High-refractive mounting media (PLEURAX / NAPHRAX / ZRAX)

Information published on the net has been checked by synthesising the above mentioned mounting media several times with slightly different methods. In case of NAPHRAX an improved synthesis is given. Here are the results:



The refractive index of a substance depends solely on the number of easily movable pairs of electrons per volume unit. The higher this number is, the higher is the refractive index. Therefore the refractive index increases in this order: polar aliphates < non-polar aliphates < non-polar aromates. In addition large atoms increase the refractive index markedly: oxygen < sulfur, nitrogen < phosphorus < arsenic, chlorine < bromine <iodine. For non-polymers this means: the higher the refractive index of a monomer is, the more toxic, the more volatile and the more sensitive it is to oxidation. The toxic and volatile methylene iodide is a good example of this rule, and also the ingredients of extremely high refractive mounting media: triphenylarsin or arsenic sulfide. So it is improbable to sythesise polymers whose refractive index exceed significantly that of PLEURAX when using "civilian" precursors.



Melt 110 grams of crystalline phenol in a 250 ml beaker on a magnetic stirrer. Then add 40 grams of powdered sulphur and slowly heat the mixture up to 150 oC (internal thermometer!). Add a spatula tip (approximately 100 mg) of anhydrous (!) sodium carbonate (catalyst) and raise the temperature to 170 oC, stirring continuously. At 160 oC the reaction starts with gentle foaming and H2S is set free. Cover the beaker with a piece of cardboard to avoid the evaporation of the phenol. Check the production of H2S with lead acetate paper from time to time.

Stir for 4 hours. From time to time take a small sample with a glass rod and dissolve the product in a test tube containing about 5 ml of isopropanol. The sample must be solved completely, otherwise sulphur will still be present. After about two hours the reaction is nearly complete resulting in a dark brown resin, which is yellow in thin layers. Continue heating for four hours and check the production on H2S with wet lead acetate paper. If even then some sulphur should be left add some more phenol and sodium carbonate and keep heating. The reaction temperature is not critical, but should not exceed 170 oC, otherwise too much phenol will evaporate and sulphur will spoil the product. When the reaction is finished remove the cardboard and heat for another hour to remove phenol. The volume will decrease significantly. The resulting product still has the smell of H2S, but this odour disappears with time. Some phenol in excess is no problem but will decrease the refractive index.

Allow the resin to cool down to 100 oC, continue stirring, then add 50 ml of isopropanol. After a homogeneous solution has formed, pour it into 50 ml vials, filling them to two-thirds. If the viscosity of the cold mixture should be too high, add some more isopropanol and heat in a laboratory oven to obtain a homogeneous solution.

Yield of pure resin: about 60 grams.

After MELLER, MIKROKOSMOS, slightly modified.


1. Phenol is usually coloured slightly red by oxidation products. These impurities do not hamper the synthesis, they even act as a catalyst. Freshly distilled in vacuum phenol is colorless, but in this particular case not reactive enough.

2. Allow exceeding sulphur to crystallise, then pour the now sulphur-free PLEURAX into another vial.

3. PLEURAX is soluble in isopropanol and acetone, but not in toluene or xylene.

4. When preparing diatoms, put a small sample on an object slide, let it dry completely, cover it with one drop of isopropanol to remove the air from the frustules, then cover with PLEURAX, add a coverslide and remove the solvent by heating. Only dry PLEURAX has the high refractive index wanted. The yellow colour is no drawback.

5. The long term stability of PLEURAX is unknown. PLEURAX consists of benzene rings with OH-groups connected via sulphur bridges. Such a polymer may be susceptible to oxidation: sulphur bridges may be oxidised to SO2-bridges (clouding of the slide), phenolic OH-groups may be oxidised to quinones (darkening of the polymer). Whether such changes actually occur in the course of time is unknown. Protecting the slides with a ring of varnish is recommended.

6. PLEURAX is said to have a refractive index of about 1.73. According to my measurements the refractive index is 1.68. This diversion may be a consequence of the grade of polymerisation dependent on the quantity of sodium carbonate used (smaller quantities should enhance the grade of polymerisation). PLEURAX has a high dispersion and shows fluorescence when blue light is used.

7. PLEURAX is unsuitable as a mounting medium for UV-microscopy because wavelengths below 400 nm are blocked.

8. The synthesis is safe and requires neither much time nor much equipment. The product is ready for immediate use without further treatment.

9. Due to the high toxicity of H2S an effective fume hood is required!

10. The synthesis of PLEURAX is preferable in comparison to NAPHRAX.




Improved synthesis


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 homogenous 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 vapours 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 colourless 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. Allowe 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 deacidification and stirr 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. Sincs 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 deacidification and the removal of the precipitate NAPHRAX thus produced is quite usable though - in thin layers the dark colour 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)!



Original link: http://www.sas.upenn.edu/~dailey/zrax.pdf . This link is now inactive, and a new one could not be found.

ZRAX was distributed by Professor Dailey, but no information concerning the synthesis is available. The above source leads to the assumption that ZRAX is nothing else than NAPHRAX, synthesized under special conditions and carefully purified . Furthermore, the source provides the information that NAPHRAX may not have a long-term stability.

Meanwhile Mr.Matthias Burba gave me a small original sample of ZRAX, so an infrared spectrum could be made. This yellow resin is obviously identical with NAPHRAX produced with p-toluenesulfonic acid as catalyst. Whether the production method is completely the same must remain open to dispute. (The slightly different shapes of the IR-peaks can be explained by the different thicknesses of the samples - what is essential are the positions of the absorption bands).

The refractive index of ZRAX is 1.68 and thus is slightly higher than that of NAPHRAX. This difference suggests a higher degree of polymerization, combined with a slightly higher density.