Equilibrist: A Browser-Based Platform for Fitting Equilibrium and Rate Constants from Optical and NMR Data

Equilibrist is a free, open-source platform for the quantitative analysis of chemical equilibria and kinetics. It runs in a local web browser, requires no installation, and uses a plain-text scripting language to specify reversible and irreversible reactions, polyprotic acid-base networks, and thermodynamic-cycle constraints. Five fitting modes cover species concentration, NMR shifts (fast exchange), NMR integrations (slow exchange), a mixed mode, and UV-vis/emission spectrophotometry, optimized by the L-BFGS-B and Nelder-Mead methods. A comprehensive statistical layer adds information criteria (AIC, AICc, BIC) with Akaike weights and a Fisher F-test for model selection, residual diagnostics (Durbin-Watson and Shapiro-Wilk statistics, Q-Q plot), and three uncertainty quantification families (bootstrap, jackknife, and Monte Carlo input propagation). Parameter identifiability is mapped by 1D and 2D RMSE profiles, sloppy-spectrum eigen decomposition, correlation heatmaps, t-tests, and a novel local sensitivity coefficient ξ. The combined diagnostics let the user move past R2 coefficients and Hessian standard errors to assess parameter estimates and the chemical model itself.