pH and pKa - Definition, Values, Calculations | [email protected]
This equation is: pH = pKa + log[A-]/[AH], where [AH] is the concentration of the acid and [A-] is the concentration of its conjugate base after. in equilibrium – there is a mixture of both sides of the equation present. Acetic Acid Ka = ×10^-5; – log Ka = , therefore pKa = pH and pKa Relationship: The Henderson-Hasselbalch Equation. Understand pH = pKa + log ([conjugate base]/[weak acid]) pH = pka+log.
The Henderson-Hasselbalch equation gives you a really quick and easy way of doing that. So what we're gonna do, is we're gonna rearrange this equation to solve for this ratio that we might be interested in.
Acids and Bases – pKa, Equilibrium Constant and logs
And I don't know about you, but I actually find, well, laughs I find logs not super-intuitive sometimes. So I'm actually going to get rid of the log by raising both sides to the 10th power.
So what does this tell us? It may not look like it tells us a whole lot more, but actually, it tells us a lot. It tells us about the relative relationship and size between A minus and HA concentration. So if we look at this, we can derive a couple relationships. So let's go ahead and look at all the possible scenarios for these three things. So anything to the zeroth power is equal to one.
pH and pKa relationship for buffers (video) | Khan Academy
Which tells us that this ratio is equal to one. And if A minus concentration over HA concentration is equal to one, that means that they have the same concentration. I forgot a minus sign there. This is a really helpful thing to remember. And this comes up a lot not just when you're talking about buffers by themselves, but also when you're doing titrations. And the point in your titration where the HA is equal to A minus is called the half-equivalence point.
And if you haven't learnt about buffers, that's okay. Oh, sorry, if you haven't learn about titrations yet, that's fully fine. The P that provides the best fit is taken to be the measured P value, which is reported as the logarithm, i. As well as log P, the log D value as a function of pH is determined from the data. The pKa of a molecule predicts the degree of ionisation the molecule will have at a particular pH. Most drugs are weak acids or weak bases and exist in solution as an equilibrium between unionised and ionised forms.
The ionisation potential of a compound affects the distribution of the chemical in solution and affects the availability of the chemical to enter into physical, chemical and biological reactions. According to the pH partition hypothesis, only unionised nonpolar drugs penetrate the cell membrane, and at equilibrium, the concentrations of the unionised species are equal on both sides.
The pKa of a compound influences properties such as logD and solubility as well as the absorption, distribution, metabolism, elimination and potency of a compound.pKa y pH - Ecuación de Henderson-Hasselbalch
How does pKa affect other pharmacokinetic parameters? Solubility — Acidic compounds tend to be more soluble at high pH values, and basic compounds tend to be more soluble at low pH values. Permeability — Acidic compounds tend to be less permeable at high pH and basic compounds tend to be less permeable at low pH.
Metabolism — Electrostatic interactions are determined by the pKa of a compound. These interactions can affect binding of the compound to the active sites of enzymes. Protein binding — Binding of drugs to plasma proteins tends to be by hydrophobic and electrostatic interactions.
Excretion — Urinary pH is an important factor in the excretion of a drug.
For example, acidic drugs are ionised at alkaline urinary pH and basic drugs are ionised at acidic urinary pH. Only unionised compounds in the tubular fluid will be reabsorbed by passive diffusion4.
How does lipophilicity influence pharmacokinetic properties of a drug? LogP partition coefficient is a measure of the lipophilicity of a compound. Lipophilicity is a key determinant of the pharmacokinetic behaviour of drugs.
It can influence distribution into tissues, absorption and the binding characteristics of a drug, as well as being an important factor in determining the solubility of a compound. How much compound will I need to measure pKa and log P? Although smaller weights are generally required for pKa and log P assays on the SiriusT3, we normally ask customers to send at least 5 mg of solid compound per parameter measured.
If only pKa is required, and if it can be measured by a UV-metric technique, then aliquots of sample in stock solution e. What are the limits to the pKa and log P you can measure?
The standard range is 2.
How to Determine pH From pKa | Sciencing
Lower or higher pKas can be measured if the sample is sufficiently soluble. The higher limit may be reduced for bases with pKas below 4 or acids with pKa above Samples must be in solution during pKa measurement. Turbidity is monitored throughout pH-metric pKa titrations to give warning of precipitation.
If the compound is insoluble then it may be necessary to perform the measurements either at a lower concentration or in the presence of a co-solvent.