Fig 1Chemical equilibriumDrag to turn · pinch or scroll to zoomiron and thiocyanate · mix the solutions to begin
Blood red [Fe(SCN)]²⁺: deeper means moreBurettes: 0 at the top, read to 0.05 mLTube a: the reference solutionFe³⁺ + SCN⁻ ⇌ [Fe(SCN)]²⁺ (blood red), K = [[Fe(SCN)]²⁺] / ([Fe³⁺][SCN⁻]). Every tube holds 2.5 mL of the blood red solution in 20 mL. Adding ferric chloride (b to e) or potassium thiocyanate (b′ to e′) makes Q smaller than K, so the equilibrium shifts forward and the red deepens: 30.1 µM in tube a, 52.3, 73.1, 92.5 and 110.8 µM in b to e. Oxalic acid takes the iron away and the red fades. [Co(H₂O)₆]²⁺ (pink) + 4Cl⁻ ⇌ [CoCl₄]²⁻ (blue) + 6H₂O: water turns it pink, chloride and heat turn it blue (the forward reaction is endothermic).