Analytical Chemistry

[EspoirTron]

Hey everyone, so as we all now know, area of study one in unit 3 chemistry has a lot of calculations associated with it. Although, I haven't really seen much on the forum about actually comparing analytical techniques and which ones are better used in which situation. So I thought I would start a topic on this. I will start off with this question and please feel free to contribute. Volumetric analysis vs Gravimetric analysis. Which one suites which situations and what are the pros and cons to each of these analytical techniques?

[clıppy]

This isn't a bad idea so i'll contribute what i know:

Gravimetric Analysis:
-Requires a precipitate be formed
-Is quantitative
-Requires a high molar mass


Volumetric Analysis:
-Is qualitative
-Requires a standard solution


Those are all I can think of off the top of my head, if anyone posts anything i'll add it to the list.

[EspoirTron]

This isn't a bad idea so i'll contribute what i know:

Gravimetric Analysis:
-Requires a precipitate be formed
-Is quantitative
-Requires a high molar mass


Volumetric Analysis:
-Is qualitative
-Requires a standard solution


Those are all I can think of off the top of my head, if anyone posts anything i'll add it to the list.

Thanks for that.
I agree all those features you have mentioned are appropriate.

I will add some more

Gravimetric Analysis:
-There could be other ions present in the sample that could also be precipitated unintentionally.

That is all I can think of the top of my head at the moment.

Volumetric analysis:
- Can be quantitative
-The pH indicator has its own pH and could affect the results dependent upon its own pH.
- The colour change that occurs, is subjective and is up to the student(s) conducting the experiment whether or not the end point has occurred. This issue could be resolved by using a pH meter though, if you knew the pH you required at neutralization.

I decided to make this thread, as Unit 3 Area Of Study 1 has a lot of calculations but browsing through exam questions, it is evident that they are coupled with theory questions, which test a students understanding the actual principles. So I thought this would be a good way to collaborate knowledge on the principles of analytical chemistry! 

[lzxnl]

Gravimetric analysis has another major flaw: adding too much solvent will dissolve some of the precipitate. Dissolution is an equilibrium process; you can't say that if I have 10.00 grams of barium sulfate initially, after placing it in 500 mL of water, I will still have 10.00 grams of barium sulfate.
As for volumetric analysis, if we're doing acid base titrations, indicators can be a problem, but if the indicators are chosen appropriately to show a sharp endpoint that is very close to the equivalence point with a marked change in colour, volumetric is fine. Remember, with a strong acid and weak base or vice versa, we will have a sharp equivalence point.

[EspoirTron]

Gravimetric analysis has another major flaw: adding too much solvent will dissolve some of the precipitate. Dissolution is an equilibrium process; you can't say that if I have 10.00 grams of barium sulfate initially, after placing it in 500 mL of water, I will still have 10.00 grams of barium sulfate.
As for volumetric analysis, if we're doing acid base titrations, indicators can be a problem, but if the indicators are chosen appropriately to show a sharp endpoint that is very close to the equivalence point with a marked change in colour, volumetric is fine. Remember, with a strong acid and weak base or vice versa, we will have a sharp equivalence point.

Quite right you are, excellent points raised there!

[dzames]

Hey guys!

Just a couple of extra hints!

Gravimetric: can also be used to determine the water content/degree of hydration for certain compounds (weight to constant mass). But this means a shortfall of gravimetric is if the compound being analysed decomposes when heated - then things become complicated.

Volumetric: while it's often used for acid/bases, it can also be used for other types of reactions like redox. But be careful about indicators! all of the indicators in the data book are for acid/base reactions. In fact, a lot of redox reactions don't even require indicators as they have their own colour change as a part of the reaction (sometimes changing oxidation state changes colour).

Finally, don't forget that we also have back-titrations, which are used for 3 scenarios:
1. Broad end point (weak acid and weak base)
2. Insoluble material - we can't put that in a burette or pipette
3. Volatile solution - a normal titration takes too long to perform and the solution could react with the atmosphere, so do a back titration instead.

Hope this helps!
James

[Limista]

Dissolution is an equilibrium process; you can't say that if I have 10.00 grams of barium sulfate initially, after placing it in 500 mL of water, I will still have 10.00 grams of barium sulfate.

I don't understand this. What is dissolution and why is it an equilibrium process? What is an equilibrium process?

Also, if you have 10grams of barium sulfate initially, and then you place it in 500mL of water, won't you still have 10grams of barium sulfate because the number of moles is the same?

I'm not challenging you here - I just don't get what you're saying.

[EspoirTron]

Hey guys!

Just a couple of extra hints!

Gravimetric: can also be used to determine the water content/degree of hydration for certain compounds (weight to constant mass). But this means a shortfall of gravimetric is if the compound being analysed decomposes when heated - then things become complicated.

Volumetric: while it's often used for acid/bases, it can also be used for other types of reactions like redox. But be careful about indicators! all of the indicators in the data book are for acid/base reactions. In fact, a lot of redox reactions don't even require indicators as they have their own colour change as a part of the reaction (sometimes changing oxidation state changes colour).

