Trouble in paradise?
I was supposed to make an update on the issue of de-alkalizing papers with sulfamic acid but I was not able to gather my mind and write something. Things were and still are a bit hectic in Turkey so sorry for the delay.
As I have passed halfway the capacity of 1M 3lt (%10) of SA solution (3 months old), I have realized that the time required to remove the chalk from the papers almost increased %50. I was worried if I made a mistake calculating the capacity of the solution. A quick inquiry lead me to this information: “aqueous solutions of sulfamic acid are much slower in action and require heating to remove scale. The sulfamic acid solution is heated to a temperature in the range of 54 to 71°C (130 to 160 °F) to obtain the same fast cleaning time that is achieved by using hydrochloric acid at room temperature.”
Apparently the slow acting nature of SA might be the reason why SA treated papers prints better than the papers treated with equally acidic HCl solution. Probably due to slow acting of SA, the fibers of the paper are less damaged by acid.
Anyway, due to its slow reaction speed at room temperature using halfway exhausted SA solution the time required to de-alkalize paper increases more than expexted imo. As always, the actual time depends on the paper and the buffer. Yet, I have not observed any irregularities or loss of d-max with the papers treated for 30mins in SA. And, as the information I have found suggests heated SA solutions react faster, but washing the papers in hot solutions is not a good idea. (30-35C might be acceptable but maintaining the temperature is also another issue)
At any rate, I wanted to point out that the time required to clear the papers’ buffer might increase as the SA solution gets used, but that is nothing to worry about. The previous molar calculations about the capacity are correct (1M 3lt solution should safely de-alkalize 45 full sheets), but the time required would increase gradually.
It is normal actually, as the capacity of the acid decreases the reaction time would increase gradually, but the increase in time was a bit dramatic than I expected. That is why I felt the need to clarify.
I have de-alkalized 28 full sheets so far with the solution I have prepared approximately 3 months ago and it is still working. If time is an issue you should always dump the old solution and prepare a new considering the low cost and effectiveness of sulfamic acid.