in the lab we detrmine the chloride concentation in a sample by titrating it with silver nitrate (1/35.5) the volume of the sample(100ml). i did standrization for the AgNo3 by a known concentration of cl- (20ppm) and the result is 21.8ppm so why this thing happend ? the reagent of AgNo3 was made year ago and still we use it in the lab.. how can i fix this problem? and what happend to the titrant to give wrong reading? and can i calculate from the percentage of error the sample concentrtion?