Making D-72, monohydrate and anhydrous - chemistry help, please
At this time, it's more of a curiosity question but I still would like to know.
I am (for the first time) going to make my own Dektol (D-72) from raw ingredients. The recipe comes from Ralph's book. I got through converting Sodium Carbonate monohydrate 80g (specified in recipe) to anhydrous (what I have) 66.64g and I started wondering.
For all the ingredients, only Sodium Sulfite and Sodium Carbonate has hydrate specification and nothing else do. Why? Surely, absorbing water will change weight and thus composition. Is there a reason why it matters to these two ingredients and for nothing else?
Also, for Sodium Carbonate, I have anhydrous. I read anhydrous contain NO water and monohydrate contain one molecule of water per molecule of the compound. Question then, how do I know if mine is truly monohydrate? It's in tightly sealed container and it's a granule, not a cake or rock. Further, how do I know (as I know it likes to suck moisture) it didn't become octo-hydrate or dozen-hydrate, or millennium-hydrate? (made up words, but you know what I mean...)
Develop, stop, fix.... wait.... where's my film?
Carbonate will absorb water in storage, but usually the anhydrous form is specified. Since it is possible for some or all of the molecules to bind with water it makes no sense to specify the hydrate. That said, the fact that there is water binding to the molecular forms dissapears in solution. The only reliable way to determine the amount of carbonate needed is through measurement of pH in the solution. Once you determine the amount needed of your stock, then you can reliably mix by weight, but for your stock only. Once mixed you should properly adjust pH as needed using either base or acid to make pH corrections.
If you are scratch mixing solutions then you should be measuring pH.
By denying the facts, any paradox can be sustained--Galileo
if you do not know the crystal water content, you can dry the sodium carbonate in the baking oven at 150 - 200░ C for about 1 - 2 hrs. Then it will be anhydrous. Sodium carbonate will not decompose. In my formulation for D-72 there is 3.1 g Metol, 45 g sodium sulfite anhydr., 12 g hydroquinone, 67.5 g sodium carbonate anhydr. and 1.9 g potassium bromide for a total volume of 1 litre.
Inorganic chemicals can contain water of crystalization.. When there may be some confusion as to which hydrate is to be used it is specified in the formula. So developers will specify sodium carbonate anhydrous or monohydrate. The heptahydrate of sodium sulfite is seldom used and so there is no need to specify anything other than the anhydrous form is understood.
A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.
~Antoine de Saint-Exupery
kodak later simplified the commercial formula by rounding off the weights in amateur publication, this was because few amateurs could weigh chemistry to 0.1g accuracy.
Originally Posted by jochen
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The best thing is to measure the water content of the carbonate you have in hands. F.e. weigh 10 gramms, put it in the oven and heat to 120 ░C or slightly more for about 30 minutes. Weigh again and you you have the difference that was water, now you have plain anhydrous. Then you can adjust your recipe. If stored in tight containers it won't change significantly. I have a tight box with anhydrous carbonate that didn't change at all in about 1 year. If it's very old you may do the test again. Other substances may behave different.
Imo it's important to determine exactly which kind of soda you have. My anydyrous carbonate has 2 % water, that's neglectible. For Arm&Hammer washing soda (waterfree) I have reports containing 2 - 9 % water ex factory. In France you get a soude cristeaux that has 50 % water, that doesn't match neither mono- nor decahydrate, it's in between. So you better check.
Measuring the pH may be much worse, because a pH-meter that is pretty exact as your scale needs frequent calibration and will coast much much more than a neat scale with 0.01 g resolution. And checking the scale is very easy with a cheap syringe and water. 1 ml = 1 gramm.
Again, I got a lot of inquiries why f.e. my Caffenol-C-L recipe didn't work at all. If the pH goes below 9 it doesn't work. Almost always(!) the wrong soda was the cause.
Maybe you want to read this: http://caffenol.blogspot.de/2011/02/...-shooting.html
Best - Reinhold
Last edited by grommi; 01-20-2013 at 05:26 PM. Click to view previous post history.
Not every compound can bind water when it crystallizes, and Metol, Hydroquinone and Potassium Bromide generally don't, therefore it is not necessary to specify "anhydrous" for them.
Originally Posted by tkamiya
Note that crystal water is only part of the story. KSCN has no crystal water yet it is so hygroscopic that it will literally go into solution in the water it grabs out of ambient air. To my best knowledge, neither Metol, Hydroquinone, nor Potassium Bromide have this problem.
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