Our lab does a lot of colorimetric tests for silica in solution (heteropoly blue method). We dilute our samples into our deionized water. This water starts as tap water, passes from a reverse osmosis system, and then through a deionization column. We are using this DI filter: https://www.uswatersystems.com/3-gpm-2-5-x-10-di-resin-cartridge-af-10-4010.html

Initially, we measure about 0.05 ppm SiO2. This is excellent and does not interfere with our sampling. However, repeatedly, after about 1 month, the water leaving the filter quickly climbs to a concentration of about 1 ppm SiO2. This is independent of flow through the filter (250 L and 10 L of RO water passed through independent filters give the same result).

This seems to happen after about 1 month of use. The filter resin does not contain SiO2, so we cannot figure out what is happening. Is there a better DI filter we can use to remove the SiO2, or do we just need to replace the filter more often? Replacing it less than a month after use seems excessive.