Stability of standard electrolytic conductivity solutions in glass containers

The stability of solutions having an electrolytic conductivity, kappa, of 5 muS/cm to 100 000 muS/cm packaged in glass screw-cap bottles, glass serum bottles, and glass ampoules was monitored for 1 year to 2 years. The conductivity was determined by measuring the ac resistance of the solution. Mass loss was also monitored for solutions packaged in bottles. The solutions were prepared using KCl in water (kappa greater than or equal to 100 muS/cm) or KCl in 30 % (by mass) n-propanol 70 % (by mass) water (kappa less than or equal to 15 muS/cm). The conductivity changes were compared by packaging type and by nominal kappa. The main causes of the kappa changes are evaporation (screw-cap bottles) and leaching (screw-cap bottles, serum bottles, and ampoules). Evaporation is determined from mass loss data; leaching occurs from the glass container with no change in mass. The choice of optimal packaging, which depends on the conductivity level, is the packaging in which kappa changes the least with time. Ampoules are the most suitable packaging for standards having nominal kappa values of 500 muS/cm to 100 000 muS/cm. Screw-cap bottles are most suitable for standards having a nominal kappa of 5 muS/cm to 100 muS/cm.