Sodium sulphite is alkaline. A small amount will do, probably in the range of a 1% solution (10g per litre), which should give a pH in the neighborhood of 7.5 or so. A 5% solution will give a pH of 8.0 or so. This could have some impact on photochemical reaction depending on how it is used. However, you'll likely be using a small amount benzotriazole so it may not matter. Isopropyl alcohol would be a good liquid to dissolve in as well.