Why is a hirsch funnel used

Robert Hirsch was born in Danzig in , the son of a prominent doctor and academic, Aron Simon Hirsch, a Jew who had converted to Catholicism. Robert became a chemist and studied for his PhD in Strasbourg with the one of the leading synthetic chemists of the day, Rudolf Fittig, graduating around with a thesis on chloroquinone imide. He soon had a job in the burgeoning dye industry, joining the firm of J W Weiler in the town of Ehrenfeld on the outskirts of Cologne.

The company embraced innovation and Hirsch worked in their research lab, publishing research into quinolines and cresols. He must have been good at his job, because in he was appointed director of the Kirkheaton Colour Works in Huddersfield in the English Midlands. He clearly loved working at the bench, since almost all of his publications appeared under his name alone, with occasional thanks to German colleagues for gifts of starting materials.

In , shortly after joining Kirkheaton, he published the little note that would immortalise his name. It is hard to believe that, until the s, the only way of recovering a solid product from a reaction mixture was either to decant off the supernatant liquor, or to pour the whole mixture into carefully fluted filter and then wait patiently until the paper dried before transferring the treasure to a suitable sample bottle.

Everything changed in when a young Swiss chemist, Jules Piccard, fresh from studies with Robert Bunsen, introduced the water vacuum aspirator, which increased the speed of filtrations enormously.

Aspirators were soon standard issue in all labs and suction filtration became the rule. But there was a problem: in conical funnels all of the force would be exerted at the tip of the cone of paper - even a skilled chemist would see his filtration ruined when the paper was punctured.

Filters were therefore fitted with small platinum cones to support the paper. By the Berlin chemist Otto N Witt had had enough. He suggested placing a flat, perforated disc Witt recommended platinum, but failing that, iron or porcelain into the funnel itself, covered by two pieces of filter paper. With the pressure distributed across the entire surface, punctures became a thing of the past. Then the solution is slowly poured slowly into the funnel.

Figure 2: Hirsch funnel with filter paper, all holes covered. When a funnel is placed on top of the filter flask, a neoprene adapter the black or gray "rubbery thing" or rubber stopper is used to ensure a good seal between the filter flask and the funnel. When the filtration is finished, the tubing from the filter flask should be removed before the vacuum is turned off. This is less important when the house vacuum line is used like in the instruction lab, but it will be when an aspirator is used.

The low pressure in the filter flask sucks the water back into the mother liquor, and if this is the part of interest, the experimenter is in big trouble. Also, a vacuum filtration is not suitable for solvents with low boiling points e. The solvent evaporates under the vacuum and the dissolved solids precipitate, clogging up the pores of the filter paper and get stuck on the insides of the funnel i. The same applies to hot solutions, since the solvent is close to its boiling point as well.

In those cases, gravity filtration is used. If a very fine precipitate has to be filtered, it is advisable to use some filter aid like diatomaceous earth or Celite. This will prevent that the holes of the filter paper will get clogged up too fast.

This technique is obviously not very useful if the solid is the interesting part, because it will be very time consuming to separate the product from the filter aid. Important Pointers : 1. Due to the fact that you are going to work with reduced pressure, special attention has to be paid to the equipment used. The glassware e. It is also imperative that the filter flask and the vacuum trap are clamped securely, so that they do not move anywhere.

The thick-walled tubing tends to act like a whip sometimes, and slams the filter flask into the hotplate or other metal objects which can cause an implosion , or just flips it over and you product is on the bench or even worse on the dirty floor. You probably don't have any desire to collect your product from the floor, do you?

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