Recent Softening of the Calcium Hardness Acceptance Limits

Complete Article from Edition 41 APG eNewsletter

Article written by Charles Torok, Quality Control Chemist, Analytical Products Group

Calcium hardness is a drinking water parameter that is often calculated from the determined calcium concentration. In this instance, the 2002 NELAC FOT regressions could easily create a situation in which laboratories could meet the acceptance criteria for calcium and not meet the criteria for calcium hardness. This can be seen in Table 1 below for two assigned values of Calcium, 40.0 mg/L and 90.0 mg/L which translate to Calcium Hardness values of 99.9 mg/L and 225 mg/L respectively. The next two columns are the acceptable study limits as calculated from the 2002 NELAC FOT's. The final two columns take the study limits of Calcium and calculate Calcium Hardness from the extreme Calcium values. As can be seen, the possible Calcium Hardness values are outside of the Acceptable Study limits. At the higher level of Calcium, 90.0 mg/L, this is especially the case for the Upper values where 245 mg/L is very much in excess of the acceptable limit of 239 mg/L.

The 2005 NELAC FOT's have gone a long way in rectifying this situation as can be seen in Table 2. The calculated Ca Hardness values are very close to the corresponding study limits.