Experiments with Metol and ascorbic acid.
The factors affecting developer activity are temperature, pH,
concentration, products of aerial oxidation, products of silver halide
reduction, and the orientations of the heavenly bodies.
According to some experimental results reported in "The Theory of the
Photographic Process" as little as 0.05 moles (6.3 grams) of sodium sulfite in
a liter of Metol developer will double the rate of development that is
obtained when no sulfite is present, pH being held at 8.7 in both cases.
It is explained that oxidation products of Metol restrain development, and
that sulfite counteracts these products by forming the sulfonate.
Ascorbic acid will do the same degree of acceleration without sulfite, also at pH 8.7. In this case the oxidized Metol is reduced back to its original form. The ascorbate is itself oxidized and pH is decreased in the process. The decrease in amount of active Metol in the one case or the decrease of pH in the other may not cause much difference in capacity and storage longevity between a Metol-sulfite and a metol-ascorbate developer. However, it would seem that a well buffered working solution might tip the balance in favor of the ascorbate.
I will explore first a 0.05 molar concentration of Metol, which is
approximately that of D-23, together with a 0.05 molar concentration of
ascorbic acid. It remains to formulate the alkali needed to make the pH less
than that at which an ascorbate becomes a developer and great enough to ensure activity of Metol.
I calculate that 0.1 moles of sodium hydroxide will neutralize the 0.05
moles of sulfuric acid attached to the Metol base plus the 0.05 moles of
ascorbic acid, leaving 0.05 moles each of sodium sulfate and 4-(methyl-
amino)phenol and 0.05 moles of sodium ascorbate. The net pH thus far is still
not alkaline enough to develop film in a reasonable time, nor does it have the
necessary buffering action to keep the pH from going lower yet. A few grams of borax should bring the pH up to about 9.2 but even 20 grams will have little
more effect on pH and should give a cushion against reduction of pH during
The resulting recipe is:
Ascorbic acid.....8.8 g
Sodium hydroxide..4 g
Water to 1 liter.
The concentration of Metol is about 13% greater than in D-23 and the
concentration of sodium sulfite is nil.
The pH of this mixture, which I measure with test strips to be about 9, is
below that at which the ascorbate is a developer of any consequence. The
initial activity of the mixture is about that which could have been obtained
with sulfite in place of the ascorbate, so it does not seem that synergism
between Metol and ascorbate is the explanation.
The ratio of borax to caustic in this formula is quite close to the
effective ratio in sodium metaborate. 14.5 grams of sodium hydroxide and 69
grams of borax in a liter make a solution that is often used as a substitute
for 10% sodium metaborate. 276 ml of such a solution, or 27.6 grams of Kodalk if you prefer, will contain the equivalent of 4 grams of sodium hydroxide and about 20 grams of borax. It may be easier to get sodium metaborate than sodium hydroxide through the UPS. The recipe then becomes:
Ascorbic acid......8.8 g
Sodium metaborate..27.6 g or 276 ml of 10% sodium metaborate solution.
Water to 1 liter.
All ingredients are quickly dissolved in room temperature water. HP5+
developed for 8 minutes at 68 F gives normal contrast, showing that the
solution could be diluted. In fact, diluting with an equal part of water
increased development time by only 25%, probably because the pH of this
solution does not change much with dilution. 125 ml diluted with 125 ml of
water did a 36 exposure roll of HP5+ to normal contrast in 10 minutes, which
means of course that a liter of the above formula will do 16 standard rolls
without reuse. As a matter of interest, twice the above recipe can be squeezed into one liter.
By the way, don't be confused by the fact that there exist 4 mol and 8 mol
metaborates. A gram of one has the same number of atoms of sodium, boron,
oxygen and hydrogen as a gram of the other. At 53.6 C, without losing any
water, Na2B2O4.8H2O becomes NaBO2.4H2O which is stable to 105 C. In other words, the distinction between 4 mol and 8 mol sodium metaborate is academic, not practical when we specify solution strength in terms of weight per unit volume. You may see this for yourself at www.borax.com.
I am attaching a scan of a 10x print from FP4+ developed 8 minutes, 68 F. This is not of very high resolution, but may serve to illustrate gradations. I have also attached a higher resolution scan of a small part of the same print. If you print the high res. scan to 6x9 inches, you will see about a 50X magnification of that part. I have no idea how these will show up, but you can imagine that the originals are better.
As it turned out, the attachments showed up at the end of the thread on superadditivity started by PE. My fault.
Last edited by gainer; 06-03-2007 at 11:23 AM. Click to view previous post history.
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!
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.
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.
Originally Posted by Photo Engineer
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...
Originally Posted by gainer
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.
Last edited by Kirk Keyes; 01-03-2008 at 02:28 AM. Click to view previous post history.
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Originally Posted by gainer
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?
Last edited by Photo Engineer; 01-03-2008 at 10:39 AM. Click to view previous post history.
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.
Originally Posted by Photo Engineer
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.
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.
I do understand that and I'm glad.
Originally Posted by gainer
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,
Yes, it does. It's not a supposition.
Originally Posted by gainer
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.
Originally Posted by gainer
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.
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.