ISOPIESTIC DETERMINATION OF THE OSMOTIC AND ACTIVITY-COEFFICIENTS OF CU(NO3)(2)(AQ) AT THE TEMPERATURE 298.15 K

The four previous isopiestic investigations of Cu(NO3)(2)(aq) at the t emperature 298.15 K exhibit a remarkable lack of agreement. Consequent ly, isopiestic vapor-pressure measurements were performed for Cu(NO3)( 2)(aq) solutions at T = 298.15 K both at the Texas Christian Universit y and the Lawrence Livermore National Laboratory. These measurements e xtend from molality m = (0.0638 to 6.9235) mol.kg(-1). Under the condi tions typically used for isopiestic experiments, it is found that solu tions of Cu(NO3)(2)(aq) at higher molalities lose HNO3 into the vapor phase, resulting in a gradual increase in the extent of hydrolysis of their solutions as the samples are re-equilibrated. This produces a pr ogressive downward drift in the apparent osmotic coefficients. Consequ ently, most previous higher-molality isopiestic results for this syste m are believed to be unreliable. Experiments were performed to quantif y and to minimize the effect of this hydrolysis on the resulting osmot ic coefficients. These experiments indicate that the error in the osmo tic coefficient from this HNO3 loss is insignificant at lower molaliti es of Cu(NO3)(2)(aq); at m = 4 mol.kg(-1) it is less than or equal to 0.2 per cent, but above m approximate to 5 mol.kg(-1) it can become qu ite significant. Recommended values of the osmotic coefficients, water activities, and mean activity coefficients of Cu(NO3)(2)(aq) are pres ented up to m = 3.00 mol.kg(-1), the maximum molality where we conside r them to be completely accurate, and, with slightly lower accuracy, u p to m = 6.9235 mol.kg(-1). Pitzer's equation was found to represent r eliably the experimental osmotic coefficients provided the third viria l coefficient was ionic-strength dependent.