Finally, don't forget that we also have back-titrations, which are used for 3 scenarios:
1. Broad end point (weak acid and weak base)
2. Insoluble material - we can't put that in a burette or pipette
3. Volatile solution - a normal titration takes too long to perform and the solution could react with the atmosphere, so do a back titration instead.

Hope this helps!
James

Thanks James, those are some brilliant hints! Also, I would believe that if we are heating the solution we may be in fact getting rid of insoluble products or portions of the analyte in the matrix structure of the substance: as substance may react with the atmosphere; and could be potentially lost.

I don't understand this. What is dissolution and why is it an equilibrium process? What is an equilibrium process?

Also, if you have 10grams of barium sulfate initially, and then you place it in 500mL of water, won't you still have 10grams of barium sulfate because the number of moles is the same?

I'm not challenging you here - I just don't get what you're saying.

Dissolution, is how ions and molecules, 'spread' in aqeous solution. If you recall from Unit 2 Chemistry, when an ionic substance such as sodium chloride is added to water, the substance will dissociate and it's ions will form ion-dipole bond with water. Simply, there will be sodium and chloride ions floating in solution. I wouldn't stress about the equilibrium process, as that is unit 4 chemistry. You will learn about that at a later stage.

You will still have 10grams of barium sulfate, but it may not all be in the form of barium sulfate. The substance may react with water. Obviously, when you add the barium sulfate to the water, both the water and barium sulfate are not present at a concentration that will remain 'constant' (equilibrium) in respect to time.
That is my spin on it, I may not be 100% correct. However, I hope that helped out a bit!

[Limista]

^ helped out a lot - thanks again lol 

[EspoirTron]

^ helped out a lot - thanks again lol 

No problems at all! 

[achre]

Volumetric analysis:
- Can be quantitative

It's almost exclusively quantitative.

[brightsky]

I don't understand this. What is dissolution and why is it an equilibrium process? What is an equilibrium process?

Also, if you have 10grams of barium sulfate initially, and then you place it in 500mL of water, won't you still have 10grams of barium sulfate because the number of moles is the same?

I'm not challenging you here - I just don't get what you're saying.

just thought i'd clarify. in junior science, the assumption would have been made that precipitates, being solids, are completely insoluble. in fact, all precipitates are sparingly soluble. barium sulfate participates in the following dissociation reaction when placed in water: BaSO4(s) <--> Ba(2+)(aq) + SO4(2-)(aq). note the double headed arrow. this reaction is an equilibrium reaction, meaning that it does not 'go to completion' so to speak; not all the barium sulfate will dissociate into barium and sulfate ions. in fact, only a very, very, very small amount of barium sulfate will dissociate (since the Ksp value is really, really small). so if you wash it with distilled water too many times, you may lose some of the precipitate. this is also the main reason why we wash the precipitate with COLD distilled water, because the solubility of solids increases with temperature.

[jgoudie]

Interesting points, however the idea that you do get some BaSO₄ dissolving during the washing process might only come in discussion as to why you have an answer that is less than the actual literature value.  Even in last years unit 3 exam, when the solubility was given, it was not taken into account during the calculation stage.  One thing to be careful with, from last years VCAA exam, is the throw away comments that some compounds are always completely soluble (Nitrates, Alkali ions).  These compounds are more soluble than others, but rehashed facts like all / completely soluble are unaccepted.

Here are a few other good comparisons for other analytical techniques:
Grav A - Cheap, easy to perform in a lab, in-expensive, quiet accurate, but needs to form a precipitate
Volu A- Cheap, easy to perform in a lab, back titrations can detect quiet low concentrations.
HPLC - moderate cost, pretty quick qualitative + quantitative
GC - moderate cost qualitative + quantitative
AAS - very low concentrations, qualitative + quantitative
UV-Vis - qualitative + quantitative
IR, NMR - qualitative only, pretty expensive.

It is good to know what EM radiation is used in each form of spectroscopy and what absorbs it:
IR - Infra-red absorbed by changing the vibration energies of bonds / molecules
UV-Vis - UV-Vis absorbed by valence electrons in molecules
AAS -  (UV) visible absorbed by valence electrons in metal ions.
NMR - radio-waves absorbed by nuclei changing spin to be in resonance with the magnetic field

Also you should know what materials can be tested in each type of analysis:
HPLC - soluble organic compounds
GC - volatile organic compounds (b.p. ~0-300^oC
AAS - metal ions
UV-Vis - coloured compounds
IR / NMR - organic compound structure

[lzxnl]

I am going to be pedantic here and say that some pairs of liquids have a property known as "miscibility", where any concentration of those two liquids will mix perfectly. Hydrochloric acid is one example; nitric and sulfuric acids are also miscible with water.
Organic liquids are also miscible with each other. I don't think solids exhibit this property.
A bit off-topic, I know.

[EspoirTron]

I am going to be pedantic here and say that some pairs of liquids have a property known as "miscibility", where any concentration of those two liquids will mix perfectly. Hydrochloric acid is one example; nitric and sulfuric acids are also miscible with water.
Organic liquids are also miscible with each other. I don't think solids exhibit this property.
A bit off-topic, I know.

That's fine, it's great to have a general discussion anyway. I don't think solids would either to be honest. There would be some degree what insolubility I would imagine, i.e., not all of the solid would dissolve.