lyotropic system with non-ionic hydrocarbon surfactants, mixing with silver nitrate. It's believed that due to the pH of the mixture, silver nitrate turned into silver oxide once it's added into the mixture. I wonder what is the mechanism involved in there?

[budullewraagh]

ah i see. what happens if you have a reaction that yields nitric acid and silver oxide. nitric acid dissociates very easily, mind you, so it becomes quite acidic i could imagine

[chemnovice]

I'm not exactly sure how the silver nitrate turned into silver oxide, due to the influence of the surfactant molecules. I tried to condition the mixture with 0.01M nitric acid, which would be pH2, but it's believed that when the surfactant molecules interacted with it, the pH went up. As a result, it triggered the oxidation of the silver nitrate.

[budullewraagh]

it depends on the conditions. are you sure they're HYDROCARBONS, or could they be other organic molecules, specifically ketones?

[chemnovice]

In particular, it is Polyoxyethylene (10) cetyl ether, hydrocarbons, formula: C16H33(OCH2CH2)nOH where n~10. My supervisor kinda said the rise in pH sorta robbed the proton on the oxygen, and the silver nitrate gets oxidated... ? No idea what happened there.

[budullewraagh]

well, your ether isn't a hydrocarbon. i would think that you nitrate the ether, yielding an NO2 group on the ether and an O on the silver, but i am not sure

[chemnovice]

If that's the case, how do I prevent it from happening?

 

My supervisor came up with a solution for me to up my nitric acid concentration, from 0.01M to 0.1M or 1M, in order to bring down the overall pH of the mixture back to pH2. Would this solve the problem?

[budullewraagh]

whats the problem?

[chemnovice]

Prevent the silver nitrate in the mixture from oxidizing.

[budullewraagh]

the silver nitrate is already oxidized as far as it can. i assume that you mean forming silver oxide. if you don't want to, try not having the organic compound in with it.

[chemnovice]

I need to have the silver nitrate and the organic compound together. So I've decided to increase the pH of the mixture by adding 1M nitric acid, so that there are excess of H+ ions to interact with the extra O after the ether has attached itself to a NO3 group. Gonna try this tomorrow in the lab, hope it would work.

 

I've tried it with 0.1M nitric acid already, mixture still turned dark very quickly after mixing with the surfactants. Unless it was the case (I'm still puzzled) that I didn't wrap the container of the mixture completely and some light was leaked into it (from an already quite dark environment) and the light sensitivity of silver nitrate taken effect.

[Guest mam65]

Hi!

 

I'm a chemist turned physicist - so I have the opposite problem...

 

When silver nitrate dissolves in water, it gives an acidic solution. I notice that your surfactant has an alcohol group (OH) at the end. In acidic solution, this will tend to be deprotonated and could conceivably (though it's not likely)react with the free Ag+ ions in solution, forming a surfactant-silver complex. However, it is more likely that the surfactant itself is reducing the silver ions to their elemental form. I am currently working on experiments where I use a very similar surfactant (which includes ether functional groups, like yours) and heat it up with a metal salt (such as palladium chloride, nickel nitrate etc), whereupon reduction occurs and you obtain the pure metal in nanoparticulate form. This typically makes the solution look dark, even black. Your surfactant has an alcohol group on the end (that's the OH bit) and I should point out that alcohols have also been used in a similar manner as reducing agents to form metal particles in solution. If you look at the electrochemical series (sort of a list of elements in the order of how easy it is to reduce them) you will find that silver ions are near the bottom i.e. they are very easy to reduce - you don't need a strong reducing agent for this to happen. Try another surfactant if you don't want this to occur (perhaps a cationic surfactant such as CTAB). There are plenty of articles on this type of reaction in the literature, look up "polyol reduction" or "metal nanoparticles".

 

Hope this helps!

[Guest mam65]

oops, and I should add that by lowering the pH (i.e. by adding acid) all you are doing is slightly increasing the availability of the Ag+ ions (look up "equilibria" in an introductory chemistry text to find out how this happens). If you add too much acid you could end up with it reacting with your surfactant, so you would no longer know what species were present in your system.