in certain redox titration,the lower meniscus is observed to take the burette reading,in some upper is used.y?

Question

when we perform redox titration for volumetric analysis,there are certain cases where the lower meniscus is observed to take the burette reading,in some upper meniscus is observed.what is the reason behind this difference?how important is it to take care of the meniscus which needs to be observed?

Answer 1

The accuracy of your measurment depends on the proper reading of the level of the meniscus.

The rules for reading the volume of a liquid for any glassware (pipet, burette, volumetric flask, cylinder etc) are:

If it forms a concave meniscus (downward curving like for water and its solutions) then you read the level of the bottom of the meniscus.
Exceptions are dark coloured solutions where it is difficult to discern the meniscus (like KMnO4 solutions) and thus you have to read level at the edge of the liquid.

If it forms a convex meniscus (upward curving, like mercury some organic solvents) then you read from the top of the meniscus (think of it as if the meniscus is inverted and so you measure again at the tip of the meniscus, though this time the tip of the meniscus is higher than the edge of the liquid)


Concave meniscus, clear solution:

V____ level of observation


Concave meniscus, dark solution
. _____ level of observation
V

Convex meniscus
. ______ level of observation
Λ

I am sorry I can't draw it more accurately but I guess you get the picture.

Answer 2

I've taken about 4 years of chemistry and the only time you need to worry about an upper meniscus is when you're dealing with mercury. If you are using a different compound/chemical other than mercury, you will always just have to worry about the lower meniscus.

the reason for this difference is that mercury is one of the very rare elements that don't have a lower meniscus. They round up. How cool is that. But I doubt that you will be dealing with mercury.

But this meniscus thing is very important, it could screw your data up if you take biochemistry later on if you don't know how to properly measure your liquids. If you're not sure, always ask the teacher.

Answer 3

when i was at school, i was told to take the lower meniscus for clear liquids and upper meniscus for dark colored liquids...for example, for a solution of KMnO4, i'll take the upper meniscus but for iron(2)sulphate, i'll take lower meniscus..

Answer 4

colourless liquids:lower colrd:upper.calc resultss not much diff but matters 2 examiner