Heidi
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Alternate methods of finding Acid/Alkali concentrations (determining pH!!)
Except for titrations what are other ways of finding the concentration of an acid or alkali?
I'll appreciate any help.
Thanks 
Edit: Title to make it more comprehensible.
[Edited on 27-7-2004 by chemoleo]
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darkflame89
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You could try and put sodium bicarbonate in bit by bit, then measure the total amount of sodium bicarbonate needed to neutralize the acid. Of course
it isn't as efficient as titration, that's why titration is the most commonly used procedure.
Ignis ubique latet, naturam amplectitur omnem.
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Esplosivo
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You could also use the pH/pOH to find the concentration of H+ or OH- ions present in solution respectively.
Assuming that we are talking about an acid which totallly decomposes such as HCl we say that seince the pH = -log(H+ ion concentration). Because the
acid decomposes totally (i.e. HCl --> H+ + Cl-) we can say that for each mole of H+ ion there is 1 mole of HCl. Therefore:
[H+] = 10^(- pH).
The same can apply for the concentration of OH- using pOH.
Btw, in case of weak acids, such as carboxylic acids the decomposition constant should be know. This also applies for weak alkali.
[Edited on 13-5-2004 by Esplosivo]
Theory guides, experiment decides.
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vulture
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Please use a descriptive topic! Things such as "HELP", "Chemistry" are no good.
Furthermore, this is in the wrong section.
Be more careful next time.
Moved.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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I am a fish
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| Quote: | Originally posted by darkflame89
You could try and put sodium bicarbonate in bit by bit, then measure the total amount of sodium bicarbonate needed to neutralize the acid. Of course
it isn't as efficient as titration, that's why titration is the most commonly used procedure. |
That is titration. You're reacting a reagent of known concentration with your sample and measuring how much is needed for the reaction
to go to completion.
1f `/0u (4|\\| |234d 7|-|15, `/0u |234||`/ |\\|33d 70 937 0u7 /\\/\\0|23.
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thunderfvck
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Density. You'll need a specific chart for that though.
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axehandle
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Using a digital PH meter. But I have no clue how they work, so they may employ some form of titration as well.
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lordnick
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Digital pH meters have an electrode made of very thin glass that only allows small ions(eg hydrogen ions) to pass through. It works by measuring the
electrical potential of these ions and converts it into a pH reading. I wouldn't use a pH meter for meausring the pH of a solution that contains
a hight concentration of small metal ions (such as Na+) because these can also pass through the electrode and stuff up your reading.
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nitroboy
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perhaps do a slight variation on a titration, by doing it gravimetrically?
you can add in excess acid or alkali and you will get a salt. then take the mass of the salt, and it's know chemical formula and you can work out
your various concentrations.
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axehandle
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| Quote: |
perhaps do a slight variation on a titration, by doing it gravimetrically?
you can add in excess acid or alkali and you will get a salt. then take the mass of the salt, and it's know chemical formula and you can work out
your various concentrations.
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That's actually ingenious, if you lack the capacity to measure volume exactly but have an accurate scale. I'm impressed.
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thunderfvck
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Yes, that is a good one. But you'd have to evaporate the solvent, no? Otherwise the salt will dissolve?
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vulture
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While a very workable solution, the gravimetric way isn't exactly accurate. Workup and filtering will cause alot of loss.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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t_Pyro
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Not really. The key is to use a precipitating reagent. eg. for sulfuric acid, use barium chloride, for HCl, use silver nitrate. The precipitate needs
only to be filtered out. Measure the weight of dry funnel+filter paper, filter the ppt, let the filter paper dry, and weigh the funnel+filter
paper+ppt. The difference in weghts is the weight of the ppt.
For even greater accuracy (beyond 7 decimal places, if you feel like), you could take into account the solubility product of the sparingly soluble
ppt, but I don't think anybody needs so accurate a value, especially as it is far outweighed by other factors like human error, and calibration
errors of instruments...
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unionised
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Hang on Vulture! Gravimetric measurements give some of the most precise analyses known. Granted, that you need to get a product that is nice and
insoluble). Gravimetry can be a bit short on specificity too, if you measure sulphuric acid by ppting BaSO4 then you can't tell if you have the
acid or a salt (eg Na2SO4).
It's easy to measure a mass to 5 digits and near impossible to measure a volume to that accuracy (which is why volumetric glassware is calibrated
by weight).
(A pH meter won't actually tell you how much acid is present (I know that sounds like a contradiction, but trust me). If you take a nice
well-behaved buffer solution, say 1 M each of acetic acid and sodium acetate and measure the pH you will get an answer that is the acidity constant
for the acid (about 4.5 IIRC). If you throw half of this away and fill the flask up with water you will have half as much acid, but the same pH (near
enough).
Another variation on the theme would be to add an excess of sodium bicarbonate and measure the CO2 that comes off (volumetrically or by mass, take
your pick)
A similar trick using methyl magnesium bromide was used to measure "active hydrogen" in organic analysis (before we had all the nice
spectroscopic techniques), you measured the volume of methane. Of course, from this point of view water is an acid.
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BromicAcid
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I was thinking about this the other day. Considering oleum and testing concentration there of.
What I was thinking of was to take a specific volume of concentrated H2SO4/SO3 mixture and place in a bomb calorimeter, not an advanced one, just a
styrofoam cup like used in begining chem classes. Then add a specific amount of water, e.g. 3 ml H2O added to 10 ml H2SO4/SO3 mix of unknown
concentration and measure the temperature change. If enough readings were taken at different temperatures a curve might form, I'd be easier then
titrating it every time. Has this method ever been used to anyone's knowledge?
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Esplosivo
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The medthod you are suggesting BromicAcid seems like thermal titration to me. During thermal titration a fixed volume of a base with unknown
concentration, for example, is mixed with a small volume of an acid with known concentration. This is carried out repeatedly, keeping the volume of
the base fixed and increasing the volume of the acid by for example 5cm3 for each titration. The change in temperature in each case is recorded and a
graph is drawn, as you described. Instead of an acid-base titration you are suggesting some sort of recording the enthalpy of dissolution if I've
understood well.
[Edited on 9-6-2004 by Esplosivo]
Theory guides, experiment decides.
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BromicAcid
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That's pretty much it exactly. I'm sure though there is a method to calculate the heat of hydration without actually testing it then using
that as a rough reference. Basically I was wondering this, if I could react 10 ml of my concentrated H2SO4 with 5 ml H2O and measure the temperature
change would I be able to see a difference if it was say 10% oleum and therefore have a quick dirty method to test if SO3 is actually getting produced
and absorbed without sticking my head in there. But I guess the answer is yes from what you've said so thank you! Although not a complete
titration it is a form of titration I guess, especially during the initial stages if SO3 is present converting the pyrosulfuric acid to sulfuric acid.
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If_6_was_9
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See
http://search.ebay.com/refractometer_W0QQsokeywordredirectZ1...
You should find some refractometers which are made for testing battery acid. For other things you'll have to calibrate it if you can't find
a table somewhere.
[Edited on 17-6-2004 by If_6_was_9]
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