Any dilutions used to prepare turbidity standards, as well as blanks, must use degassed DI water. This is especially important for accurate low-end turbidity analysis, as there is often a positive bias due to bubbles introduced into the DI water solution when it is being produced/dispensed. This effect becomes more noticeable with lower NTU readings, and can cause challenges with performing an accurate calibration. To degas DI water, it must be dispensed into a very clean bottle or container, and left to sit for ~24h. You will see bubbles form on the side of the container as the water degasses. After degassing, it is ready to use for blanks and standards preparation.
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