Experiments with Metol and ascorbic acid.

[gainer]

I thought I could stay out of this fray, but I have to say this: I did not ever publish a formula for fixer, and if I had it would not have contained borax. You did not respond to my questioning the probability of significant potassium content in technical grade borax. The refinement of raw borax ore by the producers of 20 Mule Team products involves, as you might expect, solution in hot water and crystallization. Although there may be potassium remaining as a soluble impurity among the 1300 ppm maximum allowed in technical grade, it is difficult to see how that fraction of the small amount of borax used in any of the formulae I have seen could have a drastic effect on fixation, except in the psycholigical sense. Even so, it can easily be reduced by a factor of 50 or so in the home laboratory simply by making a saturated solution of the technical grade with at least double the amount needed for saturation, discarding the clear liquid, and making another saturated solution with the sediment from the first. If the solutions are saturated at high temperature, the last solution upon cooling will produce crystals of very pure borax decahydrate. How much did it cost to do this with borax that costs 50 cents a pound at the local supermarket? These are procedures that require little attention.

Flashpoint is the temperature at which an open flame close to the surface of a liquid will ignite the liquid. I tested that with the propylene glycol. It ignited as predicted, but there was no explosion, an open flame was required for ignition, and the burning glycol was easily extinguished.

 

[Photo Engineer]

A fire is achieved when a flammable vapor reaches a hot element that is above the flashpoint of the vapor.

In this case, you heat the liquid until vapor begins to fill the container. The heavy vapor will rise over the edges of the container and drop downwards due to the density compared with the atmosphere, and will ignite when it hits a surface heated at or above the flashpoint.

This can take place over distances of 6 feet or more. I have seen it, and it is much more difficult to contain and extinguish. The sudden ignition of even a moderate amount of vapor can be nearly explosive in force.

Very often, this is seen with gasoline or solvent based paints which can ignite when even opened in a shop with a lit pilot lamp on a water heater or furnace over 6 feet away. I was a consultant for an attorney on such a case many years ago.

In many cases the ratio of air to vapor determines whether you get a fire or an explosion.

IIRC, TF-3 uses Borax for buffer. I don't have my copy of A&T handy, but if so then the Borax would have to be essentially free of Borax. Common salts vary in the Potassium and Sodium ratio around the world so it could depend on the source.

PE

 

[Photo Engineer]

Found A&T and looked up some formulas...

TF2 and TF3 contain Borax. There is Kodalk in Kodak's F6, F10 and F24. F5 contains Boric acid. All, being photo grade are free of Potassium as per the information from Haist.

PE

 

[gainer]

A fire is achieved when a flammable vapor reaches a hot element that is above the flashpoint of the vapor.

In this case, you heat the liquid until vapor begins to fill the container. The heavy vapor will rise over the edges of the container and drop downwards due to the density compared with the atmosphere, and will ignite when it hits a surface heated at or above the flashpoint.

This can take place over distances of 6 feet or more. I have seen it, and it is much more difficult to contain and extinguish. The sudden ignition of even a moderate amount of vapor can be nearly explosive in force.

Very often, this is seen with gasoline or solvent based paints which can ignite when even opened in a shop with a lit pilot lamp on a water heater or furnace over 6 feet away. I was a consultant for an attorney on such a case many years ago.

In many cases the ratio of air to vapor determines whether you get a fire or an explosion.

IIRC, TF-3 uses Borax for buffer. I don't have my copy of A&T handy, but if so then the Borax would have to be essentially free of Borax. Common salts vary in the Potassium and Sodium ratio around the world so it could depend on the source.

PE

Where did you get that definition of flashpoint? IIRC the flashpoint of propylene glycol is about the temperature of boiling water. It would be very difficult to use it as an automobile antifreeze would it not?

 

[gainer]

Wikipedia:The flashpoint of a flammable liquid is the lowest temperature at which it can form an ignitable mixture in air. At this temperature the vapor may cease to burn when the source of ignition is removed. A slightly higher temperature, the fire point, is the temperature at which the vapor continues to burn after being ignited. Neither of these parameters are related to the temperatures of the ignition source or of the burning liquid, which are much higher.

