04-09-2018, 02:55 PM

Hi guys, we covered this briefly in class but I've been looking at book which has various questions using the equations below and I don't understand when and why to use which equation.

The book states for an acidic drug

Ph-pKa=log(S-S0/S0)

Basic drug

Ph-pKa=log(S0/S-S0)

In this example below, they determine the compound is a base, I understand that, but it states to use the equation above but they rearrange it, I'm not sure why and how they determine which of the two equations to use?

In the example below, they calculate the isoelectric point and they state to use the basic equation but if you study the picture below, they plug the necessary figures into the equation and then rearrange it to the acidic equation and I don't know why.

It's states on the previous page to use the basic equation for ph's below the isoelectric point and the acidic equation for ph's above the isoelectric point.

All in all, I just don't know when to use which of the two forms of the equation. If someone could spend some time helping explain it would help me a great deal.

The book states for an acidic drug

Ph-pKa=log(S-S0/S0)

Basic drug

Ph-pKa=log(S0/S-S0)

In this example below, they determine the compound is a base, I understand that, but it states to use the equation above but they rearrange it, I'm not sure why and how they determine which of the two equations to use?

In the example below, they calculate the isoelectric point and they state to use the basic equation but if you study the picture below, they plug the necessary figures into the equation and then rearrange it to the acidic equation and I don't know why.

It's states on the previous page to use the basic equation for ph's below the isoelectric point and the acidic equation for ph's above the isoelectric point.

All in all, I just don't know when to use which of the two forms of the equation. If someone could spend some time helping explain it would help me a great deal.