Sciencemadness Discussion Board

Quick way to determine the concentration of H2SO4

underground - 7-7-2018 at 13:28

Usually my source of concentrated H2S04 is by boiling battery acid. Sometimes i am not boiling that much so i have to reboil it again. What i want is an easy way to determine if the concentration is somewhere in 97-98% so I could stop boiling it. I saw a video on YouTube that you can test it by adding some of your acid to the match head. If it ignites, it is concentrated but I don't know how reliable is this method.

CobaltChloride - 7-7-2018 at 14:06

The one which seems fairly easy to me is sucking up 10ml of the acid into a volumetric pipette and weighing that amount. If it weighs 18.36g, then it is 97-98% concentrated H2SO4, but I think you already knew this. Besides this method there are some others (like seeing whether the acid dehydrates paper), but those only indicate that the acid is above about 70% concentration.

[Edited on 7-7-2018 by CobaltChloride]

JJay - 7-7-2018 at 14:06

People use various methods such as checking the density, dripping it onto paper to see if it burns it, etc., but none of them will really give you a very reliable indication like a titration. You can prepare high-purity sodium carbonate at home and use it to do a titration, but it's easier to use a standardized solution of sodium hydroxide if you can get one. I'm not sure how much of an advantage a quick method would give you... are you dipping samples of boiling sulfuric acid out of a beaker?

MJ101 - 7-7-2018 at 14:08

I started looking around and I found this:

http://www.titrations.info/acid-base-titration-sulfuric-acid

and this:

http://www.marz-kreations.com/Chemistry/Analysis/Quantitativ...

This link has some formulae that look useful.

Hope this helps. :)

underground - 7-7-2018 at 17:03

I just want to find a simple way to find it out, not doing titrations etc. Like there must be something that reacts with a specific way with a 98% SA and will not while below 96%. Foe example 98% SA will turn paper towel black at room temp,but >95% would not right ?

happyfooddance - 7-7-2018 at 18:25

Quote: Originally posted by underground  
Foe example 98% SA will turn paper towel black at room temp,but >95% would not right ?


I can confirm that this is true, this is a great way to check the concentration of sulfuric acid, but it only works if your paper towel is ACS reagent grade.

underground - 8-7-2018 at 11:17

Quote: Originally posted by happyfooddance  
Quote: Originally posted by underground  
Foe example 98% SA will turn paper towel black at room temp,but >95% would not right ?


I can confirm that this is true, this is a great way to check the concentration of sulfuric acid, but it only works if your paper towel is ACS reagent grade.


What is ACS reagent grade ? Just common paper towel wont work ?

CobaltChloride - 8-7-2018 at 11:36

I just tested my 90% drain cleaning H2SO4 with this method and it quickly ate through the paper towel

underground - 8-7-2018 at 15:28

Hmm i think we should have to test various materials to find out which one is the best for this method.

happyfooddance - 9-7-2018 at 10:39

I see that my attempt at humor was lost on you guys.
Quote: Originally posted by CobaltChloride  
I just tested my 90% drain cleaning H2SO4 with this method and it quickly ate through the paper towel


Pretty much any concentration of H2SO4 will eat through a paper towel. That is why the suggested test is ridiculous.
Quote: Originally posted by underground  
Hmm i think we should have to test various materials to find out which one is the best for this method.


Why not spend the time and energy to learn how to titrate? All you need is a syringe and some pH indicator. Once you learn how to do it, you will be able to check the concentration of your acid in a matter of seconds. Moreover, you will know an exact concentration, which will allow you to easily make accurate dilutions.

If you are still wanting to go your route, here is an interesting link that might help or be interesting to you, from a moderator of this forum:
http://woelen.homescience.net/science/chem/riddles/copper+h2...

MöbiusMan - 13-7-2018 at 05:01

You could check the density with a hydrometer or a measuring cylinder and look up a chart, otherwise titrate it as previously said.

Tsjerk - 13-7-2018 at 09:13

If your source of sulfuric is cheap, just over do the boiling. If your sulfuric is precious, stop boiling once in a while and measure the density. It is not going to get easier than that.

I boiled down a couple of solutions on a semi-regular basis over the years, at some point you will be able to tell when it is 'done'. If you do reductions often and you don't like the cooling/heating procedure; better get good at recognizing the viscosity/speed of bubbles moving through the liquid etc. It will become like recognizing faces ;)

ave369 - 20-7-2018 at 02:09

In my experience, the match test is good enough. If it ignites matches, it is good for most experiments.

Herr Haber - 20-7-2018 at 03:26

Quote: Originally posted by MöbiusMan  
You could check the density with a hydrometer or a measuring cylinder and look up a chart, otherwise titrate it as previously said.


Only if you have a large quantity.
Otherwise it'll touch the bottom of your cylinder :-/

AJKOER - 20-7-2018 at 04:34

This is likely a new and not completely safe test. React 1 ml (or drops) of the H2SO4 with an excess of the reducing agent NaBr.

Collect the SO2 (and Br2 vapor) over just boiled water.

The more concentrated the acid, the more SO2 gas should be created which is sparingly soluble in hot water!

H2SO4 + 2 H+ + 2 Br- (aq) --> Br2 + SO2 (g) + 2 H2O

Reference: https://www.chemguide.co.uk/inorganic/group7/halideions.html

and see video at: https://www.youtube.com/watch?v=o36330Uwmeg .

My calculations suggest that 1 cm3 of 98% H2SO4 should yield over 413 cm3 of SO2 at RT, and more over hot water. If water temperature over 59 C, more than double that amount with toxic bromine vapors not adjusting for volume increases due to hot gases. I would recommend a test run with a precisely known concentration of H2SO4 to assess a correction factor to be applied to the implied concentration per observed volume of gases.
--------------------------------------------

Related idea is add Aluminum metal and collect the SO2 gas. However, a slow reaction (see https://www.youtube.com/watch?v=O1MpQv-P-24 ).

[Edited on 20-7-2018 by AJKOER]

JJay - 20-7-2018 at 08:15

Quote: Originally posted by MöbiusMan  
You could check the density with a hydrometer or a measuring cylinder and look up a chart, otherwise titrate it as previously said.


The first problem is that when using the density, you need four significant digits of weight and volume measurement to get two significant digits of acid concentration (check a sulfuric acid density table, and you'll see what I mean).

I like the idea of using a hydrometer. I suppose it would be possible to make one that would work well with sulfuric acid, but it is impossible to conclusively determine that sulfuric acid is 98% with a density measurement.

Sulaiman - 20-7-2018 at 09:36

Using this table
Attachment: NorFalco_H2SO4Property02.pdf (371kB)
This file has been downloaded 319 times
I just checked my drain un-blocker grade sulphuric acid using a Class-B 50ml volumetric flask and 300x0.01g scales

The result was 95 to 96 %w/w (91.66g @21.5oC)
which agrees with my previous titration measurement.

The table goes up to 100% w/w

JJay - 20-7-2018 at 09:55

91.66/50.0 = 1.83 (additional digits are garbage). You can do linear interpolation on that table (or polynomial interpolation if you want to get fancy).

Yet your measurement tells you nothing more than that the drain cleaner (assuming it contains no inhibitors) is between 93 and 100% sulfuric acid.