Jump to content
I Forge Iron

Acid Molarity


Recommended Posts

I'm looking to passivate some stainless forgings (although reading several of Grant's comments make me think I need to anneal first) and I have come across quite a bit of info. One thing is missing, though. What is the "standard" Molarity for the various acids?

When I asked my wife, a reluctant chemistry teacher, about some of these recipes, she asked about the Molarity of the original solution that you use before dilution

You read about anything from 5% to 30% solutions, but I don't see where the "full strength" acid is defined.

Anyone know the molar concentration of common acids such as citric, muriatic, oxcalic, phosphoric?

Link to comment
Share on other sites

The molarity of a solution is dependant on the concentration of the solution as well as the type of solution. Commercially available acids are available in many different concentrations. It has been a while since I have had to look up a periodic table to detemine the atomic weight of a substance. I do still remember Avagadro's number = 6.022 X 10^23. That being said, I can probably help you with molarity calculations, but I would think that a simple acid concentration would suffice for your purposes.

The golden rule of concentrations is C1V1=C2V2 where C=concentration and V=volume. This formula will tell you how much you need to dilute an acid of a known volume to get a different concentration of acid.

Link to comment
Share on other sites

After checking out these replies, and others, I think I've figured it out. One of the issues was that I was always thinking of acids as being liquids, but citric acid is a solid, so "pure" citric acid doesn't make sense as a bath, or as a 100% solution.

So the percent solution thing comes in two different forms:

1) Liquid acids (hydrochloric, sulfuric, etc.), otherwise known as a miscible acid. A 100% solution is just pure acid. The confusion was coming in that you can also describe the strength of the acid in terms of molarity (moles per liter of solution). So, according to the chart Jack provided, phosphoric acid is at 100% at 17.3 moles/liter. My wife was working with acids using molarity, and I was asking in terms of percent solutions.

2) Solid acids (citric). For these, the solution is figured as percentage by weight. A certain number of grams per liter. In this case, you may not get to a 100% solution, since there will be limits on solubility.

If anyone questions this, please let me know. I'm not sure, it's just what I've worked out in the last few days!

Link to comment
Share on other sites

Looks like you've got it, but I don't quite get your numbers from the table that I referenced. The first acid (acetic)had a formula wt or molecular weight of 60.052 and a sp gr (density)of 1.05 at 99.5% acid so one liter would weigh 1050 grams and 1050 x 99.5 = 1045 grams of that would be acid That would be 1045/60.052 = 17.4 moles/liter.

Notice none of the entries show 100% acid since it's difficult (and unreasonably expensive) to form "pure" substances. With phosphoric using the table values, I calculate 14.9 molarity rather than 14.8 (1710 gr/liter times .855/97.995 = 14.9 moles/liter) but probably just round off. Close enough. If you could get pure phosphoric, your number of 17.3 would be about right.

Link to comment
Share on other sites

As a solution, you cannot have 100% acid as the solvent will occupy some of that makeup. The highest you can go is saturated. Above saturation levels (at stp) and the solutes would precipitate out.

I ran into this problem when we used saturated sodium hydroxide (not an acid but same principle applies)in the chemistry lab. If it was kept in a storage room only a few degrees below 'room temperature' the a large block of soldi NaOH would form on the bottom of the containers.

Link to comment
Share on other sites

What I'm gathering here is that with a miscible (i.e. soluble in any concentration) acid, you're looking at what is essentially a liquid compound. With solids, like citric, then you run into the issues with saturation levels. But also note two things:

I'm totally oversimplifying so that I can get my head around it.

I'm only trying to figure out the chemistry well enough so that I can order the right amounts and strengths of acids for some pickles.

Link to comment
Share on other sites

use 95% nitric acid. trust me i am a chemistry nerd.

I have seen some nasty things go wrong with concentrated acids, and 95% is a bit high to my way of thinking. I would suggest the PROPER dilution for the application. If in doubt start with a weak solution.

With any suggestions of using a dangerous substance or procedure* (dangerous meaning you stand a good chance of being injured - or could be injured if proper personal safety equipment and procedures are not followed) we ALWAYS and STRONGLY suggest the proper personal safety equipment be used and the proper procedures be followed.

As a chemistry nerd you should KNOW about eye protection, full face shields, aprons, long pants, shoes, and the proper gloves before you even reach for the bottle of a concentrated acid. You should also know to have a charged garden hose or other working water source, as well as a MIXED solution of base on hand. (The Base solution will neutralize the acid.) If you do not know how to properly use baking soda then stay both out of the kitchen and away from concentrated acids.

*a dangerous substance or procedure
Everything in blacksmithing is either hot, heavy, sharp, or dangerous.
Link to comment
Share on other sites

If we’re going to talk about safety when working with acids the rule is ALWAYS add the acid to the water when making a diluted solution. NEVER add the water to the acid. The reason for this is that a large amount of heat is released when strong acids are mixed with water.

If you add the water to acid, you initially form a concentrated solution of acid as the water enters the acid. The solution can boil violently, splashing concentrated acid out of the container because so much heat is released.

When you add the acid to the water, the initial solution is very dilute and the small amount of heat released is not enough to cause the solution to boil.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...