Method and apparatus for automatic end-point detection in tripolyphosphate sequestration of hardness

摘要

A method and apparatus for automatic end-point detection in tripolyphosphate sequestration of hardness are disclosed. A desired approximate 1:1 molar ratio of the hardness elements calcium (Ca) and magnesium (Mg) to sodium tripolyphosphate in a wash water solution is achieved by adding to the wash water solution sodium tripolyphosphate at a preselected rate as the electrical conductivity of the wash water solution is monitored. The desired approximate 1:1 molar ratio of hardness element to sodium tripolyphosphate is obtained, irrespective of variations of concentration levels of hardness elements in the wash water solution or in the soils being removed by the solution, either (a) by detecting the maximum rate of change of conductivity of the wash water solution and terminating the addition of sodium tripolyphosphate when the maximum rate of change of conductivity of the wash water solution is detected or (b) when the pH of the wash solution is to be above 11; by detecting the 5:2 molar ratio of calcium hardness to tripolyphosphate by determining when the conductivity of the wash water solution first increases substantially; then adding 1.5 times more sodium tripolyphosphate than had been added when conductivity increased substantially to chelate the calcium hardness; and raising the pH to at least 11 to precipitate the magnesium hardness. Apparatus is disclosed for implementing automatically the sequestration method of the present invention in, for example, the home dishwasher environment.