Re: Burette/Thermom. Readings - Rounding?

farooq_w_at_hotmail.com
Date: 12/29/04 

Date: 28 Dec 2004 23:15:39 -0800

B wrote:
> Hello!
> Got a problem below which has been troubling me for a few days...
>
> I realised I don't know how to round values from thermometers,
burettes
> etc. if the measurement is inbetween the divisions but you have to
> (because the teacher has said so) have a value which is +-0.05 i.e.
the
> last digit / 2nd d.p ends in 0 or 5 e.g. 25.05cm3, 13.20cm3 are
> acceptable, 25.04cm3 are NOT. ---

This depends as Bob as pointed out on the personal choice of the worker
and the nature of work. Some authors do recommend recoding the volume
to 0.01 ml, so 25.04 ml would be acceptable.

>
> If one has to measure a reading on a burette to 0.05 i.e. the last
2nd
> decimal place number must be a 0 or a 5, but the burette only has
> divisions every 0.1ml, e.g. 20.0, 20.1, 20.2, then where does one
apply
> the 20.5 reading?

The last digit is for estimation and applied to the second digit after
the decimal point, your readings could be (say) 20.05 , 20.15 ml,
20.20, 20.25 ml.

>
> I have heard of a number of different approaches from various people,

> and was wondering if the chemists here could offer a definitive
opinion.
> :) [The following examples assume that the bottom of the meniscus is
> between the divisions e.g. 20.0 and 20.1cm3] N.B. I have ignored the
> fact that burettes have their numbers from top down to bottom :)
>
> Approach A] If the bottom of the meniscus is "below" halfway between
the
> divisions, then you round DOWN to 20.00; else if it is above the
halfway
> point, you round DOWN to 20.05 cm3.
> However, another approach suggested by some is: B] If the bottom of
the
> meniscus is "below" halfway between the divisions, then you round UP
to
> 20.05; else if it is above the halfway point (i.e. in the upper half
of the
> region between marked divisions on the burette), you round UP to
20.10 cm3.

>
> Another approach (we'll call it approach C) I have heard is to split
the
> area between 20.0 and 20.1 - i.e. between the divisions - into 3
regions
> (mentally that is) i.e. in the lower 1/3 you assume that the value is
> 20.00, in the middle 1/3 you assume that the value is 20.05, in the
> uppermost 1/3, you assume the value is 20.10.

Never use sub-divisions in terms of odd numbers.

> Last but not least, another approach (which I think is a tad
> inaccurate), that I have heard from many is the following rule: If it
is
> on the division / line itself, it's 20.00 or 20.10 etc. If it isn't
on
> the line, then it's 20.05,20.15 etc. regardless of how close it may
be
> to the upper or lower regions inbetween the divisions.

.........
.........
Perhaps there is no universally accepted official method for reading
burette or graduated pipette volumes.

A British analytical chemistry standard text (some 40 years ago)
suggested that "... for common purposes (burette) readings should be
made to 0.05 mL; for precision work, readings should be made to
0.01-0.02 mL..."

So say if you get the meniscus *anywhere between* say 20.0 and 20.1
ml, the only option (for not-so-precise work) by the above rule would
be to record the burette volume as 20.05 ml. If the meniscus exactly
coincides with 20.0 ml or 21.0 division your volume would be 20.00 and
20.10 ml.

For more precise work, you could divide (by imagining) the marks
between 20.0 and 20.1 into 0.02 divisions, i.e. imagine the burette was
marked in 20.00, 20.02, 20.04, 20.06, 20.08 and 20.10 ml. Now if the
mesicus lies in-between the marks 20.0 and 20.1 ml, then you should
record the volume as 20.04, 20.08 etc depending on the position of the
meniscus.

Some authors suggest reading the burette to 0.01 ml, now if the
meniscus is in between the 20.0 and 20.1 mark, then you will need to
note the volume as 20.01, 20.04, 20.07, 20.10 ml depending on the
position of the meniscus and nature of work.

If you go into more details, you will across "drainage time" usually 30
seconds written on good quality burettes & pipettes, this is the time
you should wait before noting the volume, after you finish your
titration etc.