[grammerPolice]

I can't see any buffering regions on this curve from an amino acid titration. The theoretical buffering regions suggest this amino acid is lysine, but the curve just increases linearly through these regions. Can someone give me an explanation?

[mike]

can you show us the graph?

[grammerPolice]

here it is:

[mike]

It doesn't look like a buffering graph. What is the pKa for lysine?

[mike]

Well I looked up the pKas for lysine and they are 2.2, 8.9 and 10.28, these are also the flat regions of your graph so I guess it is buffering after all.

[grammerPolice]

2.15, 9.16, 10.67

I can probably attribute the lack of the first buffering region to the solution approaching the pH of the titrant, but the other two especially the middle one I have no idea how to explain

[mike]

Here are some graphs I did, one unbuffered and one buffered with L-histidine.

[mike]

L-histidine.

Your graph does have flat area where the Lysine could be acting as a buffer.

[grammerPolice]

Really? To me mine looks more like the unbuffered graph than the buffered one. But in any case, why aren't the plateaus on mine as distinct as those on your second graph? Is there some sort of experimental error that can cause this?

[mike]

I tend to agree that your graph does look like the unbuffered one

Sometimes the scale of the graph can be changed and the graph looks different, or you could try taking smaller volume readings around the buffer zone. I am really not sure about your data.

Other possibilities are that lysine is not a good buffer.

Maybe you could try it with a more dilute NaOH solution.

I'm sorry I don't know much more to explain your graph.

Did you put the buffer solution in? Did other people do the same experiment and get the same results?

[grammerPolice]

Well, some other groups we compared with had much nicer curves. Haven't compared it with another lysine curve though. One possibility I've been contemplating is that we didn't mix well enough after adding the titrant. Could that have had such an effect? I'm intrigued by the idea that maybe lysine just isn't a good buffer. Could someone that has titrated lysine before provide some input?

[Borek]

buffer pH calculation

Read the last paragraph.

Difference between two pKas is to small for two endpoints to be separated, hence what you observe is a flat area of the curve (if the slope allows to call it 'flat').

You will see similar effect in the case of titration of citric acid, although the flat area will be visible before the endpoint, not after.

Curve calculated using BATE (2.15, 9.16, 10.67, C_lysine=0.01M, C_NaOH=0.013M) looks pretty similar to your experimental one.

[Borek]

Here goes citric acid, both concentrations 0.01M. pKa values taken from BATE database - 3.128, 4.761, 6.396.

[grammerPolice]

Thanks alot, that really helped . A big thanks to Mike too.