The four benzene boron - quaternary ammonium salt capacity measured potassium sodium content, experimental principle requirements should make analysis of potassium content in the sample control at about 5 mg. This experiment by controlling the potassium content in sample analysis was from 5 mg and control about 5 mg in error analysis, found below 5 mg potassium content in the sample analysis, the results would be larger, higher than 5 mg test results would be small, the relative error was more than 1%.