[Shehezaada]

Hey guys...we did a lab in which we found the Ksp of Calcium Hydroxide....bascally what we did is slowly titrate the HCl into a Ca(OH)2 solution until the color change. The accepted value is about 12 mL, and we got around 9.8. Corresponding directly to that however...is the Ksp values. The Accepeted Ksp for Ca(OH)2 is about 6.5 * 10 ^-6, while our figure is about 3.1 * 10^-6.....can anyone help us on the sources of error..what could have possibly went wrong?

[Chrataxe]

Impurities due to dirty equipment is a good possibility.  In my chemistry lab, most of the people just do all their experiments and never clean the stuff before.  Then, when they are done, they don't clean it then either.

[mike]

Dirty glassware is not a source of error....

[AWK]

Carbon dioxide from air increases solubility of Ca(OH)2

[Borek]

Dirty glassware is not a source of error....

Yes it is!

Drops of titrant left in the burette and pipettes, air lock preventing you from precise titration, changes in menisk shape changing volume reading. Also remnants of reagents left from previous experiments. These are all sources of error caused by dirty glassware  

[Alberto_Kravina]

Carbon dioxide from air increases solubility of Ca(OH)2

Really? Doesnt CO₂ react with Ca(OH)₂ to Calcium carbonate and water?

CO₂ + Ca(OH)₂ ------> CaCO₃ + H₂O

[Borek]

That will happen later.

At first CO₂ creates calcium hydrogen carbonate inscreasing solubility of Ca²⁺. When you titrate such solution with strong acid you will titrate OH^-, CO₃^2- and HCO₃^-, as they are all present in the solution. Thus effectively you will determine concentration of Ca²⁺ - which is higher in the presence of carbon dioxide.

[jdurg]

The fact that your calculated value is approximately half of what it should be for the Ksp, it makes me wonder if perhaps there was an error at some point in your calculations.  (Remember that there are two moles of OH- for every mole of calcium hydroxide).

[Shehezaada]

all the sources of error guys...were totally awesome! thanks so much..you guys rock!

btw, the calculations were right...we just messed up the lab bit

[mike]

Yes it is!

Drops of titrant left in the burette and pipettes, air lock preventing you from precise titration, changes in menisk shape changing volume reading. Also remnants of reagents left from previous experiments. These are all sources of error caused by dirty glassware  

No, I disagree, students should not be allowed to think that dirty glassware is a source of error.

Poor technique is not a sufficient excuse. What you have said above is not "dirty glassware". Maybe these are mistakes but they are not errors. Is your dirty glassware a random or systematic error?

[Borek]

No, I disagree, students should not be allowed to think that dirty glassware is a source of error.

Why?

Poor technique is not a sufficient excuse. What you have said above is not "dirty glassware". Maybe these are mistakes but they are not errors.

None of the things I have listed can be attributed to poor technique. If the glass is not degreased properly its surface is hydrophobic and the solution doesn't flow down, but can form drops - thus real amount of the solution (either titrated - left in pipette or titrant - left in burette) is smaller than you expect.

There is the same situation with meniscus - hydrophobic surface may distort the meniscus shape, thus making your reading incorrect.

If you will not properly rinse burette used for titration with NaOH and you fill it with HCl solution, remnants of the base will neutralize new titrant, changing its concentration.

For me all these are sources of errors. And they are all caused by dirty glassware.

Is your dirty glassware a random or systematic error?

Random. Although in case of a sloppy student, it will be systematic error - with random value

[mike]

So are you assuming that the dirt in the glassware is staying the same (or haiving the same affect) throughout the experiment? What then happens if the glassware becomes "cleaner" throughout the experiment?

Also, why do you not include having dirty glassware as poor technique, surely part of the technique is to have clean glassware!

I think I just see too many students who blame everything on "dirty glassware" before they will even think about the accuracy of a pipette or burette, parallax error reading a pipette or burette, accuracy of their balance, accuracy of their density graph

I think that having air lock in your burette is not as valid a source of error as these, what if the airlock disappears during the experiment, how do you factor this in?

Errors is always an intersting topic

[Borek]

So are you assuming that the dirt in the glassware is staying the same (or haiving the same affect) throughout the experiment? What then happens if the glassware becomes "cleaner" throughout the experiment?

It adds more unknowns to the final result, making it less predictable and more difficult to reproduce.

Also, why do you not include having dirty glassware as poor technique, surely part of the technique is to have clean glassware!

That's just selection of point of view - and you are right.

I think I just see too many students who blame everything on "dirty glassware" before they will even think about the accuracy of a pipette or burette, parallax error reading a pipette or burette, accuracy of their balance, accuracy of their density graph

Request them to clean their glassware, this way they will be able to blame only themselves

I think that having air lock in your burette is not as valid a source of error as these, what if the airlock disappears during the experiment, how do you factor this in?

No idea, but I am sure it can influence final result. Especially if the airlock is present near the end of titration and makes it impossible to add small drop of titrant - you may either add large (read: uncontrollable) amount of titrant, or don't add it at all

[xiankai]

while in quantitative analysis such impurities may or may not have much effect, in qualitative analysis they have a much more misleading effect. i once boiled an unknown liquid, later to see some precipitate. however, i only realised my error after i realised the precipitate had the same colour with a smudge of solid that obiviously did not belong to the test-tube in the first place.