Science Buddies: "Ask an Expert"

I've made a double salt of copper and phosphate, and am now analysing it to find the percentage mass of each within the double salt. For analysis of the copper, the double salt was mixed with water and heated to dissolve. This was made up to 100ml in a graduated flask and 25ml of this solution was pipetted into a conical flask. 10ml of sulphuric acid and 25ml of potassium iodide were then added, and the mixture was titrated with standard 0.1M sodium thiosulphate. Was the sulphuric acid needed to provide hydrogen ions? And was the potassium iodide needed to provide the iodide/iodine(?) ions which later acted as an indicator?

For analysis of the phosphate, the double salt and nitric acid were mixed together until clear, then 20ml of distilled water and 20 ml of lead (II) nitrate were added while stirring. 3 mol l-1 ammonia solution was added dropwise to the solution until the pH reached 5. the beaker was then covered with a watchglass and left to stand for 10 minutes (It was then filtered and the precipitate dried and weighed). Why do distilled water and lead (II) nitrate need to be added? And is the presence of the watchglass over the beaker to stop gases escaping during precipitation?

Thanks to anyone who can help.

ChemChris wrote:I've made a double salt of copper and phosphate, and am now analysing it to find the percentage mass of each within the double salt. For analysis of the copper, the double salt was mixed with water and heated to dissolve. This was made up to 100ml in a graduated flask and 25ml of this solution was pipetted into a conical flask. 10ml of sulphuric acid and 25ml of potassium iodide were then added, and the mixture was titrated with standard 0.1M sodium thiosulphate. Was the sulphuric acid needed to provide hydrogen ions? And was the potassium iodide needed to provide the iodide/iodine(?) ions which later acted as an indicator?

For analysis of the phosphate, the double salt and nitric acid were mixed together until clear, then 20ml of distilled water and 20 ml of lead (II) nitrate were added while stirring. 3 mol l-1 ammonia solution was added dropwise to the solution until the pH reached 5. the beaker was then covered with a watchglass and left to stand for 10 minutes (It was then filtered and the precipitate dried and weighed). Why do distilled water and lead (II) nitrate need to be added? And is the presence of the watchglass over the beaker to stop gases escaping during precipitation?

Thanks to anyone who can help.

This sounds like you are doing a specific lab. I suggest you ask your teacher or TA for assistance with these questions. If this lab has a specific name, you can try googling for it. Additionally, if your procedure or prelab had specific references you should read them.

Louise

Do a little thinking or research into what precipitates, oxidizes, or reduces with the ions you have.

As far as a the cover, think about what might evaporate and condense on the cover.

We could look it up for you, but you are obviously doing a specific lab experiment in qualitative chemical analysis and they are designed to make YOU think!