I have always used Arm and Hammer brand washing soda here in the US. The form most common in ths country is the monohydrate since it is the most stable form and less bulky. I wrote off to the company and they sent me literature on its purity which is about 98%. The main impurity is sodium chloride. This is to be expected due to the method of manufacture. There is a small amount of iron present. I would assume that other brands are similar.
I use the washing soda for print developers. But for film developer I either buy photograde sodium carbonate or make it from sodium bicarbonate. The amount of iron impurity in the washing soda makes it problematic for ascorbate developers. However, since you make up developer just before use there should be no problem.