Gravimetric Analysis of Chloride
In this experiment you will conduct
a gravimetric analysis of the [Cl-]
(chloride ion) mass percentage in an unknown solid. The method makes use of the
very low solubility of silver halides, as explained in SWHC Chapter 12. (Note 1) Gravimetry
has the distinct advantage of being an extremely straightforward and direct
method. The analyte of interest is converted into a known chemical form that is
purified and weighed. This is as direct a measurement as is used in
quantitative analysis, and as such, the precision and accuracy of these
determinations is usually very good.
The procedure given in SWHC 37B-1
will be followed except as noted below.
Since it can take a fairly long time
to bring the crucibles to constant weight, we will try to get a head start on
this by cleaning them and placing them in the oven (as instructed in SWHC 37B-1)
the week before. Go ahead and prepare all four crucibles although we will only
do three replicate analyses. That way if you have problems attaining constant weight
you will have a better chance of getting three. Also if you get all four to
constant weight and have an accident, you will have an extra crucible.
Concentrated nitric acid is very
corrosive and is a strong oxidant. Be
sure to wear gloves and goggles and be very careful when handling it.
The 6 M
ammonia has a pungent odor; you may wish to place the crucibles in the hoods during
the step where it is used.
You will have to be conscious of
time to get the procedure done during this lab period. One suggestion is to
immediately move your crucibles from the oven to your desiccator so that they
can begin cooling. There will be “down times” later in the lab when you can do
the weighing. (Note 2)
If you are unsure of how to set up
the filtration with the Gooch crucible, be sure to ask the TA for help. There will be scant time for you to spend
figuring it out on your own.
After the digestion, you will
probably not have enough time to allow the samples to sit in the dark
for two hours. One hour should be sufficient, but if you can go longer, you
will probably get better results. Leave
yourself at least 30 min. for the filtering and washing steps and use your best
judgment. The goal is to have the
filtered and washed silver chloride in the crucibles in the oven when you leave
today.
You will need to cool the samples
in the desiccator again before weighing, so you should do this transfer
immediately when you arrive on week 8. With luck you will only need to do one
more cycle of heating and weighing, but keep an eye on the time while doing the
spec. lab, in case you need to do more. You must get a constant weight from the
crucible plus samples before working up the results.
As with all of the previous labs,
the preLab is due before you can start your work.
You will present the results for %
chloride in your solid unknown (be sure to provide the unknown # as well) to
your lab TA during week 8. As always, your results include the mean and
relative standard deviation of your replicate determinations. You should expect
the precision to be very good (ppth level) in this
case.
1) The biggest
problem with silver halides is their tendency to form colloidal suspensions
instead of proper precipitates. Your success in this experiment will depend almost
exclusively on your ability to get a solid that you can filter.
2) Make
sure you have a way to tell the crucibles apart before you start weighing them.
This seems obvious, but it is awfully hard to get a crucible to come to
constant weight if you aren’t weighing the same one.