Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: Bill.D.Katt. on 17/03/2011 05:07:23

Title: What are good chemical filters?
Post by: Bill.D.Katt. on 17/03/2011 05:07:23

This question sort of spans two different topics. Agar (or agar-agar) can be used as a chemical filter for the salt bridge in a galvanic cell. It allows (for example) NO3 ions to pass through, but stops metallic ions such as Zn. What other filters would fulfill this role, and would the membrane from a plant work?

Title: What are good chemical filters?
Post by: Bored chemist on 17/03/2011 18:47:27

"It allows (for example) NO3 ions to pass through, but stops metallic ions such as Zn."
No it doesn't.

Title: What are good chemical filters?
Post by: Bill.D.Katt. on 18/03/2011 03:48:57

Quote from: Bored chemist on 17/03/2011 18:47:27

"It allows (for example) NO3 ions to pass through, but stops metallic ions such as Zn."
No it doesn't.

Hm. I must have been misinformed. We used a chem filter in a electrochem lab and my TA said it was agar. Do you know what filters do work?

Title: What are good chemical filters?
Post by: Bored chemist on 18/03/2011 07:05:56

Salt bridges are not filters. They are a tube full of salt water, sometimes thickened with agar or gelatine. The reason the Zn++ ions don't get through is simply that the tubes are long and thin and the total charge transferred is small.
The zinc goes in, but it doesn't get time to reach the other end.

Title: What are good chemical filters?
Post by: Bill.D.Katt. on 18/03/2011 22:06:35

I wasn't saying that they were filters. We had used some kind of thin clear film on either ends of the salt bridge, and I was informed that this was agar and was keeping the metallic cations from being transferred with the anions. I guess the method we used was nontraditional. Would a salt bridge of this sort (packed with agar or gelatine) keep observable transfer from happening if the two cells were connected for several days?

Title: What are good chemical filters?
Post by: Bored chemist on 19/03/2011 14:38:22

Diffusion is a random process so it's perfectly possible that a zinc ion might diffuse through any salt bridge in seconds. However the mean rate of diffusion would be fairly slow.
It might be interesting to get some similar tubing and fill it with agar then place it in some copper sulphate solution. You can see copper's blue colour so you can tell how fast it diffuses.

Then there's the complication that a current flow will force ions (zinc, sodium or whatever) through the bridge.
The extant of this will depend on the size and duration of the current.
About 96000 coulombs of charge will transfer half a mole of zinc or a mole of sodium ions.
If you know the current and duration you can calculate the amount of zinc that gets into the tube.