Thhe flash point of propylene glycol is 210 F by the closed cup method and 224 F by the open cup method. You can hold a rod heated to 224 F near the surface of propylene glycol all day long without igniting it. The autoignition point is 700 F. If the glycol were to boil over onto a red hot heating element, there would certainly be a fire. If the vapor from glycol at its flashpoint overflowed the container, there might or might not be a fire, depending on the partial pressure of glycol at the hot surface, which would have to be at least at the autoignition point of the glycol.

 

[Photo Engineer]

You are exactly correct, however you have discounted the fact that vapour travels, and the PG vapour is heavier than air as are the vapours of many organic chemicals.

Therefore, the vapour rises over PG to form a 'cap' which eventually flows over the vessel in which you are heating it, and it eventually overflows and drifts down to the heating element or flame. Then you have a fire. This fire can be mild or explosive depending on the ratio of air to vapour.

So, we have a textbook example, but I've stood in the middle of such a fire and fought it.

I was the first to tell you of flash point. Now you are trying to teach me? I've been there and experienced this type of fire! So your talking book learning against actual experience.

Besides which, consider the fact that flash point is exceeded by ALL open flames in the room in which the vapours exist. If a flammable vapour contacts a flame, you get a fire.

I think you can understand this!

PE

 

[Photo Engineer]

Let me explain something clearly (I hope). Almost all organic liquids are flammable in gaseous or vapour form. The point at which this takes place is the flash point.

However, if we have any volatile material form through heating, even if it is below the stated flash point, it still can diffuse through a room. Now, this vapour, if it contacts an open heating element that is at or above the flash point, the vapour will ignite. These vapours tend to hug the ground or lower levels due to their density.

If you are able to gain a sufficient concentration in air, you get either a fire, or an explosion. It takes about a 50:50 mixture of commercial gas or gasoline fumes in air to explode (IIRC), but otherwise you have a large fire. Depending on the quantity of flammable vapour involved, the fire can be small or large.

A small amount of liquid can convert into a large amount of vapour. AAMOF, 1 mole of liquid can convert to 22.5 cubic liters of vapour at room temperature(correct this if I've disremembered, but I think that is the correct conversion). At higher temps, this volume is larger by the equation PV = nrT.

So, if this is correct, then 18 grams of water will make 22.5 cubic liters of gas (at room temp). PG will do much the same from one mole of PG liquid.

So, be careful when heating any organic solvent that has the potential of being flammable. Any open flame at or above the flash point in the same room can cause a flash fire if you form vapours.

PE

 

[gainer]

You are exactly correct, however you have discounted the fact that vapour travels, and the PG vapour is heavier than air as are the vapours of many organic chemicals.

Therefore, the vapour rises over PG to form a 'cap' which eventually flows over the vessel in which you are heating it, and it eventually overflows and drifts down to the heating element or flame. Then you have a fire. This fire can be mild or explosive depending on the ratio of air to vapour.

So, we have a textbook example, but I've stood in the middle of such a fire and fought it.

I was the first to tell you of flash point. Now you are trying to teach me? I've been there and experienced this type of fire! So your talking book learning against actual experience.

Besides which, consider the fact that flash point is exceeded by ALL open flames in the room in which the vapours exist. If a flammable vapour contacts a flame, you get a fire.

I think you can understand this!

PE

You were not the first by a long shot. I did study engineering, and I studied welding, machine shop, strength of materials and many other things besides that ill fated organic chemistry course. I also learned many things about the physical world at NACA-NASA. You defined flash point in your previous post and it was not the scientifically correct definition. If I can't get away with doing that with borax, then neither can you with anything else. You did not say that if a flammable vapor contacts a flame, you said if a flammable vapor contacts an object at its flash point you get a fire. I'm fed up with double talk.

 

[Kirk Keyes]

If the glycol were to boil over onto a red hot heating element, there would certainly be a fire. If the vapor from glycol at its flashpoint overflowed the container, there might or might not be a fire, depending on the partial pressure of glycol at the hot surface, which would have to be at least at the autoignition point of the glycol.

Pat, it has nothing to do with the "partial pressure of glycol at the hot surface" here, it's simply the concentration of the flammable vapour in air that is important and an ignition source. You're trying to make things too complicated...

