Gravimetric Analysis of Chloride

 

In this experiment you will conduct a gravimetric analysis of the [Cl-] (chloride ion) concentration 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 extremely straightforward.  The analyte of interest is concentrated in a known form and weighed.  This is as direct a measurement as is used in quantitative analysis, and as such, the precision of these determinations is usually very good. 

 

The procedure given in SWHC 37B-1 will be followed except as noted below. 

 

Bringing Gooch crucibles to constant weight

            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.

 

Precipitating Cl- from solution

            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.  They are happy to help and 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.

 

Bringing Gooch crucibles with AgCl to constant weight and completing the analysis

            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.   

 

REQUIRED MEASUREMENTS

            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 in this case. 

 

NOTES

 

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.