10% Formalin (4% Formaldehyde)
From Paraformaldehyde

10% Formalin from paraformaldehyde
Paraformaldehyde 40 g   Heat the mixture at 60°C – 80°C until it becomes clear. Allow to cool, and adjust the pH to 7.2 – 7.4.
2.26% sodium dihydrogen phosphate 830 mL
2.52% sodium hydroxide 170 mL

 

Methanol and formic acid free 10% NBF
Make a solution of the buffer salts used for NBF.
Add 40 grams paraformaldehyde per litre and heat at 60°C – 80°C until it becomes clear.
Allow to cool.

Formalin solutions made from paraformaldehyde are identical to formalin solutions made from strong formalin with two exceptions, that they do not contain any methanol, and when first made they do not contain any formic acid.

Methanol is added to strong formalin during manufacture to inhibit polymerisation of the formaldehyde to paraformaldehyde and a residue remains in diluted formalin mixtures. This may be up to 1%, as the strong formalin may contain up to 10% and the usual dilution is 1:10. Apart from this insignificant amount of methanol all solutions of formalin are the same whether made by diluting strong formalin or from heating paraformaldehyde. The only justification for using a solution made from paraformaldehyde in preference to one made by dilution is if the material being investigated is affected in some fashion by the presence of trace amounts of methanol, otherwise dilution made formalin solutions should be satisfactory.

Formic acid developes in formalin solutions on standing. Although a formalin solution made from paraformaldehyde will be formic acid free when freshly made, it will begin to form it immediately, and as it ages the amount will increase, as with any formalin solution. Buffer salts can compensate for this.

For all other uses of formalin made from paraformaldehyde, please refer to the formalin variant fixatives. They may be substituted directly, keeping in mind that it requires 40 grams paraformaldehyde in a litre of water to make 10% formalin (4% formaldehyde). It will usually be found more convenient to depolymerise the paraformaldehyde if the mixture is slightly alkaline while being heated. The pH may then be adjusted after the solution has cooled.

 

Reference
Bancroft, J.D. and Stevens, A.,
Theory and Practice of Histological Techniques, Ed. 2,
Churchill Livingstone, Edinburgh, UK.

Kiernan, J.A.,
Histological & Histochemical Methods, Ed. 3,
Butterworth Heinemann, Oxford, UK.

 


 

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