Pleurax synthesis?

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Pleurax synthesis?

#1 Post by MicroBob » Fri Jul 31, 2020 6:21 pm

Hi together,
apparently there are variables that can lead to different product qualities when synthesizing Pleurax. One obvious difference is the colour of the mountant that can be between light yellow and a lot darker brown.
Colleagues of our microscopy group have made Pleurax and Naphrax several years ago and described the Pleurax process here: ... ittel.html
The resulting Pleurax works fine but is fairly brown in colour.

Before you think about doing this: You would need the necessary knowledge to do is yourself and a good laboratory fume hood - garden and wind from the back could prove deadly!

Does somebody have done this himself or does somebody have a recipe that goes beyond the recipe of my colleagues?

Pleurax 1 1024.jpg
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Re: Pleurax synthesis?

#2 Post by Charles » Fri Jul 31, 2020 8:14 pm

I've never made either but here is the steps for Naphrax. I haven't seen any warnings like for Pleurax but I would recommend a fume hood.


-Glacial acetic acid 200 ml
-Naphthalene 50 g
-Paraformaldehyd 12,5 g
-p-Toluenesulfonic acid (monohydrate) 5 g

500 ml flask, water bath, centrifuge, magnetic stirrer, distilling apparatus, heating mantle

Mix naphthalene, toluenesulfonic acid and glacial acetic acid and heat in a water bath until a homogeneous mixture is obtained, then add paraformaldehyde, distribute the powder by gentle shaking and heat for 72 hours in a water bath at 95 oC. Cover the water with 1 cm of oil to prevent evaporation. Immerse the flask only partially. Insert a plug and put a piece of cardboard between the plug and the neck to allow vapors to escape, Shake the flask gently from time to time..
The praformaldehyd is completely dissolved after a few hours and after about 18 hours a colorless liquid resin starts to precipitate, which becomes solid even at 95 oC. Towards the end of the reaction the resin is honey yellow and opaque. Allow the mixture to cool, pour off the liquid phase and wash quickly with toluene. Loss of material must not be feared because the resin dissolves very slowly in cold toluene.

Purification / Method 1
Dissolve the resin in toluene at 95 oC and heat until a clear honey-yellow solution is obtained. Add a teaspoon of marble grains for de-acidification and stir at room temperature (magnetic stirrer) at least for three days. The white powder precipitated during this time is centrifuged (filtration is not recommended). Then the toluene is removed by distilling (heating mantle!) along with a small amount of water and some acetic acid. The result is a honey-yellow, clear, hard resin. This is covered again with about 1 cm of toluene and again heated up to 90 oC . The solution so obtained is ready for use even if some traces of acetic acid should be still present.
Addendum: Wait for another two weeks to allow precipitation (the white precipitate will adhere on the inner surface of the flask), then pour the now crystal-clear solution into small vials.

Purification / Method 2
Pour the crude product, dissolved in toluene, into a separating funnel and add about 100 ml of water. Shake gently. After a few hours three layers will separate: the top phase contains the resin dissolved in toluene, the bottom phase is a mixture of water and acids. In between you get a stable white paste-like emulsion. Since this paste cannot pass the tap of the separating funnel the toluene layer must be siphoned off. Stir the separated resin solution for several hours with granular calcium chloride to absorb water, then, the toluene will be largely removed by distilling. Then proceed as described in the section "Method 1". The solution is free of acids, it secretes no precipitate even after a long time, but this method is associated with a loss of material because of emulsion formation.

1. The method published recommends 20 ml HCl conc. and 5 ml of H3PO4 conc. as a catalyst. The resin so obtained is nearly black and the solved resin exhales HCl for a long time. The method does not avoid the formation of the white precipitate. After de-acidification and the removal of the precipitate NAPHRAX thus produced is quite usable though - in thin layers the dark color is of no disadvantage - but the resin may still contain reactive CH2Cl-groups which may lead to aging. The use of p-toluenesulfonic acid represents a significant improvement!
2. The entire synthesis can be done in an apartment. But the synthesis is very time-consuming, the raw product must be purified in any case and a variety of laboratory equipment is needed.
3. The composition of the white precipitate could not be determined yet. The IR-spectrum indicates a mixture of several substances, paraformaldehyde is definitely absent.
4. When preparing diatoms, put a small sample on an object slide, let it dry completely, cover it with one drop of toluene to remove the air from the frustules, then cover with NAPHRAX, add a coverslip and remove the solvent by heating (100 oC). Only dry NAPHRAX has the high refractive index wanted. Thin layers of this mounting medium are nearly colorless.
5. NAPHRAX has a refractive index of 1.66, shows less dispersion than PLEURAX but is stimulated to fluorescence by blue light .
6. NAPHRAX is suitable for UV-microscopy, as wavelengths down to 340 nm are transmitted.
7. NAPHRAX consists of naphthalene rings linked together by CH2-groups - so the structure is similar to polystyrene. Such hydrocarbons are chemically extremely inert, the resin should have a high long-term stability. It is however important to remove the white precipitate completely (see above)!

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Re: Pleurax synthesis?

#3 Post by Hobbyst46 » Fri Jul 31, 2020 8:50 pm

From reading about Pleurax:

Pleurax is the product of heating phenol with sulfur in the presence of caustic soda or carbonate as catalyst. The reaction is deceivingly simple: The resin is most probably a mixture of several compounds of phenol and sulfur. The protocol does not suggest any accurate indicator of the exact final composition of the resin. The quality of the starting materials - phenol and sulfur - does not guarantee a defined purity level of the product. Index of refraction and viscosity are qualitative, not quantitative measures. I believe that the final color of the resin could change from batch to batch. Moreover, tiny residues of phenol in the final product can become oxidized with time and develop a into a dark tint of the resin.

So I keep my Pleurax in the dark, and hope that it will still be usable after years.

Edit: to clarify my point - the protocol does not specify exactly which molecule (of the various possibilities) is responsible for the index of refraction and the viscosity of the resin (and color and any other feature).

Edit no 2: Just revealed this protocol - from Czechia
Last edited by Hobbyst46 on Sat Aug 01, 2020 6:35 am, edited 1 time in total.
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Re: Pleurax synthesis?

#4 Post by MicroBob » Sat Aug 01, 2020 6:16 am

Good morning Charles and Doron,
thank you for your replies!
I have a bottle of Naphrax that has been made after this recipe. I also have slides from about 20 years ago, made with Naphrax, where crystallisation creeps in from the edges. As far as I know this is not uncommon with Naphrax.

In practical use I like Pleurax a lot. It seems to fill the frustules especially well and it is comparatively easy to make good looking slides. A member of our group thinks about making some Pleurax and it would be nice to profit from experiences that have been made in recent years. The Pleurax from the Czech article turned out brown. The available dutch Pleurax apparently can be yellow or brown depending on the batch. In any case one seriously should have sound chemical knowledge and a good fume cabinet to try this. I don't have both and wouldn't attempt to cook Pleurax.


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