We have developed a method to observe the destruction rate of H-3(+) in the positive column, which is limited by ambipolar diffusion. This method is based on the competition between the ambipolar diffusion rate and destruction rate by added impurities. The decrease of a velocity modulated H-3(+) absorption signal was studied when small amounts of CH4, N-2, and CO were added to a pure hydrogen AC positive column discharge. The destruction rate constant of H-3(+) due to ambipolar diffusion was then obtained using a steady-state model and previously reported ion-neutral reaction rate constants.