Sciencemadness Discussion Board

Is there any way I can determine my acid/base concentrations from what is on hand?

alking - 17-6-2017 at 06:21

I currently have no known concentrations in which to titrate with to determine that of another acid or base. I have 29% Ammonia solution, ~35% HCl, ~98% H2SO4, Sodium and Potassium Hydroxide, Glacial Acetic Acid, and about 5-10 other common to less common weak acids and bases which I won't list as they're probably not useful for calibration. At this point they're all old enough that I can't trust their initial concentrations to still be reliable.

I would imagine the only ones that would be feasible to titrate from are strong acids or bases so that just leaves HCl, H2SO4, KOH, and NaOH. The problem with those is they're all impossible to either fully dry or determine their existing concentration w/o another reference point to measure it from.

There are ways I can come up with to at least get an idea, but nothing stands out as a clear solution to me. The two best ideas I have so far would be to start by measuring the density of either the HCl, H2SO4, or volume of the hydroxides in water to determine the water content. That would sure be better than nothing, but it then will only be as accurate as the combination of my scale, which is unreliable as of late, my glassware, and my eyes, none of which I'd give a lot of merit for accuracy.

edit: Another idea would be to make a saturated solution of a hydroxide salt so that some precipitates out. Theoretically I could then determine the exact concentration based on the temperature. Again that would only be so reliable though unless I held it at an exact temperature for a number of days until most things that are going to precipitate have done so.

[Edited on 17-6-2017 by alking]

JJay - 17-6-2017 at 07:33

I'm planning on running some titrations today. My understanding is that sodium carbonate makes a good primary standard. It's a little tricky to titrate with it, but it is easy to make from baking soda and also easy to dehydrate. So I'm going to use it for my primary standard.

Azeotropic HCl solution is also used as a primary standard, but to do so with precision requires paying close attention to atmospheric pressure when distilling it.

It's possible to use NaOH as a primary standard... I think the usual procedure involves recrystallization from methanol and drying under vacuum in an oven, which requires apparatus not always readily available to the amateur. It tends to absorb both water and carbon dioxide from the air, so it's not an ideal primary standard, but it is easier to titrate with it than sodium carbonate since it is monobasic.

KOH and H2SO4 are probably not good primary standards because they are hard to dehydrate, and determining their water content with precision requires analytical procedures.

Sulaiman - 17-6-2017 at 07:36

or heat commercial/domestic/kitchen sodium bicarbonate (sodium hydrogen carbonate) in an oven to get anhydrous sodium carbonate,
which can be made into a 'standard' solution with good accuracy.

EDIT; sum1 beat me to it

[Edited on 17-6-2017 by Sulaiman]

UC235 - 17-6-2017 at 08:40

If you can get a hold of some phthalic acid, you can make potassium hydrogen phthalate, easily recrystallized from water, and a primary standard for standardizing a solution of NaOH or KOH along with phenolphthalein. You can go on to use the standardized NaOH solution to standardize your acids.

I have tried to use highly purified potassium hydrogen tartrate (cream of tartar) as a substitute but could not get good results due to its very poor water solubility.

alking - 17-6-2017 at 09:07

I actually just ran out of sodium carbonate yesterday! I'll probably wait until I get more rather than make some from the bisulfate, but good to know that's a common method, I'll give that a shot. Thanks!

unionised - 18-6-2017 at 08:05

Quote: Originally posted by UC235  
If you can get a hold of some phthalic acid, you can make potassium hydrogen phthalate, easily recrystallized from water, and a primary standard for standardizing a solution of NaOH or KOH along with phenolphthalein. You can go on to use the standardized NaOH solution to standardize your acids.

I have tried to use highly purified potassium hydrogen tartrate (cream of tartar) as a substitute but could not get good results due to its very poor water solubility.

The poor solubility is much less of a problem if you do a back titration.