## pka

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2468
Garter
Posts: 23
Joined: Thu Nov 01, 2012 12:48 pm

### pka

Hey,

need help working this out:

pH: 4
molar extinction coefficient: 0.0187142857 m-1cm-1
absorbance at 500nm: 1.492

[Acid form]?? (work out using molar extinction coefficient)
[base form]?
lg([acid]/[base])?
pKa?

If someone can guide me on how to do this especially the acid form....i'd greatly appreciate it...so i can use the basics fr it to work out the rest of my ques...

thank you

JackBean
Inland Taipan
Posts: 5694
Joined: Mon Sep 14, 2009 7:12 pm
don't you need more values?
http://www.biolib.cz/en/main/

Cis or trans? That's what matters.

2468
Garter
Posts: 23
Joined: Thu Nov 01, 2012 12:48 pm

### Re:

JackBean wrote:don't you need more values?

oh yeah sorry and methyl red concentration was 12.5um....3ml of buffer used and 1ml of stock methyl red..if that helps?...really cant get my head round this..i've tried searching

JackBean
Inland Taipan
Posts: 5694
Joined: Mon Sep 14, 2009 7:12 pm
LOL, you're really expert.

From Lambert-Beer law (A = e.c.l) calculate concentration of the color form. Concentration of colorless form is c(total) - c(color)
http://www.biolib.cz/en/main/

Cis or trans? That's what matters.

2468
Garter
Posts: 23
Joined: Thu Nov 01, 2012 12:48 pm

### Re:

JackBean wrote:LOL, you're really expert.

From Lambert-Beer law (A = e.c.l) calculate concentration of the color form. Concentration of colorless form is c(total) - c(color)

'Love' the sarcasm.. :/

Elaborate? Dont undertsand it.

JackBean
Inland Taipan
Posts: 5694
Joined: Mon Sep 14, 2009 7:12 pm
This could be of use for you
http://www.usca.edu/chemistry/spectra/methlred.htm (if you can open *.spc files)
http://www.colby.edu/chemistry/PChem/la ... hylRed.pdf (especially picture on p. 3 where you can see that the extinction coefficient is around 8000 M-1 cm-1)
http://www-ec.njit.edu/~grow/Methylred.pdf
http://www.biolib.cz/en/main/

Cis or trans? That's what matters.

2468
Garter
Posts: 23
Joined: Thu Nov 01, 2012 12:48 pm
Thank you!

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