Determining shelf life of chemical solutions

[Nodda Duma]

I'd like to determine expiration of chemicals for which the shelf life is unknown or unreported. I've been sort of winging it and "hoping" that the chemicals I mix for emulsion batches are still good, but as a scientific professional (and from a "paying for materials and not wasting money" perspective) "hope" is not a satisfactory process control.

Here is a procedure that I recall from about 15-25 years ago and I'm wondering if the chemists here can weigh in on the merits of this procedure.

1. Prepare a fresh solution
2. Measure the absorbance (on a spectrophotometer)
3. Store it as I will in practice.
4. Measure the absorbance weekly (may be daily)
5. Shelf life is established when absorbance changes 10 to 15% compared to fresh solution

This makes a lot of sense so I'm wondering if the chemists in the room would agree. Once I've measured shelf life then I'll know how long I can keep chemicals around and be much more satisfied that I can crank out consistent emulsion batches.

Note the process requires a spectrophotometer so this is more of an "advanced" procedure. I'd like to get a spectrophotometer anyways because my basement is also turning into a nice optical testing lab so if I go down this path I'll share results (for stuff like Steigmann's, dyes, etc, as I test them).

Cheers,
Jason

 

[Photo Engineer]

Few chemicals that you may use making an emulsion go bad. OTOMH, the worst are KI and Hypo. Second to that would be sensitizing dyes and those are measured by spectrophotometry. Other than that, you are generally good.

PE

 

[ThePhotoChemist]

...KI solutions go bad? Uh oh. How does one know when this happens? I've had a pretty dilute one that I've been using regularly for about two years...

 

[Photo Engineer]

Yep, KI goes bad. NaI goes bad so fast that they usually don't use it in chemical reactions. KI is preferred as it has better stability, but that is relative.

Solid KI is bad when it begins to yellow or smell of Iodine. The solution is good for about 1 month. You begin to form Iodates.

PE

 

[bernard_L]

Dektol, Eukobrom, Dokumol become dark as they turn bad. D-76 goes bad while remaining clear.

 

[Luckless]

Are there any chemicals or solutions that are worth running cross testings with before use? I've seen the industrial lab a friend works at, and they have long chains of tests to confirm samples of their input chemicals conform to spec just prior to use. [But they didn't seem to have a lot in common with photo chemistry.]

Testing Chemical A:
Mix A + B: If reaction-x happens it is good
Mix A + C: If no reaction happens, it is good, if it reacts then you know it has oxidized/decayed/etc

Kind of thing.

 

[Photo Engineer]

Dektol, Eukobrom, Dokumol become dark as they turn bad. D-76 goes bad while remaining clear.

Sorry, this is about Emulsion Making. I'd be happy to comment in the correct forum.

PE

 

[Photo Engineer]

Are there any chemicals or solutions that are worth running cross testings with before use? I've seen the industrial lab a friend works at, and they have long chains of tests to confirm samples of their input chemicals conform to spec just prior to use. [But they didn't seem to have a lot in common with photo chemistry.]

Testing Chemical A:
Mix A + B: If reaction-x happens it is good
Mix A + C: If no reaction happens, it is good, if it reacts then you know it has oxidized/decayed/etc

Kind of thing.

Probably only sensitizing dyes are expensive enough to need something like this.

PE

 

[NedL]

Yep, KI goes bad. NaI goes bad so fast that they usually don't use it in chemical reactions. KI is preferred as it has better stability, but that is relative.

Solid KI is bad when it begins to yellow or smell of Iodine. The solution is good for about 1 month. You begin to form Iodates.

PE

This is very interesting. Not exactly emulsion making, but in making calotypes I use about a 6% KI solution ( it also contains about 2% KBr, and is made up with DH2O ). The calotype is made by first soaking paper in this solution, letting it dry and then later "sensitizing" in silver nitrate solution. I've often wondered how long the KI solution keeps and I've been very inconsistent about how/when it is made anew. A lot of solution is used up in the process of "iodizing" the paper, and often I've simply filtered what remains and then topped off with fresh KI solution before the next "iodizing" session. Other times I've made new fresh KI solution. One time I kept a bottle going for about 1-1/2 or 2 years, but I got worried when it took on a purple iodine color and started over. It sounds like I may have been "getting by" just because usually 1/2 or more of the solution needs to be fresh in order to top off the bottle.

I have questions!

• If iodates form, would they still combine with silver nitrate to form AgI?

More important question:

• If paper has been soaked in KI solution and then dried, does it make sense that the KI might still "decompose" to iodates or something else over time?

( I know that paper is never truly "dry" and a lot of slow reactions can take place in it .... )

That would explain a lot of what I've seen. Calotypists often iodize large batches of paper and then use it over a long period of time, but I've seen problems appear if the paper is stored too long. All the papers I've ever iodized have changed color over time during storage. Usually they turn a lavender or purple or purplish-brown color, which doesn't necessarily mean they are unusable. But sometimes they can't be used at all after about 6 months ( very dependent on type of paper, acidification, etc ). I always assumed the KI was reacting with something in the paper, but it could be that it degrades all by itself.

 

[Photo Engineer]

Ned, I've never tested Iodates with Ag+ so I really cannot answer this. I would only be guessing.

Dry is much better than wet, but as you note, there are limits.

PE

 

[NedL]

Thanks. I'm always interested in causes of problems, maybe the next time I make up some KI solution, I'll make an extra bottle for testing a few years from now!
Cheers!

 

[Nodda Duma]

Ned that’s a tried and true method to test stuff like this. Control (fresh KI) and variable (aged KI)


If you make up a big batch, then you could draw from it and see how things change over time and not just one data point a month or whatever out.

 

[NedL]

True, although I'd need to go through the lengthy process of making and developing a "fresh KI" and "aged KI" calotype for each test. And too often, if you try to devise a shorter simpler test, some critical unexpected variable comes into play. But I've been thinking about modifying the KI solution to see if I can add povidone iodine as an indicator ( a visual clue to show during sensitizing that all the iodine has been converted to AgI; traditionally they used elemental iodine which I don't want to store or work with ), so it would be a good time to start several KI-related tests.

 

[Photo Engineer]

I'm not sure that Povidone would work. It is so tightly bound by the polymer. Maybe.

Actually, Iodine itself is not that bad, but it does sublime quickly and you lose it.

PE

 

[NedL]

I don't know why, but many drug stores here don't sell tincture of iodine anymore, but sell povidone instead. Only a small amount of iodine is needed, enough to give the paper a visible tint.