I know you love MSDS sheets, you may want to look closer at one for propylene glycol. Try this one: http://www.jtbaker.com/msds/englishhtml/p6928.htm Look in Section 5 - fire fighting info. You will see right after the flashpoint and the autoignition point values, it gives the info for explosive mixtures of propylene glycol in air. It says, "Flammable limits in air % by volume: lel: 2.6; uel: 12.5" That's a lower explosive limit (lel) of 2.6% propylene glycol in air, and an upper explosive limit of 12.5%. Not enough vapour in the air and there cannot be a fire or explosion, between the lel and uel and you can get a fire/explosion, and above the uel and there cannot be an fire/explosion.

PE's concern is you were not concerned about heating propylene glycol in a microwave to temperatures above the flashpoint. You said, "I question whether the vapor seen at about 250 degees is propylene glycol. The boiling point of the glycol is much higher than that. I think it is likely that the glycol had some water in it which distilled out." http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00Bhk2 You may want to think about that statement some more.

Flashpoint is a test used to measure the temperature at which a material will form an explosive mixture in air. I've done many flashpoint measurements of both waste materials and commercial products. I can tell you that when a sample has enough flammable material in it to give a strong flashpoint, you get flames several inches long coming from the closed cup tester. It's a lot of fun when they go. You'd like it.

The vapour from propylene glycol heaed to 250F is most likely from the glycol, and not water as you proposed in the thread I linked to above. Think about those flashpoint temps that you quoted above. "Thhe flash point of propylene glycol is 210 F by the closed cup method and 224 F by the open cup method." There must be at least 2.6% propylene glycol vapor present in the flash point apparatus in order for the glycol to flash. The liquid does not have to be anywhere near its boiling point. And do you have any suggestions as to why the closed cup flashpoint is lower than the open cup flash? Perhaps is has to do with the propylene glycol vapour being more concentrated at a lower temperature in the closed cup test vs. the open cup test, which as the names imply, the open cup test is done with a confined area above the material that is being tested while the open cup is open to the air.

So there can be enough propylene glycol vapour present at the temperature of 210 in a closed cup in order for the vapor to catch fire or even explode given a suitable ignition source, like a spark or an open flame. (And yes, I agree with you that the glycol will not autoignite at the flashpoint temp and that it does need an ignition source.)

Anyway, heating propylene glycol in a microwave does present a hazardous condition similar to that in the closed cup flashpoint tester and it should not be recommended due to safety concerns. We can't know that there is internal sparking in the microwave that could present an ignition source. (And I have had a microwave melt down and catch fire breifly due to failed electronics. I'm glad it did not contain a flammable vapour in it as well when it failed.)

And I'm glad that you did eventually agree that propylene glycol did not need to be heated to such high temperatures in the p.net thread - even though you agreed because you decided that such high temperatures were bad for dissolving ascorbic acid, and not because it was a safety issue...

PS - it would be fun to see a microwave blow itself up by heating a flammable material above the flashpoint. Perhaps we can get the Mythbusters guys on this one.

 

[Photo Engineer]

You were not the first by a long shot. I did study engineering, and I studied welding, machine shop, strength of materials and many other things besides that ill fated organic chemistry course. I also learned many things about the physical world at NACA-NASA. You defined flash point in your previous post and it was not the scientifically correct definition. If I can't get away with doing that with borax, then neither can you with anything else. You did not say that if a flammable vapor contacts a flame, you said if a flammable vapor contacts an object at its flash point you get a fire. I'm fed up with double talk.

Patrick;

All I can say is that my definition was correct from an actual practical standpoint in simple, not technical terms. For more detail, read what Kirk said and I hope you understand. He has given a complete technical definition and explanation.

Remember that your hotplate or burner is HOTTER by far than the flashpoint if you are heating as you said, and therefore the vapour will ignite if it reaches the heater. The vapour can even be below the flashpoint when it reaches a hot element.

But to get back to your comment about me being wrong in my post. I said nothing about the heating element being AT the flash point. As I have contended all along, if you heat anything on a burner or hotplate, the heating element is HOTTER than the temperature of the liquid and therefore is usuallly hotter than the flashpoint of the liquid.

