Gum Silver - what does the acid do?

[Jerevan]

I was looking at the (there was a url link here which no longer exists) and I wonder what the glacial acetic acid does in solution 2? Does it only lower the Ph or does it do something else? Could it be substituted?

(forever inching closer to emulsion-making...)

 

[Kirk Keyes]

THis is similar to printing out paper. It looks to me that the acetate is the photosensitive agent for the paper. POP papers used silver chloride and organic salts of silver (often citrate or tartrate). Silver nitrate was used as a "sensitizing agent" in POPs.

So you're adding silver nitrate, but no bromide or other halide as you do for dry or wet plate emulsions. You have an organic silver salt with the acetate.

Silver acetate is more photosensitive than silver nitrate salt alone, but not as fast as a halide.

 

[Jerevan]

Thanks, Kirk. I wasn't aware there was something called silver acetate - always learn something new around here. Now, I guess that it has to be glacial acetic acid and not some other variety, ie it has to be a non-diluted acid to work?

 

[Dwane]

I was looking at the (there was a url link here which no longer exists) and I wonder what the glacial acetic acid does in solution 2? Does it only lower the Ph or does it do something else? Could it be substituted?

(forever inching closer to emulsion-making...)

I've wondered about this too. The following is pure speculation on my part.

I suspect the glacial acetic acid does lower the pH, and it would be simple enough to check - measure the pH before and after adding the acid.

Does the acid do something else? I do know that gum arabic is a very complex substance, and it's possible that it's physical properties may change with pH. Maybe an aqueous solution of silver nitrate is more soluble in gum arabic in the presence of an acid?

Could it be substituted? This was the first thing I wondered about. Why glacial acetic acid? Why not vinegar? Or hydrochloric acid, which would require a less amount of acid. Does the silver nitrate react with the acetic acid in the presence of the gum arabic, forming silver acetate? Would the hydrochloric acid form silver chloride in the presence of the gum? Both salts are photosensitive, but so is the silver nitrate. And silver nitrate is more soluble in water than silver acetate, which is more soluble than silver chloride ...

Many questions. Too few answers. I'm sorry I can't give a definitive answer to these questions. I honestly don't know. Hopefully someone else can help out here.

 

[Dwane]

Wow! While I was typing my response (with my two fingers, using the backspace key more often than any other key), Kirk jumped right in and pretty much took care of the questions posed. Now I can't wait to see how many other people have added their responses while I'm hacking away at my keyboard, trying to get this into a coherent response.

 

[Jerevan]

Thanks, Dwane! I was thinking along the lines of substituting the Glacial Acetic Acid with the white vinegar I have at hand, which is 24%. But I'll see if I can get the GAA. According to what Kirk says, you were thinking correctly that the solution forms silver actetate.

 

[Kirk Keyes]

Gum arabic (AKA gum acacia which is a common ingredient of some soft drinks like Fresca (mmm - I love Fresca)) is a glycoprotien, which will expand and contract just like gelatin does with changes in pH. I have no idea at which pH this effect occurs. But I'm guessing that the behaviour of the gum with respect to pH is a key to what's happening here.

The acetic acid will certainly make the mix acidic. There's not any real buffering there to hold the pH at any specific range, so you add an acid like the acetic acid, and the solution will be acidic.

Silver nitrate will react with the acetic acid to form silver acetate by itself. The gum is not needed for that. It's a simple ionic reaction.

If you use hydrochloric to adjust the pH, you will have made silver chloride, and dropped the silver out of the solution. Then you would have an "emulsion" and you would be onto making a different process (more like dry plate than POP).

To substitute the acetic acid, you could try citric or tataric acid as I mentioned in the comment about POPs. And to substitute glacial acetic for less concentrated, sure, why not? Just get the amount of acetate into the solution so it matches what's in the glacial acetic.

 

[wildbillbugman]

Hi,
I recently read somewhere that salicylic acid(sp?),asprin, can be used instead of thymol. Any opinions?
Bill

 

[Kirk Keyes]

Bill - do you mean as a preservative for emulsions, or as a substitute for acetate in the gum method being discussed above?