From a critical review of the existing experimental data, the free energy equations are presented. A new estimate is made of the standard free energy of formation of the hypothetical pure liquid P 2 O 5 , which is used in calculating the P 2 O 5 activity coefficients from the experimental data of independent studies of slagmetal reactions. It is found that in low P 2 O 5 slags, log(g P 2 O 5 ) is a linear function of CaO from 0 to 60 %, independent of temperature. Whereas, in slags containing P

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... 2 O 5 Ͼ10 %, log(g P 2 O 5 ) is a linear function of CaO con-tentϾ40 % only and increases with an increasing temperature. A novel concept of a slag-P 2 O 5 emf sensor is presented for experimental evaluation. KEY WORDS: Free energies of calcium phosphates and P 2 O 5 ; P 2 O 5 activity coefficients in slags; Slag-P 2 O 5 sensor; Phosphate capacity of slags; Limeϩcalcium phosphate saturated slags.