Gas to Mass Volatilization Gravimetry Techniques Quiz

In volatilization gravimetry the analyte or its decomposition products are volatilized at a suitable temperature. The mass of the volatile product is determined either directly by collecting it or indirectly by measuring the loss in mass of the original sample.

The indirect method involves weighing the sample before and after heating. The difference in mass represents the amount of volatile component like water of hydration that has escaped into the atmosphere.

In the direct method the volatile substance is passed through a tube containing a chemical absorbent that specifically reacts with or traps that substance. The increase in the mass of the absorbent corresponds to the mass of the volatile analyte.

By heating a known mass of a hydrated salt to a constant mass the amount of water lost can be calculated. This is one of the most common laboratory applications of this technique.

Accuracy depends on ensuring that the mass loss is exclusively due to the analyte. If the remaining solid also decomposes or reacts with air while hot the results will be skewed. Samples should always be cooled in a desiccator before weighing.

A desiccator contains a drying agent that removes moisture from the air. This prevents the hot dry residue from reabsorbing water vapor from the atmosphere as it cools to room temperature.

Adding an acid to a carbonate sample triggers a chemical reaction that releases carbon dioxide. In a direct method this gas is then swept into an absorption tube containing a base like sodium hydroxide.

Constant mass is achieved when two consecutive weighings differ by less than a tiny specified amount. This ensures that all volatile components have been completely removed.

In organic microanalysis the sample is burned in oxygen. The carbon is converted to carbon dioxide and hydrogen to water. These are then trapped in separate absorption tubes to determine the original mass of carbon and hydrogen.

These substances have a high affinity for water and are used to maintain a dry atmosphere inside a desiccator. Distilled water would provide moisture defeating the purpose of the device.

A thermogravimetric curve plots mass as a function of temperature. It shows the specific temperature ranges where different components volatilize allowing the chemist to choose the correct heating temperature.

When sodium hydrogen carbonate is heated it decomposes into sodium carbonate water vapor and carbon dioxide gas. Both water and carbon dioxide are lost so the calculation must account for the stoichiometry of both volatile products.

Some samples may splatter as moisture escapes rapidly. A lid prevents the physical loss of the solid residue which would result in an erroneously high calculated mass loss.

By using a specific absorbent the direct method ensures that only the analyte of interest is measured. The indirect method is nonspecific because if any other impurity volatilizes along with the analyte the mass loss will be attributed entirely to the analyte.

The percentage is calculated as mass loss divided by initial mass multiplied by 100. In this case 0.180 divided by 1.000 multiplied by 100 equals 18 percent. This is a fundamental skill in analytical laboratory work.