MANTECH recommends the use of turbidity standards made with suspensions of microspheres of styrene-divinylbenzene copolymer for all turbidity applications. Standard Methods dictate that “Secondary standards made with suspensions of microspheres of styrene-divinylbenzene copolymer typically are as stable as concentrated formazin and are much more stable than diluted formazin.”
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.
