pKa is the pH at which 50% of the compound is in an ionised form and 50% is un-ionised.
As membranes are positively and negatively charged, transmembrane movement of un-ionised compounds is faster than ionised compounds.

Correction of acidosis increases ionised salicylate reducing its movement into cells and reducing clinical toxicity. Alkaline urine increases the ionised/ un-ionised ratio in the urine reducing tubular reabsorption and increases renal excretion.