Washing, Drying, and Ignition of the Precipitate

The ingredient sought is transformed into a material (of known composition) that may be isolated from the sample and weighed in gravimetric analysis. Preparing a solution that contains a set weight of the sample, weighing the constituent that is isolated, setting apart the desired constituent, and computing the amount of a particular constituent in the sample from the observed weight of the isolated substance are the steps commonly followed in gravimetric analysis.

Precipitation—that is, transformation into a material that is not soluble in the solution—is the most common approach for separating the desired constituent from a solution of a sample. A reagent is added that creates an insoluble compound with the target constituent while leaving other sample constituents alone. Filtration separates the precipitate, which is then washed free of soluble contaminants, dried or burned to remove water, and weighed. In this post, we will study washing, drying, and ignition in gravimetric analysis.

Washing of Precipitate

A precipitate can be washed directly on the filter or partially on the filter and partly by decantation. The precipitate is allowed to settle after decantation, and the supernatant liquid is placed onto the filter. After the precipitate has settled, wash water is added, and the decantation is done a few times before the precipitate is transferred to the paper or Gooch crucible.

Whatever procedure is employed, it must be thorough; both the precipitate and the paper or asbestos must be thoroughly rinsed to remove all traces of the original solution. This must be determined through meticulous testing; speculation based on the “good enough” approach is a definite way to inaccuracy and must never be allowed. It may appear that the chemist makes educated judgments about a washing’s thoroughness. This is not the case because he knows from the experience gathered from many analyses that four washings are sufficient to remove some salts.

Unless otherwise specified, one should use hot water to wash the precipitate. If you are using filter paper, then each washing must be allowed to drain before adding another. In this approach, a smaller amount of water is necessary to complete the task. This is critical because a large majority of the filtrate is undesirable when subsequently processed. When using the Gooch method, the washing water is added before the crucible is emptied. A small amount of liquid remains above the filter until the final wash, after which the crucible is drained.

Precipitate particles stuck to a beaker can be rubbed away with a piece of rubber tubing on the end of a glass rod. One of the highly gentle qualitative tests must be used to determine the thoroughness of a washing. If the filtrate is being saved for subsequent processing, tests should not be performed until the third wash, as a portion of the filtrate will be lost.

If both chloride and nitrate are present in the solution, it is permissible to test the washings for chlorine alone and assume that if the chloride is eliminated, the nitrate has been removed as well because of their solubility. This idea is supported by the fact that chloride qualitative tests are more delicate than nitrate qualitative testing.

Drying and Igniting Of Precipitate

After removing the precipitate from the solution, it must be dried and weighed in some circumstances and burned in others before being weighed. The drying and igniting processes, as well as the essential equipment, will now be discussed. Water or air ovens are commonly used to dry precipitates. The water oven is used when the drying temperature does not reach 100° C, and the air oven is used when the temperature does surpass 100° C.

When directed to dry a precipitate in the water oven at 100° C., it is sufficient to heat the water to the boiling point and transfer the precipitate to the oven. The boiling point of the liquid in the jacket can be enhanced by adding a pinch of salt to the water if required.

When a precipitate is to be dried on paper, the funnel and paper are placed in the oven, the stem of the funnel is passed through one of the holes in the shelf, and the funnel’s top is covered with filter paper to avoid contamination. The oven’s burner is lit, and heat is applied until the precipitate appears to be dry and no more vapour rises from its surface. The door is closed during the drying process and only opened for removal or inspection.

The drying process can also be done conveniently and rapidly in an air oven if the temperature is carefully controlled. With practice, the learner will be able to adjust the Bunsen flame to keep the oven temperature within a few degrees of the desired temperature, but for more precise work, a gas regulator (also known as a thermostat) should be installed in the oven. 

What is the Difference Between Drying and Ignition?

After understanding the concept of washing, drying, and ignition in gravimetric analysis, let’s differentiate between drying and ignition of the precipitate.

What is the Purpose of Igniting the Precipitate?

Having understood the difference between drying and ignition, let’s understand the purpose of igniting the precipitate.

The goal of drying and ignition is to create a compound with a known and constant composition. Ignition can also be used to break down a solid into a known-composition chemical. The ignited sample is cooled for a few minutes before being placed in a desiccator to prevent moisture adsorption and weighing on a chemical balance. ]

Conclusion

That’s a wrap to the washing, drying, and ignition in gravimetric analysis. The final precipitate must have a well-defined composition for a quantitative application. A precipitate containing volatile ions or significant volumes of hydrated water is typically dried at a high enough temperature to eliminate these volatile species. One typical gravimetric method for determining magnesium, for example, is to precipitate it as MgNH4PO4•6H2O. Unfortunately, drying this precipitate without losing an irr=egular amount of hydrated water and ammonia at lower temperatures is challenging. Instead, the precipitate is dried at temperatures above 1000 degrees Celsius when it decomposes into Mg2P2O7, magnesium pyrophosphate.