Abundances of the ammonium cation with respect to H₂O ice plotted against the τ_intC4/τ_intC3 ratios of ices toward isolated dense clouds (left, blue) and protostars (right, red). To facilitate comparison, the protostellar and cloud values are also plotted in the left and right plots, respectively, in light gray. A higher τ_intC4/τ_intC3 is indicative of a greater redshift in the peak position of the 6.85 µm feature. The plotted values are taken from Boogert et al. (2011) (clouds) and Boogert et al. (2008) (protostars). The Spearman’s rank correlation coefficient ρ and the p-value indicating probability of noncorrelation are provided in the top right corner. The plotted NH₄⁺ abundances have been multiplied by a correction factor of 4.4/3.6 to account for the difference between NH₄⁺ ν₄ mode band strength used by Boogert et al. (2008) and Boogert et al. (2011) (4.4× 10⁻¹⁷ cm molec⁻¹ from Schutte & Khanna 2003) and the band strengths calculated in this work to enable a direct comparison to our NH₄⁺ ice upper limits in Table 5. Upper limits have been excluded from this plot, as well as data from the sources 2MASS J19214480+1121203 (due to the high error of its τ_intC4/τ_intC3 ratio) and IRAS 03301+3111 (due to its τ_intC4/τ_intC3 ratio being undefined because its τ_intC3 = 0).