Metal sols are often stabilized in solution by the presence of a charged double layer surrounding each colloidal nanoparticle that produces a coulomb barrier to aggregation. The nanoparticles can be induced to aggregate by replacing the charged surface species by uncharged adsorbate. We present a simple analysis that produces an expression for the initial rate constant describing the aggregation process, which is shown to depend nontrivially on the adsorbate concentration. The expression is tested experimentally by following and analyzing the time rate of decrease of the surface plasmon resonance absorption of isolated Ag nanoparticles in aqueous solution. The experimental result accords well with theory, producing a rough estimate of the (adsorbate-concentration dependent) barrier to aggregation that in the absence of added adsorbate is approximately equal to 0.08 eV.