I might also mention that in an earlier post you commented on how I brought the importance of the flashpoint of PG to your attention. If you knew about it, why did I have to bring it to your attention?

PE

 

[Kirk Keyes]

Patrick;
The vapour can even be below the flashpoint when it reaches a hot element.

Right - the vapour can be whatever the ambient temp is, it just needs to be concentrated enough to get into the lel-uel range for it to be flammable. Just like any other flammable gas in air.

 

[gainer]

The fact remains that in all the times I have heated glycol above its specified flashpoint, I have not had a fire that I did not set intentionally, and that was the one I mentioned in a previous post. There are other things to consider: does the vapor actually behave as you suppose? There are temperature gradients to consider. The container will be at a higher temperature than its contents during heating. The air-vapor mixture above the surface will not be at a higher temperature than the surface of the fluid from which it came. There will be a partial pressure of the vapor. It will be an indicator of flammability. The flashpoint is not the autoignition temperature. The autoignition temperature of propylene glycol is much higher. than the flashpoint. I have read the MSDS for PG, several times over and from different source. In addition, I have read the manufacturers literature about these solvents.

Aside from all that, I no longer recommend heating above the flashpoint and have not for quite some time.

Where is the hot element in a microwave oven? It is in fact the material that is being heated., and probably not at the surface that is in contact with air. In a microwave oven, heating to or even above the flashpoint will not be sufficient to ignite the glycol. Try it if you don't believe it. You will need an ignition temperature that is not found at any point in the oven.

 

[Photo Engineer]

Patrick;

You do what you wish, and advise people as you wish. In this type of work, all it takes is one big accident. So, you will have to bear all of the responsibility, not me if anything happens. I urge everyone to caution and err on the side of caution to prevent accidents.

PE

 

[Kirk Keyes]

Aside from all that, I no longer recommend heating above the flashpoint and have not for quite some time.

I do understand that and I'm glad.

But the description of the behaviour of flammable liquids (and yes, glycols are flammable liquids) that PE and I have given here are accurate.

The only requirement you need to achieve to get a flash fire is to heat the flammable liquid enough to get a suffient quantity into the air AND an ignition source. The best way to avoid one of these fires is to not have either condition met.

At a lab I worked in, we used to do fat extractions using petroleum ether which has a 30-60C boiling range. It is a very volatile solvent. As a safety precaution, we used light bulbs to heat the refux flasks. Safer right? No Bunsen burners.

Well, the flash point of Pet Ether is less than room temp, and the autoignition temp is 473F. The Pet Ether the guy spilled evaporated and worked its way over to one of the hot light bulb in the apparatus and ignited. No open flame, and it was not spilled directly on to the hot bulb.

So I'm just trying to point out that it's pretty easy to meet the conditions needed to start a flash fire. And if the conditions are just right, an explosion can occur as well. It really is best to work with flammable materials well below the flash point whenever possible.

As to your question,

does the vapor actually behave as you suppose?

Yes, it does. It's not a supposition.

There are temperature gradients to consider. The container will be at a higher temperature than its contents during heating.

No, you do not need to worry about temperature gradients of the container. You need to worry about the concentration of the vapour in the air.

I'm not an expert in microwave oven design. But I have personally experieinced a fire in a microwave due to failed electronics. Also, we don't know if the person doing this will be smoking which would give ample source of ignition if enough vapour is in the air when the door to the microwave is opened. Lots of things we can't control that may happen.

Again, glad you don't recommend this practice anymore. But why are you so stubborn about general safety recommendations? We should all be promoting practises that will make everyone's photo lab experience safe and enjoyable.

 

[gainer]

Boiling occurs when the partial pressure of vapor at the surface of the liquid in question equals the pressure of the atmosphere at the liquid surface. Do we agree on that? The flashpoint is reached when the partial pressure of the liquid in question forms an inflammable mixture with the air at the surface. A very simple way to keep this from happening is to cover the container with microwave-safe Glad wrap or equal. Leave enough slack so that the wrap lies at least partly on the surface of the liquid. There can be no contact then between inflammable liquid and air and the first evidence of boiling will be when the plastic lifts up. There will be no creeping of vapor over the side onto the nonexistent heating element. I have learned by experiment that the airflow through the heating chamber passes over the hot light bulb BEFORE entering the chamber.

