[jdelligatti]

Good Afternoon,

I admittedly don't have a great chemistry background, but need to understand the best way to calculate flash point for a few mixtures of which I am authoring safety data sheets. Most chemicals I am authoring these for I am able to simply copy my suppliers safety data sheet, but we mix 2 or 3 chemicals in house which I will need to know this calculation for in order to classify properly.

One example is I have a mixture where 58% is water, 12% is ethanol, and then there are some small percentages of other chemicals. The ethanol is the only chemical rated as a flammability hazard under the GHS.

Can anyone point me in the right direction?

Thanks!

[Enthalpy]

Hi jdelligatti, welcome here!

I'm not convinced that the flash point of a mixture can be properly calculated. A vague estimate would adapt the liquid's temperature so the (only) flammable compound has the sae vapour pressure as if it were alone, but this is a bit bold; in your example, water vapour would help quench the ethanol vapour.

Anyway, as a user of safety datasheets, I expect to find accurate measured data in them, not estimates. Especially for data directly relevant to safety. A flash point isn't difficult to measure; contract someone if necessary.

[pgk]

Let’s clarify a few things:
1). If the liberated energy by the combustion of a part of a compound, is higher than the activation energy of the compound then, the compound is totally combusted.
2). If the liberated energy by the combustion of a part of a compound, is about equal than the activation energy of the compound then, the combustion stops.
3). If the liberated energy by the combustion of a part of a compound, is lower than the activation energy of the compound then, there is no combustion due to the contact with a spark.
4). Obviously, vapor pressure and partial vapor pressurein a given temperature, play an important role to the above described. Volatile compounds are generally flammable but there are exceptions, e.g. dichloromethane, chloroform.
5). Viscosity, as well as surface tension in a mixture, also play an important role, at given temperature, regarding combustion.
6). Humidity content at that temperature, too.
7). Combustion behavior is different in “open fire” (wildfire) than in a “closed one” (a small fire in a building). Therefore, there are two methods of flash point determination, the open-cup and the closed-cup.
As a conclusion, flash point, only roughly and indicatively can be estimated, under a lot of assumptions and there is no linear function between the flash points of the components and the flash point of the mixture. However, the use of specific software might lead to more accurate estimations.

[Enthalpy]

The flash point has little to do with the heat of combustion.

[pgk]

I do not agree.
The flash point is the temperature of ignition in presence of a spark.
The spark offers the activation energy for the combustion of an amount of the material. If the heat of combustion of that amount is higher than the activation energy of combustion of the material, the combustion continues, otherwise combustion (and ignition) stops.
Flammable materials are usually volatile compounds that have a significant vapor pressure. Vapor is easily combusted by the spark.
The mass of vapor over the material depends on the vapor pressure which is temperature depended.
Usually, the flash point corresponds about to the temperature of that critical vapor tension and therefore, there is a direct relationship between the flash point and the combustion heat in relation to the activation energy of the combustion.
For similar reasons, the flash point of non-volatile materials usually corresponds to the decomposition temperature to the formation of volatile compounds.
But all above are not general rules because many other factors can also interfere (e.g. high thermal capacity, endothermic decomposition, endothermic dehalogenation, dehydration reaction, hydrates decomposition, free radicals formation, etc.).
Thus, the flash points of materials and their mixtures cannot easily be estimated with accuracy, by that method.

[Enthalpy]

Nature uses to defeat our nicest attempts to understand it... So one should be wary of simple ideas like "if the produced heat suffices, the combustion continues".

For instance, halons quench a fire even if present in small amounts, wit no important efefct on the amount of oxygen available nor on the heat of combustion.

Or just materials like copper, nickel, hafnium... Their heat of combustion would easily bring them to monoatomic gases which would oxidize immediately, but they're impossible to light.

Other materials burn without first decomposing nor even vaporizing, for instance iron powder. Not a matter of vapour pressure then.

In short: neither the heat of combustion nor the vapour pressure are robust indicators of a flash point. Flash points must be measured.

[pgk]

Finally, I say the same thing, with different words:
“Although there is a direct relationship between the flash point and the combustion heat in relation to the activation energy of the combustion, the flash points of materials and their mixtures cannot so easily be estimated with accuracy, due to the interference of various factors, depending on case.”
Conclusion: This cannot be a general method and thus, flash points must be measured.
Explanation: I am referring to organic materials. Ignition and autoignition of minerals often follow different rules (e.g. ignition of metallic sodium in contact with water or ignition of white phosphorus in contact with the hand).

[Corribus]

When in doubt, it helps to view real data. Because the discussion piqued my interest, I plotted the heats of combustion of a number of common organic solvents as a function of their corresponding flash points. The heats of combustion I found predominantly from NIST sources, and the flash points were taken from my CRC where available, and Wikipedia where they weren't. The result is the top scatter plot at the end of this post.

For my part, I discern very little correlation between the two parameters, leading me to throw in my support to the argument that combustion enthalpies aren't very relevant in a discussion about flash points. At the least, it seems virtually impossible to make any qualitative, let along quantitative, prediction about the flash point purely from heat of combustion values. We may turn to some representative examples, highlighted conveniently in blue and green for the butanols and amyl alcohols (a few of them, anyway), respectively. It is apparent that although members of these groups have virtually identical heats of combustion (within their respective classes), the flash points vary quite a bit.

In hindsight, all of this makes pretty good sense. Combustion happens in the gas phase (even for a liquid, which needs to be aerosolized or vaporized; solids typically undergo pyrolysis first to produce gas-phase combustible fuels). The heat of combustion is predominantly a representation of the difference between the sum total bond energies for the reactant hydrocarbon (and oxygen) and those for the products, minus any angle strain energies that might be involved, all in the gas phase. That is to say, it involves almost exclusively intramolecular electrostatic energy contributions. Any interactions between molecules are negligible, because combustion happens in the gas phase. Where the oxygen is.

