Dear Christian,

yes, it is possible to dilute a salt solution to increase its saturated vapour pressure, but it is difficult to keep your solution concentration constant when your rock takes up a significant volume of water with respect to the volume of solution in your desiccator. The concentration of the salt solution will increase as water evaporates, which causes the vapour pressure to decrease. Evidently an equilibrium is reached at some point, but it is practically difficult to know what the relative humidity will be at that point. This issue is avoided by using saturated solutions with excess solid salt, those dissolve/precipitate salt as the volume of water changes.

I don't really get what you mean by 'the range of RH produced by the solutions is too large'? Saturated salt solutions produce vapour pressures that are very constant and well documented. We use these values to calibrate vapour sorption equipement. Several salts give RH in the range you need (90-100%RH) at room temperature (K2SO4, KNO3), the exact values are in Greenspan 1977:

http://nvlpubs.nist.gov/nistpubs/jres/81A/jresv81An1p89_A1b.pdf

and 100% RH is the easiest, just put a cup of pure water in your desiccator.

regards,

Pieter

Hello Pieter,

thank you for your explanation. It is really helpful.

What I mean regarding the "the range is too large" is that I need RH between 90% and 99,9%. According to Greenspan the salts, which generate an equilibrium condition in this range are KNO3, K2SO4 and K2CrO4. But I want to study the effects in the whole range of relative humidities (especially in the range of 96-99,9% RH) and three "data points" are too little.

Regards

Christian

OK, I thought you just wanted to store them.

There are a lot more salts that generate humidities in that range, in saturated solution. There is a list in Perry's Chemical Engineers Handbook (table 12-8), and that one is based on “International Critical Tables,” vol. 1, p. 68.

If you want to measure the water uptake, you'd be much better off with a water sorption device. I had a student doing this for some 15 mudrocks/shales. Our device has an RH resolution better than 0.5%, so you can do some 20 points between 90-100%RH. Our experiences are that these measurements are not easy on this kind of rock at those high humidities. Above 98% it takes extremely long to reach an equilibrium. Most samples showed a substantial ad/desorption hysteresis at high RH. We compared to desiccator equilibrations and saw that those give the uptake corresponding to the desorption branch, also when dry samples are equilibrated, unless the equilibration and weighing etc is done at very, very constant temperature.

Pieter

Dear Pieter how about if I wanna use RH 80% with temperature 100 Celsius? Is it possible

Dear Sonia,

According to the Critical Tables, Na2CO3 would be your best option, with about 82 %RH at 100°C. The original reference is Gerasimov, 1913, Journal of the Russian Physico-Chemical Society, v. 45, p. 1666-, but I can't find that one online.

In Perry's handbook, TlSO4 is listed at 84.8 %RH with max. temperature 104.7°C, but without reference. Greenspan does not list anything in the 80% range at 100°C, but extrapolation of his NaCl, KCl and KBr seem to get close.

You might want to try out some of these and measure the RH or pressure. Since 100°C is close to boiling point for most of these solutions, you'll need a container that can take the 0.8 bar overpressure. I wouldn't do this in a regular desiccator.

Alternatively, you also get a humidifier.

kind regards,

Pieter

Put the rocks is a dessicator with a salt saturated solution in the bottom and the samples placed on a shelf... Ill get back to you about which salt I used when I did it but it was published in my 2016 SPWLA paper

Dielectric permittivity and anisotropy of intact multi-saturated organic shales

M Josh, B Clennell, M Cauchefert, T Han

SPWLA 57th Annual Logging Symposium (Reykjavik 25-29 June 2016), 1-14

132016

Hey Alexander,

The bigger you make the water surface, the faster your desiccator will equilibrate. I usually take the largest bowl I have that fits the desiccator. Then add about 1 cm of DI water and put a pile of salt in the middle of a couple cm high. Wait a bit while the salt dissolves, then add more salt once the water is saturated, if needed. It is best to have a large excess of salt, especially when you are equilibrating at low RH, because then the deliquescent solution will take up water from the atmosphere every time you open the desiccator. After some time there can be enough water that way to dissolve all available salt, which means your solution is not saturated anymore and thus has a higher equilibrium humidity than expected. When you are equilibrating with a high humidity salt solution, the opposite can happen, you might need to add water from time to time.

For some salts, like LiCl, the dissolution releases a lot of heat. That can cause the solution to heat up quite a bit, which also offsets the humidity. It can take several hours then for the solution to cool down and your desiccator to reach equilibrium RH.

hope that helps,

Pieter

Hey Thibaut,

the equilibrium of a solution with its vapour can pretty much be seen as an equilibrium reaction, I think. It will follow Le Chatelier's principle and try to restore the equilibrium when disturbed. The exchange of molecules between solution and vapour will be subject to kinetics, for which the rate equations will be fairly complex I'd assume, because there are so many parameters that affect it. The obvious ones I'd say will be temperature, surface area of vapour-liquid contact, volume of system, mixing in the vapour phase and distance to equilibrium.

