Acid Strength

[Jimk]

How many corpses are you trying to dissolve?

[Mike McLeod]

Hi,

Go to a builders' suppliers or a plumbers' merchants and you can buy concentrated sulphuric and concentrated hydrochloric acid. Sulphuric is concidered concentrated at 98% or more; hydrochloric at only about

35% IIRC. Nitric, OTOH, is concentrated at 73% or thereabouts so it's all very confusing. What's the best way to measure the strength of these acids in practice, though? would a hydrometer do it?

cheers, Mike

[Pancho]

AIUI, acid concentration and "strength" are different things

[Pancho]

I meant to add, back when I did A'level we measured concentration by titration.

[PeterC]

ISTR it's to do with equilibrium: sulphuric is v. concentrated before the fumes (sulphur trioxide?) coming off balance the SO3 dissolving; Nitric fumes rather more; HCl fumes a lot, even when not fully concentrated. They are all strong acids, even at v. low concentrations but are quite easy to rinse out. Organic acids (citric, acetic etc.) are weak acids, do not fully dissociate in solution and retain the same concentration of ions until a certain dilution is reached, i.e. buffered, so can be harder to rinse away. As an aside, pieces of fruit stuck between teeth will, apart from providing sugar(s) for bacteria, also remain acid for quite a while in spite of dilution by saliva and reaction with enamel.

[Cursitor Doom]

You can tell the strength of an acid by the strength of the conjugate base it forms. Strong acids form weak conjugate bases and vice versa. It's all to do with protons and how likely they are to cleave and stay cleaved. That won't help you one iota, though, I just thought I'd mention it to show off.

[Max Demian]

That's the way you do it for car battery acid. You can get special hydrometers with a rubber bulb to suck the acid from the cells.

I don't know how you would convert the specific gravity to concentration. Ask a chemist.

[Paul]

Just take the concentration on the bottle as accurate, do the maths for dilution by volume, to get it to where you want.

"Why is acid always added to water, and not the reverse"

You won't have the glassware to do a titration (volumetric flasks, gram scale, burette, phenolphthalein, beakers).

Preps to a certain concentration, are based on manufacturing practice or practical limits. We don't make substances that explode spontaneously at high concentrations, on purpose that way. We make things with known and stable characteristics.

Of the "strong" materials you might run into, I might want to "assay" ammonia in a bottle. I had a bottle of that which I used to dilute and make window cleaner (without any "polish" in it), and one day I opened that bottle, and only water was in there. The ammonia had "vanished" :-)

Since ammonia is a watched substance, the bottle I had, I can't buy another. That's the pisser of the thing.

Paul

[Tim Streater]

I thought that was only the case for sulphuric?

[The Natural Philosopher]

I used to swallow the blotting paper as it came

[newshound]

Oh please

[Paul]

If there's a noteworthy exothermy, you'd probably follow the rule.

You've probably noticed how warm the water gets when you mix up Drano.

In a chem lab, I've only seen one loon manage to coat the lab ceiling (and these are tall ceilings too) with chemicals cooking in a refluxing setup. The number of "oopsy" incidents was remarkably low. So not too many "liquid headed for orbit" events in the lab. I'm sure lots of people didn't follow the rules on acid order.

Sulphuric acid is a bitch. All my jeans in lab had holes in 'em, thanks to sulphuric. You cannot neutralize it fast enough, so don't bother... Sulphuric can react with cotton in under a second, to its ruination.

If you're pipetting acid, *always* use a rubber bulb to operate the pipette. No cowboy stuff.

Paul

[Tim Streater]

Don't spill this:

[Jeff Layman]

That's more-or-less correct. It's to do with the maximum stable amount of their oxide reacting with water. For sulphuric acid, it's about 98% sulphur trioxide content; for nitric acid about 73% nitrogen dioxide content. Both can "dissolve" further amounts of their oxides, but they are not stable, and form fuming sulphuric and fuming nitric acid. Hydrochloric acid is simply a solution of hydrogen chloride in water - it does not react with the water in the same way that the oxides do.

If you want a really strong acid, have a look here:

[Cursitor Doom]

I'm guessing this list of "watched substances" grows longer every year. Is there an up-to-date list on the net anywhere? Brick cleaner is strong hydrochloric acid. Diluted down, it makes a very effective and dirt-cheap limescale remover; no scrubbing, just dissolves it on contact forming a salt which is readily soluble - being polar - in water and easily washed away. Most of the fancy brand-name cleaning products you find in the supermarkets are just heavily-diluted industrial chemicals you can buy far cheaper and make much more effective if you know a bit of basic chemistry and aren't afraid of builders merchants.

[Cursitor Doom]

ISTR that the problem with hydrochloric is it forms an azeotropic mixture at a relatively low concentration which makes further purification very difficult. I don't think that occurs with the other common mineral acids.

[Davey]

I once worked at an aircraft wing treatment plant, and one of the tanks contained a mixture of hydroflouric, nitric and chromic acids. Heated to 140 deg. F. The sump under the circulating pumps contained a sludge which indicated on the pH test paper at less than 0 pH.

[Paul]

What was the tank made out of ?

Paul

[The Natural Philosopher]

it also discolours chrome very badly. You have been warned.

[Jeff Layman]

I'm surprised you had time to read the pH before the test paper dissolved! What was that acid mixture needed for?