<div dir="ltr">Dries,<br>As always I appreciate your observations based on your experience. A few comments:<br><br>On Thu, Jul 3, 2014 at 1:46 AM, <span dir="ltr"><<a href="mailto:A.J.van_Dam@lumc.nl" target="_blank">A.J.van_Dam@lumc.nl</a>></span> wrote:<br>
<div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div style="direction:ltr;font-family:Tahoma;color:#000000;font-size:10pt">Formalin is not stable! </div></div></blockquote><div>Of course formalin is not stable, that is why it has to be buffered. If it was stable, we could merely neutralize it and be done. In any case, I said the buffering system was stable, not the formaldehyde.<br>
</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div style="direction:ltr;font-family:Tahoma;color:#000000;font-size:10pt">We noticed in our fetus collection stored on phosphate buffered formalin (pH 7.3) that after ten years pH is around 6, after 20 years around 5, and after
30 years around 4 (close to unbuffered formalin). </div></div></blockquote><div>What you are describing is acidification of a formaldehyde solution; what Greg described as a solution becoming sufficiently alkaline to cause clearing of tissues. One has to be cautious about extrapolating from one sort of preserved specimen to another. A human fetus has a very different surface-to-volume ratio than do amphibian larvae, more lipids (your formaldehyde may be acidifying due to the breakdown of lipids into fatty acids), and the overall specimen volume to preservative volume is likely to be very different for a human fetus and a clutch of amphibian larvae--all of these can be factors in pH change in formaldehyde solutions.<br>
</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div style="direction:ltr;font-family:Tahoma;color:#000000;font-size:10pt">Therefore, when using buffered formalin as a preservation fluid, I recommend to change the fluid every 10 years.<br>
</div></div></blockquote><div> I advise monitoring the pH of the formaldehyde solution as well as visually watching for signs of clearing (which would indicate a shift to alkalinity) or evidence of acidification. If there are problems, then either adjust the buffer or, in worse case scenarios, change the formaldehyde solution. I do not recommend changing preservatives unless you have identified some problem that a fresh preservative solution is likely to resolve. Changing to fresh preservative solutions (whether formaldehyde-based or alcohol-based) can create other problems.<br>
<br></div><div>--John<br></div></div></div></div>