The flash point of a liquid, on the other hand, depends to great extent on its vapor pressure, which in turn has a lot to do with intermolecular interactions in the liquid state. This is why the four amyl alcohols can have almost the same combustion enthalpy but very different flash points. When they are in the gas phase, the differences between these alcohols are very subtle, at least as far as structural energies are concerned, but in the liquid phase the position of the alcohol group changes the nature of intermolecular interactions substantially, leading to very big different in properties like boiling point and vapor pressure, which impact the flash point to a considerable degree. In fact, if we plot the boiling points of these solvents by flash point, you will see there is a reasonably good, though not perfect, correlation. (Bottom figure below.)

So, while there might be a very weak positive correlation between heat of combustion and flash point evident in the scatter plot, the impact of intermolecular interactions in the liquid state obscures this correlation almost completely. Insofar as the intermolecular interactions in the liquid state would be very hard to calculate a priori without sophisticated computer packages, flash point is certainly a quantity that is better off measured experimentally, especially for mixtures. If anything, you should be using the boiling point as a rough estimate of what the flash point would be.

(NB, By mistake, I colored the amyl alcohol points green in the upper plot but teal in the lower plot.)

[curiouscat]

What might be relevant to flash point is the minimum temperature at which the vapor phase composition falls in a range that is between the lower & upper flamability limits.

Any estimate is likely to be poor but if I had to try then I'd start from the Antoine equation for the components. Then estimate the vapor pressure of a given mixture assuming ideality (Σ x_i * P_i_sat ), & hence the gas phase composition. Then from stoichiometry of combustion estimate the amount of O2 theoretically needed for combustion. I think the LEL occurs typically at 50% of this stoichiometric amount.

This procedure might give you a decent estimate. Though I'm too lazy to test my hypothesis.

[Corribus]

NB #2: I also noticed that I mistyped my green diamond labels in the top figure. Those should be 1-pentanol, 2-pentanol, 3-pentanol. Not 1-, 2-, and 3-propanol. My apologies for the sloppiness. /sigh

[DrCMS]

Corribus - very interesting bit of work.  I wonder if there is a better correlation between flash point and heat of combustion per mass or per volume of material? 

To be honest the best thing to do is measure the flash points of any mixture of interest. 

I do sometimes take an educated guess based on the various flash points, boiling points and viscosities of the components involved but if my guess is close to the boundaries of the different flammable classifications I get it measured. 
 

[Corribus]

I did convert all the combustion enthalpies to J/g because I wondered if that would change anything. By inspection of the numbers I didn't see anything obvious but I confess I didn't actually plot them out. Converting to J/g won't, for example, change the fact that different conformational isomers have very different flash points despite having nearly identical combustion enthalpies (the molecular weights are identical, so a mole to gram conversion doesn't change their values relative to each other). If I get a chance this morning I'll go ahead and plot them out, though.

[Enthalpy]

Some isomers exhibit worse differences than through the position of an alcohol.

Diethyl ether boils at +35°C. Its flash point is -45°C.
n-Butanol boils at +118°C because of hydrogen bonds. Its flash point is +35°C.
The heat of combustion is very close, per mole or per gram.

Then some composition details like bromine can quench the flame, with little relationship with the heat of combustion. Comparing the vapour pressure with the flammability limit looks better, yes, but in a mixture where an other component is a flame retardant, it wouldn't be accurate enough for an Msds.

Nice job with the diagrams!

[pgk]

1). Combustion heat must be related with activation energy and vapor pressure. Otherwise and as clearly mentioned above, combustion heat alone offers nothing.
2). Vapor pressure of flammable mixtures can be calculated by Dalton and Henri laws.
3). Mixture of flammable and non flammable compounds.
indicative example: Presence of alkyl chlorides
Combustion is a free-radical reaction that occurs via initiation, propagation and termination steps.
During combustion, there is a dissociation of the C-Cl bond, to radical C. and radical Cl.
C-Cl →  C. + Cl.
The so produced free radicals favor the termination step and thus, the combustion stops.
C-Cl bond dissociation (and formation) energy is about
81 kcal/mole and is taken from the combustion heat of the mixture. Therefore, the offered energy from the combustion is significantly decreased. Such energy decrease can easily be estimated per mole per gram.

[pgk]

As a reminder, autoignition temperature is a “real” property of the material; meaning that the material automatically burns when the temperature reaches the autoignition one.
While, flash point is an “artificial” property of the material; meaning that a given mass of the material is put in a given cup that is filled by a given way, according the corresponding ASTM standard. Then, in presence of a given flame, the temperature is gradually increased and measured when combustion starts, according the corresponding ASTM standard.
Thus, it obvious that the flash point is higher when is measured in an open cup than measured in a closed cup. It also obvious that n-butanol in an open container, might not burn at 35oC, by throwing a lighted cigarette, therein; while it will surly burn at 35oC in a closed container, by the same action.
Coming back to the point! The mass of the liquid is known, the volume of the cup is known, the vapor pressure at the given temperature,  if not known,  it can be estimated by the  Clausius-Clapeyron equation, the evaporation enthalpy (if not known) can be estimated by the Trouton’s rule, the heat of combustion can be estimated by the Hess law (if not found in tables) and the activation energy of combustion (if not found in the literature) can be correlated with the left part of the Hess law.
What else do you need, in order to estimate the flash point by the gas law?