As long as your system is closed, T is constant, and there is saturated solution present, an equilibrium will be established. For most systems the release or uptake of vapour from the liquid is fast enough to keep up with the moisture exchange of the sample, at least on the longer term. If it isn't, you better redimension the system, use a bigger volume, larger water surface, smaller amount of sample etc.

Opening the system will temporarily disturb it and it will take a while to reestablish the equilibrium. If that is important to you, you could measure it with a humidity sensor, i.e. the magnitude of the disturbance and the time it takes to reestablish. If you experiment a bit with it you can set up an equation to predict time to equilibrium for a specific disturbance of your system.

If you want to avoid large disturbances because of opening, you can work in a glove box with a lock system to enter or remove samples. A glove box with a balance makes it easy to decide when your samples are fully equilibrated and to accurately determine the water uptake or loss.

hope that answers your question?

regards,

Pieter

You're welcome Alex,

I'm happy to contribute.

But the credit for the paper you mention should go to Leopold Lahn and Timo Seemann , they did all the hard work, I just provided the idea and helped them a bit with the interpretation.

1. We used very small desiccators, 0.5 L total volume. I'd guess that pulling a vacuum on such a small desiccator will maybe save you 20 minutes of time, because the equilibrium RH is established very fast in those at 30°C. Since the samples we used were equilibrated for one week, it wasn't worth the effort to evacuate.

2. Those classic desiccators are good but expensive and difficult to stack. If you want accurate uptake measurements it is very important to equilibrate at constant temperature (ideally better than +/- 0.1 °C). So you need to fit all those desiccators in a cabinet. Equilibrating rock samples in an environment of which the temperature is not constant will give you uptakes that are difficult to reproduce. They are generally higher than expected, due to the hysteresis of water sorption isotherms.

We use a product named ‘Korken’ from a certain Swedish furniture store that I don’t want to advertise, but I’m sure there’s a store close to you. Though not their intended purpose, these are excellent desiccators and you can stack dozens in a heating cabinet. The salt solution goes into the jar and we made small stages from the bottom of small plastic bottles that stand in the solution. The samples are put on the stage, in small cups we made from aluminium foil.

3. We didn't monitor humidity. There's no need, deliquescence of saturated salt solutions is hard physics, the data is well established and easy enough to cross check with different methods if you'd really want. Keep in mind that for the Leo's paper our goal wasn't to measure water isotherms. We wanted to establish a certain water saturation in those samples by sorptive uptake and then assess the distribution of the water by physisorption measurements. In a previous paper, by Timo, we measured detailed water isotherms on the same samples by DVS. If isotherms are your goal, you'll get much better results with DVS or manometric sorption measurements.

Hygrometers, like the one you refer to, are in most cases also calibrated or at least evaluated against traceable standards, and those are generally saturated salt solutions. There are some hygrometers based on fundamentally independent methods, like chilled mirror dew point analysers, but those come in a different price range.

4. The samples were placed in small aluminium foil cups, dish shaped actually. A thin bed of powder in a dish with low walls will give you the fasted equilibration. In a vial with higher walls you'll get an RH gradient between sample and atmosphere, which will slow down equilibration a bit. It will work, but might take a bit longer.

For the weighing we took the desiccator out, placed it next to the balance, open, put the cup on the balance, record, and put the entire thing back as fast as possible. This is not optimal, but we tested reproducibility and saw that that is good.

Ideally you would do the weighing in a glove box brought to the same humidity as the desiccator. We just developed a reasonably affordable, small (80x50x60 cm) glovebox for weighing purposes, with accurate active RH and T control at Dynchem. If you're interested in this, I'd be happy to provide you more details.

hope that answers your questions.

regards,

Pieter

I don't get what you're trying to do there, Alex, with that bottle on a heating(?) or stirring plate.

These salts have very fast dissolution kinetics, most of them will saturate water in a matter of minutes. There is no need for mixing or heating. If you want to contain the salt in a separate container in your desiccator, normally you just take a bowl, pour a small pile of salt in the middle, I typically make them some 5 cm high, and then add 1-2 cm DI water. The water dissolves the salt, what you get is an island of salt in a pool of brine. If too much of the salt dissolves, the island gets too small, then you just add a bit of salt. If the eq. RH of your salt is far away from lab RH, it is best not to leave the bowl in the lab atmosphere for too long, it may take up or give off water relatively fast. Just put it in the desiccator, close it and give it an hour. Then check if there is still enough salt, solid, if not, just add some.

The best way to ensure that the solution remains saturated is just to have a large excess of undissolved salt in the solution. If water is taken up by the solution, some salt will dissolve, if the solution evaporates some salt will precipitate, but the dissolution kinetics and homogenisation of the solution by diffusion (or osmosis) is fast enough to keep up with the changes of the water volume. Depending on the uptake/loss of water from your samples, you may need to add some water or salt during the equilibration process, but for rock samples, that probably won't be needed.

Pieter