A World Survey of Water Activity of Selected Saturated Salt Solutions used as Standards at 25°C SILVIA L. RESNIK, GUILLERMO FAVETTO, JORGE CHIRIFE and CONSTANTINO FERRO FONTAN ABSTRACT This paper presents and discussesthe results of a world survey of water activity (aw) of selected saturated salt solutions used as stan- dards by different researchers engaged in a, determinations for food-related applications. Salt slurries cover the range of a, about 0.57-0.97 at 25°C and are NaBr, NaCl (NH4)#04, KCl, BaC12. 2H20, KN03 and KzS04. The results indicate that there is a good agreement on the exact value to be assigned to NaBr, NaCl, KC1 and BaC12.2H20, and to a lesser extent, to K2SO4. However, a signifi- cant discrepancy exists between researchers on the a, value assigned to (NH4)2S04 and KN03. INTRODUCTION MOST METHODS frequently used for measuring water activity, such as gravimetric or hygrometric ones, involve the need of reference standard sources of relative water vapor pressures in the range of interest. The reference sources are used for maintaining predetermined relative wtiter vapor pressures in the space around the samples (gravimetric methods) or for proper calibration of electric or fiber-dimensional hygrometers (Gal, 198 1; Troller, 1977; Labuza et al., 1976; Stoloff, 1978; Chirife and Ferro Fontrin, 1980). Saturated salt solutions (salt slurries) have been recom- mended by numerous workers as a convenient, easy, and accurate way to provide solutions of known a, (Young, 1967; Labuza et al., 1976; Robinson and Stokes, 1965,; Greenspan, 1977). They are reproducible reference stan- dards because no measurement of concentration is needed and if the salts are properly chosen no interfering vapors are present (Stoloff, 1978). Several literature data and reviews are available on the a, of standard salt slurries (Stokes and Robinson, 1949; Wexler and Hasegawa, 1954; Young, 1967; Rockland, 1960; International Critical Tables, 1926; Labuza et al., 1976; Greenspan, 1977). Unfortunately, most reports in the literature do not agree in the exact a, of each saturated salt solution, and so it is difficult to calibrate instruments and to evaluate different a, measuring devices. Considerable doubt has arisen about the accuracy of water activity of saturated salt solutions in view of the recent review by Greenspan (1977). His work reflects very well the uncer- tainties of literature values from various workers. It is obvious that for different researchers to reproduce each other’s a, measurements there must be an agree- ment on the iralues to be assignedto reference standards of saturated salt solutions. Unfortunately, literature “stan- dard” a, values do. not always agree and each researcher selects the data on the basis of his own preference. It is the purpose of this paper to present and discuss the results of a world survey of a, values of certain stan- dard saturated salt solutions actually used by different researchers. These results may help to find ways to obtain Authors Resnik, Favetto and Chirife are affiliated with Depto. de Industrias, and Author Ferro Fontan is with the Depto. de Fhica, Facultad de Ciencias Exactas y Naturales, Univ. de Buenos Aires, Ciudad Universitaria, (1428) Buenos Aires, Argentina. a world-wide agreement on the values to be assigned to salt slurries studied. Details of the survey The survey, conducted between Dec. 1982 and March 1983, included a large number of laboratories engaged in a, measurements in food-related applications in North America, South America, Europe, Australia, and Asia. A form was sent to well-kndwn researchers inquiring about the literature a, values for selected saturated salt solutions that they use as standards in their laboratories. Selected salt slurries covered the range of a, about 0.57-0.97 at 25’C and included the following: sodium bromide, sodium chloride, ammonium sulfate, potassium chloride, barium chloride, potassium nitrate, and potassium Table 1 -List of laboratories which answered the form UNITED STATES and CANADA (11 US Army Natick Research & Development Center, Food Chemistry Group, Physical Sciences Div., Science 81 Advanced Technology Lab., Dept. of the Army, Nat]ck, MA 01760 (USA) (2) Dept. of’ Food Science, Univ. of Wisconsin, Madison, WI 53706 (USA) (3) Dept. of Food Science, Univ. of Illinois, Urbana, IL 61081 (USA) (41 Dept. of Nutrition & Food Science, Masachusetts Institute of Technology, Cambridge, MA 02139 (USA) (5) Dept. of Food Science & Human Nutrition, Univ. of Delaware, Newark, DE 19711 (USA) (6) Food Science Research Center, Chapman College, Orange, CA 92666 (USA) (7) Dept. of Food Science, Rutgers Univ., New Brunswick, NJ 08903 (USA) (8) Dept. of Food Science, The Univ. of Georgia College of Agri- culture, Experiment Stations-Georgia Station, Experiment, GA 30212 (USA) (9) The Pillsbury Company, Research & Development Lab., Minneapolis, Ml 55414 (USA) (IO) The Quaker Oats Company, John Stuart Research Lab., Bar- rington, IL 60010 (USA) (I I) The Procter & Gamble Co., Winton Hill Technical Center, Food Product Development, Cincinnati, OH 45224 (USA) (12) Dept. of Food Science, The Univ. of Alberta, Edmonton, Alberta T6G 2P5 (Canada) (13) Agricultural Canada Research Station, Morden, Manitoba ROG IJO (Canada) SOUTH AMERICA (14) Depto. de Industrias, Facultad de Ciencias Exactas y Naturales, Univ. de Buenos Aires, (1428) Buenos Aires, Argentina (15) Depto. de Tecnologia de Procesos Bioldgicos y Bioquimicos, Univ. Sim6n Bolivar, Divisi6n de Ciencias Biol6gicas Caracas, Venezuela (16) lnstituto de Tecnologi’a de Alimentos. Campinas, Sao Paulo, CEP 13100, Brasil 510-JOURNAL OF FOOD SCIENCE-Volume 49 (1984)