LIQUID PHASE EXTRACTION: PH
Another property of certain chemical compounds is their acidity or alkalinity relative to water. An acidic substance is the one that releases hydrogen ions H+ (they become hydrated in the presence of water, so they are in the form of H2O. H+ or H3O+ called hydronium ions) when dissolved in water. The amount of H3O+ in an aqueous solution is measured by its pH. This is the negative logarithm of the H3O+ concentration in moles per liter. Acids have pH values between 0 and 7. An alkaline or basic substance is the one that releases hydroxide (OH−) ions when dissolved in water. Its pH is between 7 and 14. A neutral substance is the one that releases neither H3O+ nor OH− ions when dissolved. Its pH is 7. Drugs, for example, can be classified as acidic, basic, or neutral. Cocaine is a basic drug. When it is dissolved in water or an aqueous solvent, it attracts H+ from the solvent leaving OH− behind. On the other hand, barbiturates are acidic drugs. They attract hydroxide ions leaving behind excess hydrogen ions. Caffeine is an example of a neutral drug. Sugars and carbohydrates (common cutting agents in street drugs) are also neutral. Figure 6.3 is an example of the reaction of a basic compound, in this case, cocaine in water making it an acidic salt.
FIGURE 1: O Cocaine hydrochloride. If cocaine is dissolved in an acidic solution, an extra proton (H+) attaches itself to the amine group (NH2R) on the cocaine. This makes the cocaine much more polar than it is in its free form.
FIGURE 2: A liquid extraction process. Making use of a separatory funnel, this process takes advantage of the tendency of polar solutes to dissolve in polar solvents. The same holds true for nonpolar solutes and solvents.
Note that in Figure 2 above, the cocaine has been converted to a salt form. The cocaine molecule has obtained a positive charge and the chloride ion from the hydro chloric acid (HCl) has a negative charge. This form of cocaine is much more polar than free cocaine. This property can be exploited in separating cocaine from a mixture. Liquid phase extractions are commonly used to separate mixtures of solids. They are ideal for separating substances where one is much more soluble than the rest or if they are of different polarity and pH. If, however, the mixture contains two sub stances of similar polarity and both are acidic or basic, such as cocaine and heroin, then a liquid phase extraction would likely fail to achieve a good separation. A different type of separation process would be required for such mixtures.
