Sciencemadness Discussion Board

An interesting way to find H2S04 strength

gregxy - 5-9-2010 at 13:41

The question often comes up of how concentrated an
unknown source of H2SO4 (like Rooto) is.
Density measurement is difficult since the density does
not change that much going from 90% to 98%, likewise
titration would need a good quality standard and accurate

Then I thought of the heat of hydration. Some googling led
to the following graph of "sulphuric acid enthalpy"

This method looks like it should be quite accurate.
Diluting 98% acid with an equal weight of H20, both
starting at 32F should result in the mixture having
a temperature of 225F, while the same experiment with
95% acid results in 200F and 90% acid only 182F.
Temperatures can be measured accurately, use a light
weight insulating container and measure quickly to avoid
heat loss.

DDTea - 5-9-2010 at 23:09

Good idea!

It may be better to use two containers (something like a bomb calorimeter, but not quite): a conducting container that holds the sulfuric acid inside a second, well-insulated container (a thermos or vacuum-insulated coffee mug would be good for this) that contains water at a known initial temperature. Stirring of the water would be critically important, though.

However, if I may step up on my pedantic analytical soapbox for a moment, I would still want to compare the concentration calculated via this method to the concentration calculated via titration with a standard of known concentration. If you're just interested in calculating a percent concentration, you really don't need an extroardinarily accurate standard. Titration is probably going to be the most accurate method, but for some, it may not be the easiest or cheapest, so your idea is worth investigating.

I do not have the means to do so, though, so I will just comment from the sidelines :P

franklyn - 6-9-2010 at 20:28

The procedure then would be start with equal measured weights of ice cold H2SO4
and H2O in separate containers , dump the acid into the water and measure peak
temperature rise with an infrared thermometer ?
Now all one need do is make sense of the chart. The zero reference point of the
entalpy scale ( point 4 ) remains for all plots always using the bottom most curve
32 ºF intersection with the verticle % strength. Where the connecting line of the two
endpoints intersects the 5O % verticle line , one reads the temperature off the most
proximate curve there , interpreting as needed in between curves. Neat idea.

The inverse procedure would be to plot the intersection of % strength and 32 ºF
curve from the resulting measured temperature and ( point 4 )

I make the temperature of the diluted 90 % solution at 175 ºF. Please re-check your
measurement to see if the error is your's otherwise I must be in error.

Why is this in energetics section ?


Microtek - 7-9-2010 at 01:26

I get dilution of 90 % acid with an equal weight of water (both at 32 F) to end up at about 162 F. Remember that the resulting acid will be 45 % H2SO4, not 50 % (since it was only 90 % to begin with).

franklyn - 7-9-2010 at 09:39

Quote: Originally posted by Microtek  
I get dilution of 90 % acid with an equal weight of water (both at 32 F) to end up at about 162 F. Remember that the resulting acid will be 45 % H2SO4, not 50 % (since it was only 90 % to begin with).

Ahh - That's why


gregxy - 20-9-2010 at 12:24

I did some experiments:

Mixing 23g of Rooto with 23g of water (both at 32F),
the temperature reached a peak of 162F after about
30 seconds. (I was surprised it took this long to peak,
I used an IR thermometer to measure it). This corresponds
to an acid concentration of 90% which is inline with what
others have said.

I also tried 22g of Rooto + 10g of NH4NO3 mixed and cooled
to 32F and then mixed with 22g of water. The peak temp
was now 93F, So the "acid+salt" acts like an equivalant
weight of 75% H2SO4.

An experiment with KNO3 instead of NH4NO3 gave about
the same result.

It would be interesting for someone to do similar
comparisons with lab grade H2SO4 and HNO3+H2SO4.

Rosco Bodine - 20-9-2010 at 14:39

A figure in an old lab notebook I noted a measured density 1.826 for Santee Rooto which corresponds with a charted acid strength of 92.5% H2SO4, 58.07 ml contains 1 mole H2SO4. I did not titrate the sample to confirm. That is probably pretty close, but that figure is from at least ten years ago. Sample was described as clear light tan color. Density, specific gravity is a good general pretty accurate method of determination. Then you can bulk neutralize 99% of the density derived strength and titrate the final 1 percent or so to endpoint to get a more exact figure. Never really checked to see how close it can be gotten by observing for ceasing of effervescence upon addition of sodium bicarbonate, but it is probably closer than 1% and may be as good as 0.1% or less acidity as a visual endpoint.

blazter - 20-9-2010 at 15:31

This method is used industrially in continuous acid "make down" acid dilution systems. Temperature of neat acid is taken before, and temperature after mixing with water is watched. Control valves of acid and water are run to maintain this Dt and thus the final acid strength. IIRC the final dilute flow is all that is needed for this control scheme.