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.
"The secret to life is to keep your mind full and your bowels empty. Unfortunately, the converse is true for most people."
Benzotriazole is used in boilers as a corrosion inhibitor. Since it lasts well in those conditions it should last quite well in a bottle of solution in the home. It dissolves better in mild alkaline solutons. Sodium sulphite is alkaline enough to improve its solubility. This what the old Orthazite solution was.