Wisdom, Drops

In actual practice, however, such titrations are more or less restricted to those involving precipitation of Ag+ with anions, for instance halogens (Cl , Br, I-) and thiocyanate (SCN-). Generally, it is quite difficult and tedious to locate the exact point at which further addition of reagent affords no more precipitation. Therefore, the choice and wisdom of a chemical reaction is preferably sought so as to result in either a coloured solution or a coloured precipitate at the end point. A typical instance may be cited by application of potassium chromate (K2Cr04) solution in the above case whereby any extra drop of silver nitrate, after all the chloride has been precipitated, immediately causes precipitation of red chromate showing that the end point has been duly achieved. 

A teenage boy is seen at the office of a dental surgeon for extraction of an impacted wisdom tooth. He is so nervous that the dentist decides to administer a sedative to calm the boy. After intravenous administration of the sedative (promethazine), the boy relaxes and the extraction is accomplished with no complications. However, when the boy stands up from the dental chair, he turns very pale and faints. Lying on the floor, he rapidly regains consciousness, but has a rapid heart rate of 120 bpm and a blood pressure of only 110/70 mm Hg. When he sits up, his heart rate increases to 140 bpm, his pressure drops to 80/40 mm Hg, and he complains of faintness. He is helped to a couch in the reception area, where he rests for 30 minutes. At the end of this time the boy is able to sit up without symptoms and, after an additional 15 minutes, is able to stand without difficulty. What autonomic effects might promethazine have that would explain the patient s signs and symptoms Why did his heart rate increase when his blood pressure dropped ... 

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