Sodium Sulfite Mono/Anhydrous : LUSENET : Large format photography : One Thread

I have sodium sulfite mono, but the formula calls for sodium sulfite anhydrous. How much sodium sulfite mono do I need to equal the anhydrous?

-- William Marderness (, March 31, 2002


if i understand the table in Anchell, 1 monohydrate=1.17 anhydrous

-- Wayne (, March 31, 2002.

The molecular weight of anhydrous sodium sulfite is 126 whereas that of monohydrate is 126+the water 18 = 144, so you need 144/126 = 1.14 times the amount of monohydrate to get the equal amount of the chemical itself. The monohydrate must be a rare form of this chemical, as i could not find it in the catalogs of any big producers. In addition to the anhydrous sodium sulfite the other available form is the crystalline, containing seven water molecules. The molecular weight of this form is (126 + 7x18) 252, so double amount (252/126) of crystalline sodium sulfite is needed as compared to the anhydrous form.

The previous post referred to Anchells table no. 5. I have to admit that I do not understand the content of this table, as to my knowledge there can not be universal factor for calculation of the correspondence of anhydrous to monohydrate salts. The factor has to be dependent on the relative proportion of the water in the salt, i.e. on the molecular weight of the anhydrous chemical.


-- Olli Jaakkola (, April 02, 2002.

>there can not be universal factor for calculation of the correspondence of anhydrous to monohydrate salts<

Thank you for confirming my suspicion of that table. I did quite poorly in chemistry, but it just didnt make sense to me for the reasons you state. I think the table is for sodium carbonate, but it pretends to be universal...

-- Wayne (, April 02, 2002.

Wayne, you have right! I did some calculations and found that the table is for sodium carbonate. The molecular weights for anhydrous and monohydrated forms are 106 and 124 respectively, and the crystalline form is Na2CO3*10H2O, its molecular weight is 286. These numbers lead to the factors given in the table.

-- Olli (, April 04, 2002.

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