Cycling and bicycle discussion forums. 
   Click here to join our community Log in to access your Control Panel  


Go Back   > >

Foo Off-Topic chit chat with no general subject.

User Tag List

Reply
 
Thread Tools Search this Thread
Old 02-07-09, 03:00 PM   #1
Snicklefritz
Senior Member
Thread Starter
 
Snicklefritz's Avatar
 
Join Date: Mar 2005
Location: In the middle of horse country, in The Garden State
Bikes:
Posts: 3,159
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
any Soil science people here?

Hey is anyone out there into soil science or environmental chemistry? I'm helping someone
with some soil science homework but some of the topics are outside my area of expertise.
It has to do with calculating and deriving Eh as a function of PH. Send me a PM if you've done this stuff.
Snicklefritz is offline   Reply With Quote
Old 02-07-09, 03:10 PM   #2
randya
Senior Member
 
randya's Avatar
 
Join Date: Jul 2003
Location: in bed with your mom
Bikes: who cares?
Posts: 13,689
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
you generally plot Eh vs. pH to determine the stability fields for various ionic compounds of elements like calcium, iron, etc. in the natural environment. I'm not sure you can actually calculate Eh from pH though, they are usually both field-measured parameters, at least when collecting groundwater quality samples.
randya is offline   Reply With Quote
Old 02-07-09, 03:27 PM   #3
Snicklefritz
Senior Member
Thread Starter
 
Snicklefritz's Avatar
 
Join Date: Mar 2005
Location: In the middle of horse country, in The Garden State
Bikes:
Posts: 3,159
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
yeah, that makes sense. I'm actually helping my brother-in-law with a graduate level course in soil science and most of the stuff I have some idea about even though soil science is not my area, but the Eh and pH thing was kind of weird as you said.

the problem goes something like this:

Fe(OH)3(s) = Fe3+ + 3OH- log K = -39.1

The teacher then asks them to reexpress the equation for Eh so that it is explicitly defined in terms of pH

Eh = Eho - (0.059/n)log(Fe2+/Fe3+)

I never asked any of my students to do a calculation like this and I don't see it in any of my textbooks either. The prof then wants people to calculate Eh for the half-cell in the first equation as a function of pH. Half cell calculations, no problem, but I just wondered about the pH part.
Snicklefritz is offline   Reply With Quote
Old 02-07-09, 10:48 PM   #4
randya
Senior Member
 
randya's Avatar
 
Join Date: Jul 2003
Location: in bed with your mom
Bikes: who cares?
Posts: 13,689
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
well, I suppose there is a mathematical relationship between Eh and pH for any point on an Eh-Ph plot, but I couldn't tell you much about how that would be defined or expressed mathematically. I do know that pH is the negative log of the H+ ion concentration, which explains the log part of your equation...

randya is offline   Reply With Quote
Old 02-08-09, 01:27 AM   #5
Tinuz
Junior Member
 
Join Date: Oct 2008
Bikes:
Posts: 21
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Hmmm, let's see howmuch I remember (it's been a loooong time):

So,the problem is that your equation

Eh = Eho - (0.059/n)log(Fe2+/Fe3+)

Need to be a function of pH? Well, isn't the ratio between Fe2+ and Fe3+ dependent on pH?
By looking at how the ratio Fe2+/Fe3+ behaves under different pH's, you could subsitute that part with a function of pH.

But then again, I do much larger scale soil science (well, geo-hydrology these days) and the chemistry is nothing but a painfull memory.......(I hated it with a passion)
Tinuz is offline   Reply With Quote
Old 02-08-09, 02:00 AM   #6
coasting 
Still can't climb
 
Join Date: Sep 2006
Location: Limey in Taiwan
Bikes:
Posts: 22,645
Mentioned: 4 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
i thought foo was just for silly posts. This thread hurts my brain, but that doesn't take much these days.
coasting is offline   Reply With Quote
Old 02-08-09, 09:57 AM   #7
Tinuz
Junior Member
 
Join Date: Oct 2008
Bikes:
Posts: 21
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Hey, did you check out this wiki page?

http://en.wikipedia.org/wiki/Eh_(chemistry)

It talks about this, and includes an equation very similar to yours called the Nernst equation.

PS.If I making yself look like an idiot....I am a 'fallen' soil scientist, I do mostly statistics these days, and am loving it
Tinuz is offline   Reply With Quote
Old 02-08-09, 11:53 AM   #8
randya
Senior Member
 
randya's Avatar
 
Join Date: Jul 2003
Location: in bed with your mom
Bikes: who cares?
Posts: 13,689
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
There's a bunch more info if you google 'Nernst Equation'

http://www.google.com/search?client=...UTF-8&oe=UTF-8
randya is offline   Reply With Quote
Old 02-08-09, 02:19 PM   #9
Snicklefritz
Senior Member
Thread Starter
 
Snicklefritz's Avatar
 
Join Date: Mar 2005
Location: In the middle of horse country, in The Garden State
Bikes:
Posts: 3,159
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
yeah Tinuz, that's it. I wasn't sure how to incorporate H+ into the equation.

the "Foo" forum is awesome!!!



thanks guys!!!!

Last edited by Snicklefritz; 02-08-09 at 03:14 PM.
Snicklefritz is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -6. The time now is 04:57 AM.