For TG measurements, it usually suffices if the sample containers were previously baked at elevated temperatures. Any inert residuals generally do not negatively influence the measurement result.
The situation, however, is quite different when it comes to DTA or DSC investigations. To achieve good baseline stability, it is important to use clean, baked crucibles with roughly the same masses. Only if the heat capacities of the sample and reference side are somewhat similar to each other will the baseline drift for correction measurements be accordingly small.
Crucibles of alumina or platinum (or more precisely of platinum /iridium for DTA and of platinum/rhodium for DSC measurements), which are mainly employed in higher temperature ranges, are reusable and designed for a longer working life. Unfortunately, measurements often leave contaminations in the crucibles which must afterwards be removed. The cleaning process recommended will depend very strongly on the type of contamination.
If the residuals are of organic nature or are deposits of carbon black, e.g. pyrolytic carbon as a decomposition product of plastics or elastomers in a nitrogen atmosphere, they can be burnt by heating in an air flow. This preferably takes place in the instrument itself or in a separate (electric) furnace at moderate temperatures, typically between 600 and 800°C. Baking out should be done at temperatures above the planned working temperature. If a furnace (muffle furnace or similar) is not available, Al2O3 crucibles can alternatively be baked with a Bunsen burner in the oxidizing range of the flame (outer range).
The temperatures on the surface and at the edge of the Bunsen burner flame reach values of up to approx. 1400 – 1500°C. In platinum, contaminations can diffuse into the metal during baking in this temperature range. The Bunsen burner technique is therefore less suited for platinum crucibles.
Platinum crucibles are conventionally cleaned by boiling the sample containers in appropriate solvents such as water or acids. (Please refer to the compatibility tables published here). Encrustations and surface deposits can be removed in an ultrasonic bath or by mechanical treatment (scouring) with a fine grained powder (e.g. Al2O3 powder or sea sand). Then soak in 10% hydrochloric acid (many oxides form water-soluble chlorides in hydrochloricacid), rinse with water and bake.
For cleaning very heavily contaminated alumina crucibles, the following process has proven itself: soak the crucibles for at least 24 hours in aqua regia (3 parts concentrated hydrochloric acid and 1 part concentrated nitric acid) and boil for 3 hours in this solution (aqua regia dissolution). After cooling, rinse with water and – if necessary – then clean mechanically in an ultrasonic bath. To neutralize the crucibles, boil in a 2% –5% ammonia solution, rinse again with water, and finally bring to a boil in distilled water. Prior to reuse, bake at 1500°C for one hour.
Besides smelting with potassium pyrophosphate (where the crucibles are filled with it and then are slowly baked to red heat), the following cleaning process is also recommended for stubborn contaminations in platinum crucibles:
Soak the crucible in diluted hydrofluoric acid for at least 24 hours and then boil for 3 hours. After cooling, rinse carefully with water and – if necessary – mechanically clean in an ultrasonic bath. Rinse again and finally boil in distilled water to completely remove the hydrofluoric acid. Prior to reuse, bake at 900°C for one hour. Since hydrofluoric acid corrodes glass, it is recommended to use PTFE containers.
Please keep in mind that both aqua regia and hydrofluoric acid are extremely corrosive and toxic. Hydrofluoric acid is a contact poison which is immediately resorbed by the skin and is therefore classified as being in the“very toxic” hazard category. Please take safety measures when working with these substances and use safety goggles, protective clothing and gloves.