Sensitizing Carbon tissue. Weird pricipitation in Aceton/Dichromate solution

Hi.

For about half a year now ive been using a mixture of Acetone and Amm. Dichromate to sensitize my tissues for carbon printing. This has never posed any problem. However some time ago i mixed up a new batch of saturated amm. dichromate solution. Also i have bought a new bottle of aceton. Oddly since about the same time when i mix the solution, that being the dichro and aceton. A weird pricipation forms in the bottom of the syringe i use to make the mixture. It could best be described as some sort of white flakes. In addition the solution colors less vividly orange then it used to. Almost as if the dichromate enters a reaction with the aceton. Even though as much as i understand of chemistry aceton is already in an oxidised state and shouldnt readily form a reaction with the dichro.

Is this likely to be a problem of impurity? Even though i used distilled water to make the dichro solution. Or is this an issue that more of you encounter?

Any ideas or suggestions at this point would be greatly appreciated.

Note: Altough the syringe is made of plastic. I am positive it doesn't melt in the aceton. Ive thoroughly tested for that.

Sounds like it might be an impurity in the Acetone.

PE

Interesting. Well i guess the first step is then to find another source of acetone. Its kinda odd though. If i look at the amount of flakes appearing out of 6ml of acetone. Makes me wonder what there using to dilute or pollute the aceton with.

Thanks for your response^^

If you've truly made a saturated ammonium dichromate solution, then what I think is happening is that the acetone is displacing some of the dichromate in solution. So what looks white might actually be orange flakes in an orange solution? I've seen this happen with an 8% solution of potassium dichromate (close to the stauration point at room temp.)

Try this experiment: in a small dish/beaker/etc. mix up your solution with the same proportions as you always do. After the precipitate forms. Slowly add water, while stirring. I would expect that at some point you will be able re-dissolve the precipitate.

This is one of the reasons, IMO, to not use a truly saturated solution. As the temperature of the solution changes, the concentration can also change, as some amount of dichromate can precipitate. I chose 8% for potassium dichromate because it was below the saturation point at 15C, the coolest it was ever likely to get in my work-room.

The solubility of am. dichromate is much higher, but it should be relatively easy to find saturation tables for it, to figure out where you want your concentration to be.

--G

Yes, this is possible. Acetone can cause Amm. Dichromate to ppt out of a saturated Dichromate solution.

Sorry, I missed that.

PE

Interesting. Il definitely try the by you suggested experiment. Personally i found the saturated solution easy to work with. Less water and therefor shorter drying times. And since my working place is a small darkroom(just a thin chipboard wall) inside by living space it seldomly will get below or above 20-22 deg celsius.

However diluting by stock solution back by a few percent might in that case do the trick. Ive calculated that at room temperature 26% weight/weight makes a saturated solution. bringing that back to like 15% or so shouldnt be a problem.

But in this case if i understand correctly. The aceton didnt get decreased in purity, but rather increased. Therefore easier displacing the dichro.

Il report back as soon as i can try out the above.

Thanks alot so far guys!

I've been using AD at 6% and have heard from respected folks that a typical range of use is 3-8% of AD.

How short a drying time are you usually getting with such a strong solution?

I'm seeing 2-3 hours with 6% (mixed half and half with acetone) at temps around 20C and RH in the 65 - 75% range (damp west coast Canada)

Well. By estimate i would say about 1.5-2 hours of drying time. It is probably dry enough after 1 hour. But i kinda had bad experiences with tissue sticking to my negative. So i give it a bit longer. On avarage i use a 4-5% dilution. Meaning about 1.8ml of saturated dichro + 6.5ml of acetone.

Just like over there in canada its pretty damp here. Ranging from 75% upto 90% RH. Luckily during the winter that goes down a bit due to usage of central heating.

I would not think that the concentration of the sensitizer would have any affect of drying time. The amount of water used remains the same, thus the drying time for 2% and 8% should be the same.

Or at least it does in the way I work. Might be different for others. For an 8x10 I dilute 5ml of my stock AD solution (2% up to 8%) with 15ml acetone. So no matter what dilution I use, I am putting on 5 ml of water and 15ml of acetone. Only the amount of AD in the water changes.

I use to use AD and Isopropyl alcohol, but the mixture started to turn dark before I could brush it on. So I switched to acetone because of that and other reasons (faster drying). Impurities in the isopropyl I always thought.