Does the HI729 measure fluorosilicic acid?

No, the HI729 Low Range Fluoride Coorimeter - Checker® HC does not measure fluorosilicic acid.

The HI729 checker will only measure fluoride and not fluorosilicic acid. If fluorosilicic acid is present in your sample it will not participate in the chemical reaction between the checker reagents and your sample to create a color change. Thus, it will not be measured.