I wanted to add something points that I have shared in alt-photo-process mail list here as well.
I have listed my observations and thoughts on some of the acids I have tested for paper acidification (and one acid I haven't tested). I did not include these in the initial post, because I do not want to start a debate on everyone's favorite acids.
My observations are based on the new cyanotype, argyrotype and VDB (I used it as a control to observe if there has been any improvement with the paper in general).
Sulfamic Acid vs Hydrochloric Acid (HCl)
-Both are equally strong acids, both are ph 0 at 1M solutions
-Reactions with both acids produce highly water soluble byproducts
-HCl evaporates easily, so it is hazardous to use without proper ventilation, it has irritating odor.
-Sulfamic acid has no odor, I have not come across any warning on SA fumes.
HCl treated papers produced weaker dmax with the new cyanotype and argyrotype. I must say I had deeper blues with the same papers using traditional cyanotype formula. Papers were definetely clear of buffer but did not perform well with new cyanotype, argyrotype and even with VDB.
Anyway, that was the turning point that I decided to look for another acid, otherwise there was no point in making prints with such poor dmax.
Sulfamic Acid vs Citric Acid
-Citric acid is weaker compared to sulfamic acid, I have not tested its capacity but theoratically it will exhaust faster than sulfamic acid.
-1M CA solution has ph of 5 (source: internet), SA has 0.
-Reaction with citric acid produces, water insoluble calcium citrate inside the paper. In dry form it is a white powder substance. I have not noticed its presence even with the thinnest papers I have tested. (200gsm FATW and 180gsm Schoellerschammer)
-To prepare 1M 1lt citric acid solution 192 grams of acid is needed, 1M 1lt of Sulfamic acid requires only 97 grams. Roughly it is double the amount for a considerably weaker acid. Their costs per kilograms are similar.
I started experimenting with citric acid at the same time, I started testing sulfamic acid. Citric acid results were definetely far better than HCl, they were as good as my initial results with SA (for new cyanotype and argyrotype), but I did not continue testing it. Because double the amount to prepare 1M solution which is far weaker in acidity and who knows how few papers it would neutralize seemed too costly. If I was going to spend such money on acids, I would rather spend it on awagami masa paper and order some from USA. That is why I stopped testing citric acid, and continued with sulfamic acid.
So I must clarify, citric acid works quite good too, but it is too costly to justify.
Sulfamic acid vs Acetic Acid/Vinegar (Rather, why I haven't tested it)
-Vinegar is unreliable due to additives IMO, and the acidity changes from brand to brand
-I did not want to try concentrated acetic acid due to its strong irritating odor.
-Acetic acid is also a weak acid (around ph4 as I recal)
-Byproduct of reaction is highly water soluble calcium acetate.
That was it actually, why I haven't even tested acetic acid/vinegar. Strong odor, weak acidity and probable cost due to its weakness.
I know some people use vinegar or acetic acid, and if it works for them that is great , but cost wise, I find SA to be a better choice.
Sulfamic Acid vs Oxalic Acid
-Oxalic acid is a strong acid (I haven't measured it, wiki says it is stronger than acetic acid), but SA is stronger, has a ph of 0
-1M 1lt OA solution requires 126grams of acid. 1M 1lt SA requires 97grams
-The reaction produces water insoluble calcium oxalate crystals inside the paper. Calcium oxalate is soluble in EDTA.
Although, oxalic acid is probably the most popular acid recommended for neutralizing buffer, these crystals are a considerable problem for processes that does not involve a EDTA wash step.
These crystals were not noticable with thick papers (CansonMontval 300gsm or Gvarro 420gsm), but thin hot pressed papers like FabrianoArtistico TW HP 200gsm and Schoellerschammer 180gsm practically turned to sand-papers. Especially after coating new cyanotype, I think the crystals become more pronounced (may be from the oxalate in new cyanotype chemistry, just a wild guess). So, OA treated thin hot pressed papers became useless IMO, and not just for new cyanotype or argyrotype. As long as there are crystals on the surface it is useless for any process.
The prints had satisfactory dmax, although the surface of the paper was filled with tiny white crystals. But the Dmax with new cyanotype on 200 gsm FATW HP and on CansonMontval were not good as the dmax I got with sulfamic acid. I must mention FATW HP is a quite a poor performer among all the papers I have tested even with VDB (before and after SA treatment). But the actual problem was the presence of shiny white crystals poking out from the paper.
Considering all these,
I had best dmax with sulfamic acid treatment for new cyanotype, argyortype. There is a slight increase in dmax with VDB too and shift in color towards cooler maronish browns.
There is no insoluble byproduct in paper, no nasty odor. It has strong acidity and it is cheap. It can be stored indefinetely in powder form and it has a great capacity (1M 3lt would neutralize 45 full sheets).
These are the reasons why I considered sulfamic acid to be a better choice of acid for paper acidification.
P.S. Feel free to correct me, I have written a lot of chemistry stuff and I am quite a newbie at such things.