Hach does not publish a specific pH restriction for the Low Range (LR) Hardness APA6000 sample. However, the sample in the APA must be buffered to a specific pH in order for the indicator to react properly. The buffer reagent takes care of buffering the sample, but samples with extremely high alkalinity or acidity might exhaust the buffer's capacity; in this case the analyser would likely read incorrectly.
The buffer reagent in the analyser adjusts the sample to 11 pH. This pH is necessary to turn the calmagite indicator from red to blue. A portion of the calmagite will convert back to red depending on hardness concentration in the sample.
The best way to determine if the sample is too acid or alkaline for the analyser is to capture some of the spent sample expelled from the analyser drain and measure its pH. If it is not 11 pH then the sample is exhausting the buffering capacity of the reagent.