Are you talking about caustic soda ? If so its sodium hydroxide & is an alkali (rather than an acid). How toxic it is depends up on its concentration. Even a week solution would be an irritant though. Do you mean toxic if you swallow some or toxic to the environment though
From http://www.soda-crystals.co.uk/: (believe it or not!)
Soda Crystals - a brief history
Soda Crystals have been used around the home, by farmers and in commerce and industry for over 150 years. Soda Crystals are extremely versatile and are relatively inexpensive. One of the reasons Soda Crystals have remained so popular is that, unlike many modern cleaners and detergents, Soda Crystals contain no phosphates, enzymes or bleach.
One Product - Many Uses
Soda Crystals are alkaline. The main uses of Soda Crystals are to dissolve grease, soften water, loosen dirt and reduce acidity. Soda Crystals, along with soap and Soap Flakes, were virtually the only domestic cleaners at the turn of the twentieth century. They were used for cleaning and laundry and can even be used in cooking. Still available in free-flowing crystal variety, Soda Crystals are now also available as a liquid version - ideal as a surface cleaner, a trigger spray version and for commercial use in an anti-bacterial formula.
Soda Crystals are very effective but are also gentle. However, if you are submerging your hands in a soda crystal solution for a long time, it is advisable to wear rubber gloves - especially if you have sensitive skin.