Effect of competitive complex formation on patterning and front propagation in periodic precipitation

The Co(OH)2 Liesegang pattern (from Co2+ and NH4OH) propagates in space by periodic band formation at the head due to precipitation and band disappearance at the tail due to dissolution in excess NH4OH. Introduction of Ni2+, which competes with Co2+ for complex formation with ammonia, into the system led to several interesting observations: slower propagation, fewer bands, and increased spacing between them. Above a cutoff concentration of Ni2+ (0.13 M for [Co2+]0 = 0.100 M), only a uniform precipitation zone was observed. The quantity delta X = |XNi-Xfree|, namely, the difference in the position of last band in the system with nickel relative to that without nickel attained at a fixed time (93 h), was found to oscillate as the initial concentration of NH4OH ([NH4OH]0) was varied. It was shown that variations in the concentration of intermediate NH4+ act as precursors for these oscillations: [NH4+] was measured with an ammonia-specific electrode, and the difference delta [NH4+] = |[NH4+]Ni-[NH4+]free| at 93 h exhibited oscillations with [NH4OH]0 similar to those obtained in delta X. Theoretical calculations based on the model of Müller and Polezhaev agreed with the experimental results.