Measurement and modelling of water activity of aqueous poly (ethylene glycol) or poly (propylene glycol) + sodium potassium tartrate and corresponding binary salt solutions

•Water activity of {(Na–K Tar) + PEG or PPG + H2O} system is measured.•Experimental data were correlated with some local composition and Pitzer models.•Molal activity coefficients were calculated.•The results were discussed on basis of water, polymer and salt interactions.

Water activity values of ternary aqueous sodium potassium tartrate (Na–K Tar) + poly (ethylene glycol) or poly (propylene glycol) solutions were measured using the isopiestic method at T = 298.15 K. Also the water activity for corresponding binary aqueous sodium potassium tartrate (Na–K Tar) solutions was measured at T = (298.15, 308.15, 318.15) K. The obtained water activity values for binary aqueous sodium potassium tartrate (Na–K Tar) system were successfully fitted with the local composition and Pitzer models. The mean activity coefficients for the studied binary salt solutions have been calculated. The thermodynamic behaviour of aqueous {(Na–K Tar) + poly (ethylene glycol) or poly (propylene glycol)} solutions was discussed on basis of water, polymer and salt interactions.