On Tue, 23 May 2000, John C. Dennis wrote:

> A 4% paraform./PBS prep has gotten out of hand.  The paraformalin will not
> go into solution.  The pH is at least 10 (by ph paper).  Temp. is 65
> deg. C.

 I've had this happen too, but not for many years. It was the
 paraformaldehyde that was at fault. The stuff from another
 stock bottle behaved correctly. I don't think the degree of
 alkalinity has much effect on the depolymerization ("dissolving").
 Either the basic component of the phosphate buffer or a little
 NaOH, seems to work equally well, with the suspension quite
 quickly becoming transparent when the temp reaches 60C or a
 few degrees higher.

 I don't know what goes wrong with paraformaldehyde. Possibly
 occasional batches are excessively polymerized when made, but you'd
 expect the vendors to check that the stuff is capable of generating
 a transparent formaldehyde solution as part of their quality
 control. The age of the powder doesn't seem to matter. 

 While thinking of age: some older bottles of paraformaldehyde 
 powder have "(Trioxane)" as a synonym. This is wrong. Trioxane
 is a cyclic trimer of formaldehyde, said to have a pleasant
 chloroform-like smell. It dissolves in water but doesn't
 depolymerize unless quite a high concentration of a mineral
 acid is present, and then only slowly: for example, it takes
 20 hours to release 10% of the formaldehyde from trioxane
 in 1.0N sulphuric acid at 70C.  No, I didn't recall this from
 a high-school chemistry lesson. It's in J. F. Walker's book,
 "Formaldehyde" (3rd ed. 1964, p.196. Krieger, New York). There's
 a 1975 reprint of this classic - the version I have because I
 spotted it in a 2nd-hand book shop - also published by Krieger.
 What I do still remember from chemmy teacher (the late Mr Arthur
 Mears) is that paraldehyde is the cyclic trimer of acetaldehyde,
 and that it is quite easily hydrolysed by dilute mineral acids.
 This reaction is, of course the source of the acetaldehyde that
 changes pararosaniline into another compound when you make up
 the aldehyde-fuchsine stain.

 Enough trivia.

 John A. Kiernan,
 Department of Anatomy & Cell Biology,
 The University of Western Ontario,
 LONDON,  Canada  N6A 5C1