rain

[quedgeley]

Silly Question I know!

A group of us were having a discussion,when one came away with the suggestion that ordinary rain is a (very very) diluted form of (sulphuric acid?)

How accurate was he?

Thanks in anticipation.

[gen_2]

That is not strictly speaking true.

In an unpolluted atmosphere, the predominant acid will be carbonic acid (from dissolved CO2).

In thunderstorms, the lightning generates oxides of nitrogen which dissolve in rain to form nitrous acid and nitric acid.

Pollution adds oxides of nitrogen and oxides of sulphur to the air. These dissolve (and oxidise) to form nitrous acid and nitric acid (as previously stated) and sulphurous acid and sulphuric acid.

In industrialised countries, rainwater is therefore a dilute form of at least 5 acids.

[beso]

Not always very dilute. Acid rain from industrial pollutants has been measured as more acidic than vinegar.

[gen_2]

beso, you are confusing two different measurements. The strength of an acid does NOT relate directly to its concentration.

The strength of an acid is measured on the pH scale and depends upon how ionised it becomes in water. Mineral acids such as nitric, sulphuric and hydrochloric acids are highly ionised and give pH readings around 1 or 2. Organic acids such as vinegar are less strongly ionised and give a pH around 3 or 4.

Dilution / concentration depends upon the number of molecules of solute present in a fixed volume of solvent (in this case - water). Acid rain, even in polluted industrialised regions will not be as concentrated as vinegar.

[Organgrinder]

gen_2 The strength of an acid depends on the concentration of hydrogen ions (simplifying slightly) in solution and so depends on both concentration and degree of ionisation.

I understood that the acid smog in London before the Clean Air Act had a pH of about 3, which is about the same as vinegar and lemon juice.

[john91]

Certainly at high altitudes there are sulphur based acids in the atmosphere, these are one of the causes of the deterioration of aircraft windows, crazing and clouding, these come from volcanoes but whether any fall to make acid rain I don't know.

[theprof]

gen_2, standard table vinegar has a pH in the range 2.5 to 3.0. In low quality vinegars, the pH can be as high as 3.5. My Hanna HI-253 bench pH meter provided a reading of 2.536 earlier when the electrode was immersed in a container of a well-known vinegar brand. Whilst vinegar brands with a pH of "around 4" might well exist, I'd suggest they are not the norm and their strikingly low acidic taste would put people off purchasing them again - unless they're running a fish and chip shop.

In heavily industrialised areas of the world such as the PRC, acid rain with a pH of 2.5 to 3.0 is reasonably commonplace since legislation is not in place to control such pollution.

Even in the USA until fairly recently, acid rain with a pH of 3.0 was not regarded as unusual. Besides, I have first hand knowledge of it myself as I was seconded to UCLA for a few months in the early eighties when the uni was monitoring acid rain pH for the state governor: a pH of 3.0 was about average.

http://www.aces.edu/pubs/docs/A/ANR-1229/

As far as smog is concerned, Organgrinder is quite correct that it's pH was often more or less equivalent to that of vinegar.

http://www.newscientist.com/article/mg12216581 .600-smogbound-los-angeles-to-ban-petroldriven -cars-.html

Incidentally, I note that your New Year's resolution seems to have included changing your previous username on AB. Perhaps you would care to tell us why.

[gen_2]

Hi prof

Firstly, I was a biologist rather than a chemist and so concede your more accurate figures. What I was trying to do was distinguish between strength (meaning concentration) and strength (meaning pH - strong/weak acids). beso seemed to be confusing these terms.

As for the name-change, that was enforced by AB. I am no longer able to post questions or answers as gen2 and answers I gave last weekend disappeared from threads although they still show up in my profile. I have emailed AB Editor and technicians but so far no response. I re-registered in order to continue using the site.

[theprof]

Hi gen_2

Yes, I agree that beso did seem to be confusing the two and it did need clarification. I wouldn't say that my figures are more accurate, because when it comes to things like this it's too easy for me to remember that I was once a chemist, then a biologist like you and finally a biochemist! What does that make me?

I'm sorry to here that the name change was enforced upon you. With luck, when the editor and technicians are back in work tomorrow, they can put the matter right. You've helped me a lot over in Q&P as I'm fond of crosswords myself.

Best wishes gen2 and a Happy New Year to you sir.

[quedgeley]

Hi,

I take it then the answer is NOT sulphuric acid then.

Anyway many thanks,and I did get you talking to each other.

Happy New Year to you all!

[theprof]

quedgeley, as gen2 said in his initial answer, sulphuric acid is only one of five possible acids that occur in rainwater. In no particular order these five acids are:

1. Carbonic Acid. This is formed during the burning of coal in power stations etc where carbon is oxidised to carbon dioxide. The carbon dioxide dissolves in rainwater to form carbonic acid.

2. Sulphurous acid. This is again formed from coal as most kinds of coal contain sulphur as impurities. When sulphur burns, it is oxidised to form sulphur dioxide. this dissolves in rainwater to form sulphurous acid. Sulphur also occurs in some grades of petrol. When the petrol is burnt, sulphur dioxide is emitted in the exhaust fumes but there are low-sulphur fuels now available which reduce the gas emmision.

3. Sulphuric acid is formed in clouds from sulpher trioxide, another gas. This gas is formed from a complex series of reactions that occur within clouds in the presence of catalysts such as oxygen and ozone.

4. Nitric acid. This is formed from various nitrogen oxides dissolving in rainwater. The nitrogen oxides are created from the reaction between nitrogen and oxygen in the air caused by the heat of burning of fossil fuels. Cars also produce nitrogen oxides but nowadays catalytic converters reduce the amount of the gases in exhaust fumes.

5. Nitrous acid. This is formed from nitrogen oxides as a result of similar reactions to the formation of Nitric Acid. Thunderstorms also play a part in the formation of both as gen2 said.

(continued)

[theprof]

That's about the gist of it quedgeley but it's still a fairly simplistic answer as some of the reactions that occur to form acid rain are quite complex. These are also the main components of acid rain but other acids can occasionally occur depending on the particular type of pollution in the area.

So, was your friend right in saying that rainwater was a very dilute form of sulphuric acid. Well, the answer is yes and no. You could say he's right because sulphuric acid can be detected in virtually all rainwater to some extent even in that falling in Scandinavia. However, he's wrong because it's not the only acid present and it's just as logical to say that rainwater is dilute carbonic acid or any of the other acids outlined above in areas of acid rain.

You see, "ordinary rain" as you put it does not really exist, as even in the most unpolluted areas of the world, carbon dioxide dissolves in it causing some degree of acidification. Once you start including any heavy industry, refineries or power stations, these serve to push up the acidity of the rain - they don't have to be on your doorstep for acid rain to occur as winds are more than capable of blowing the clouds over you..