I have wondered ever since A & H quit selling the old washing soda that we used for cleaning radiators, etc, which was Na2CO3 + 10 H2O, why it is now claimed to be the monohydrate in its resurrected form. The CRC Handbook says that the decahydrate is washing soda. I have not been able to get the A & H Washing Soda, so cannot test it. It is easier for me to get the pHPlus for adjusting pH of swimming pools. It is close enough to anhydrous for government work. In any case, as long as you know what you have, you can use more or less than the recipe calls for. In a developer like Ansco 130 or D-72, you will probably not notice the difference between anhydrous and monohydrated, but will see a difference between monohydrated and the true washing soda, as the ratio of molecular weights is 2.31. You need 2.31 times the weight of what CRC calls washing soda as of monohydrated sodium carbonate. The difference is water, of course.