Need Advice on Material Weighting

You need an enclosure that dehumidifies the air. I've seen these things in laboratories. The scale is inside a glass case. You open the case, put in your samples, and close the door.

In our factory, one of the laboratory's rooms is humidity controlled. This is where they weigh hygroscopic materials.

You have made a good observation.

In interpreting your observation, Vulcan is correct. Calcium chloride is widely used as a desiccant. I remember cannisters for crackers which had an indicator-colored calcium chloride filled glass knob in the lid. When it turned pink you put it into the oven to regenerate it (which turned it blue). My old Chemical Rubber Company Handbook of Chemistry and Physics says that if your air's moisture content is above 1.5 mg / liter of air at 30 deg. Celsius then it will be absorbing moisture and its weight will be increasing. That is only 5% relative humidity! The method to use, if I remember my lab days correctly, was to dry your specimen by heating at a temperature above that at which the bound water is driven off, then cover it while it cools to room temperature and weigh it. Subtract the tare weight of the covered container to get your specimen weight. Repeated cycles of heating and weighing will ensure that you are dealing with the actual dry weight. If you are trying to determine the amount of water it has absorbed, then you need to cover it (minimizing the amount of air inside the cover) before weighing, then dry it after this and determine its dry weight. Otherwise, you will continue to see the shifting of weight you have observed.

CaCl2 (anhydride) has a melting point of 772 deg C. The tetra hydrate loses two waters at 30 deg C and the final two at 200 deg C, so if you want a weight of the CaCl2 only, heating to over 200 deg C is required. If you weigh on a balance inside a glass case, all you will exclude is the moving air; the weight gain due to its pulling water from the air will continue because the balance's enclosure is fairly big compared to your sample. Vulcan's suggestion here was good, but not good enough. At the sensitivity you mentioned (0.1mg), approximately 10 cm3 of air at 40% relative humidity and 30 deg C will contribute 0.1mg of moisture to your sample! Try using a glass petri dish with a mating glass cover--the volume of air enclosed in this will be much closer to the desired 10 cm3 of air over your sample.

Keep asking good questions and keep looking for the things which seem to "not fit". That was how "teflon" was discovered. JMM

Jmueller,

Thanks for the correction. 'Don't want to be giving bad info here.

However, the glass enclosed scale that I mentioned seemed to be different. I know about the type that prevents air movement. The one I saw had a tube running into the case. I asked the lab tech what it was and she said it was supplying dehumidified air. Of course, now that I think about it, a precision scale's reading would be affected by the draft but I can't remember the precision of the one I saw. 'Don't go into the lab anymore. Haven't been in there for several years in fact!

Vulcan-- Rapid responses are something I am getting used to, being a newbie on internet discussion rooms. From the original post, I couldn't determine if the question was to determine the mass of a dry sample, or to determine the mass of a partially hydrated sample, hence the suggestion of minimizing the amount of air exposure by covering the sample. At the higher levels of sensitivity mentioned, thermal equilibrium is important, so the sample may have to sit a while before being placed on the balance. I didn't know about the use of a tightly-closed scale enclosure supplied with a source of dry air--pretty good! Its been many years since I was in the chemistry laboratories, so accepted practices as described in good textbooks now probably cover the topic better than my musings. . .

Thanks--John M.

Thanks for both of you very much on the answer.

At Lab, I working, we have a very limit eqiupment (here in thailand -_-)

but i will try for both alternative that you provide.

The information about calcium chloride that you provided help me a lot.

Now I looking for some chemistry text to refresh my memory.

It's been a couple years since I finished my B.Eng. Chemical

Thanks again Vulcan and Jmueller

A little late answering, but just in case you're still listening:

A good proceedure before working with any chemical substance with which you are
unfamiliar is to obtain the MSDS, (Material Safety Data Sheet), from a chemical
supply house. Merck for instance. - They're on the web.

Go to the list of products, and with each product you will find there is a path to
'view MSDS`.

It gives you a lot of cautions more useful to bulk handlers than experementers, or lab. tech.s but also a list of physical characteristics, (in this case including the various hydration states, their molecular weights, and the temperatures at which they reverse.)

It also lists substance class, (Corrosive, oxidizer, acid, Base, etc.), things to avoid mixing with, good things to know when working with unfamiliar substances.

As a hobbyist, I make it a practice to read the MSDS for everything I propose to work with before I even open the package.