 

[Ryuji]

Another point is that the curve from Mees & James shows that the effect of either ascorbic acid or sodium sulfite increases as mole fraction rises to about 0.05, then levels off with further increase. Thus 10 g/l sulfite is well above that point while 1.9 g/l even of full strength ascorbic acid is far below. If you were to bring the ascorbic acid up to 8.8 g/l (as an ascorbate, of course) you might see the activity increase without sulfite. However. I don't think you have quite enough pH without the sulfite. It should be about 9.

I have a feeling that it is the molar concentration of either the ascorbic acid or the sulfite that counts, not the ratio of either to the Metol, as long as the pH is right.

You have focused on reducing properties of the two agents, but you might want to know that ascorbate is a much much stronger reducing agent than sulfite. The kind of reactions they undergo are also different.

One point that's also omitted from your discussion is the emulsion type. For the purpose of this discussion, what you want to know is where the latent image centers are made. The most recent technology allows emulsions that make latent image on the surface of the grain, where developer is directly accessible, especially when exposure is relatively slow (which means anything but very brief flash exposure in modern standard). However, in old technology emulsions, latent image centers can be made in the grain or on the surface. Especially when the grain is exposed with very brief exposure of light (which means 1/60 or faster in some long exposure type emulsions), or high energy radiations, more latent image centers form inside the grain. Also, direct positive emulsions have latent image (intentionally created fog centers, rather) in the interior. These internal latent images are not directly accessible to your developers and therefore the sensitometric curves can differ from those obtained with developers that can access internal latent images.

I have made several different emulsions, some of which form latent image in the interior, and others near the corners of the grains outside, and I do see quite noticeable differences with the kind of factors you are looking at, including the sulfite concentration.

 

[gainer]

I was intrigued by the fact that sulfite and ascorbate showed almost identical curves in the excerpt that was shown in "The Theory.... " while hydroquinone showed no effect. Most simple analyses of MQ developers omit mention of the importance of sulfite to the "regeneration" of Metol by hydroquinone. The operation of ascorbate is certainly different in the long run from that of sulfite, and the regeneration of metol by hydroquinone appears not to take place without either sulfite or ascorbate. Ascorbate and Metol without sulfite, with the proper pH, can make a developer that is as active as Metol and hydroquinone with sulfite, which would lead one at least to guess that ascorbate is both developing agent and antioxidant.

IIRC, the antioxidant and the reducing properties of ascorbate use different parts of the molecule. The body does remarkable things with it to allow it to pass into the brain through the blood supply. We can use either ascorbic or isoascorbic acid in our developers, but the body needs ascorbic acid for vitamin C, from what I have read.

 

[monkeykoder]

Sorry to post in such an old thread but I have to say with my limited knowledge I would hesitate quite strongly to heat anything flammable in a microwave. If you have spent a considerable amount of money on the microwave you MIGHT be safer but as far as I know now microwave heats even remotely evenly. Just say you're microwaving a burrito (at least it is a relatively safe bet) if you stick the burrito towards the center of the microwave you'll usually notice (at least in cheaper models) that the endpoints of the burrito are MUCH cooler than the center. Even given that for PG this is a (relatively) safe method (assuming there is no metal in there and that the vapor will never reach the radiation source which if I'm not mistaken does spark) if you're microwave is failing usually this leads to even greater temperature gradients which may or may not cause it to get above the autoignition temperature (this is of course a college student approaching this from the microwave burrito perspective... Your mileage may vary).

 

[gainer]

There is a variety of plastic wrap that is advertised as "Microwave safe". Cover the vessel you are heating with this type of wrap so that the edges are sealed but the center is loose enough to contact the fluid being heated. Before ANY vapor can escape the vessel to be ignited, the wrap will have to rise from contact with the fluid and baloon up. This action ought to be easily visible throuh the window. Heating will instantly stop when the microwave is turned off. (No more energy will be supplied to the fluid.) Uneven heating now plays no part in the process. If the wrap swells or even just barely leaves contact with the fluid surface, the vapor pressure is at least equal to atmospheric and the fluid is boiling. Until that point is reached, there is no possibility of the vapor being ignited by spark or open flame.