NIST prepares and certifies electrolytic conductivity standards inthe range of 5 [mu]S/cm to 100 000 [mu]S/cm as SRMs (Standard ReferenceMaterials) 3190 to 3199. The certificates for these SRMs typicallyexpire in 1 year to 2 years because of the difficulties in maintainingtheir long-term stability. Stability is one of the factors in thecertified uncertainties, which vary from 0.07 % to 4 % in the mostrecent certifications of the highest and lowest conductivity standardsin this group, respectively. Neglecting the contribution of instability,the certified uncertainties of these SRMs would be in the range of 0.07% to 2 % (Table 1). The goal of this study is to achieve an uncertaintyclose to the target values for each SRM listed in Table 1.
According to Newton's law, a constant force exerted on a particle will it, causing it to move faster and faster unless it is restrained by an opposing force. In the case of electrolytic conductance, the opposing force is frictional drag as the ion makes its way through the medium. The magnitude of this force depends on the radius of the ion and its primary hydration shell, and on the viscosity of the solution.
A practical consideration in the accuracy of electrolyticconductivity standards, as with all standards, is their stability, orchange in certified value versus time. Although there is a large body ofdata with regard to standard electrolytic conductivity solutions, e.g.,Refs. (1-10), data regarding the long-term stability of the standardsolutions are lacking. Obviously, any change from the certified valuewill compromise the accuracy of the standard at the time of use and mustbe considered in establishing both the uncertainty in [kappa] and theexpiration date of the reference material. This paper reports the changeof [kappa] in solutions packaged in a variety of container typesobserved for several years.
In electrolytic conduction, ions having different charge signs move in opposite directions. Conductivity measurements give only the sum of the positive and negative ionic conductivities according to Kohlrausch's law, but they do not reveal how much of the charge is carried by each kind of ion. Unless their mobilities are the same, cations and anions do not contribute equally to the total electric current flowing through the cell.