Re: PEG corrodes iron Carowax 4000 became 3350

[Guest guest]

wow! will add to future issues list.

* Sometime around 1981, the following changes were made:Carbowas 1540 became 1450Carbowax 4000 became 3350Carbowax 6000 became 8000

* PEG corrodes iron (MacLeod, I. D., Kenna C., 1990.)

* PEG corrodes all metals but especially iron (Hamilton 1998)

Iron compounds will depolymerize PEG at high temperatures (Hoffmann1982)

Seems that iron salts degrade higher molecular weight PEG (3350) abit, but not as notable for lower mw like PEG 400. (Bilz et al 1993)

* Mineral deposition or replacement in outer layers is common inarchaeological wood, especially with iron. Sometimes these can be seenas casts of material that is now lost. Chelating agents etc remove theiron. But iron also interacts badly with PEG. Early warning in thisarticle of problems with iron salts and PEG. Even if the soil has lowiron content, there is sometimes high iron content in the wood. (1982)

pH decreases with artificial aging and the decrease is greater forhigher temperatures, this might be indicating formation of acidicdegradation products. Excluding air (oxyen) slows the formation ofacids. Higher concentrations of PEG have less dissolved oxygen.However, because it is a mixed solvent system, 4 might be neutral andnot acidic since pH refers to a water system? (Bilz et al 1993)

* In general, the molecular weight (measured as intrinsic viscosity)decreased as PEG was artificially aged and more so with heat. (Bilz etal 1993)

* Heating during PEG impregnation causes darkening of the surface

* Size of the PEG molecule is hard to determine because it can coil, fold, twist etc.(Brownstein 1982)

* PEG does not degrade much with changes in pH although it happens a bitmore if heated. (Brownstein 1982)

* Thermal aging increases PEG degredation (Bilz et al 1994)

* For PEG use in chromatography, it is important that PEG not degradeand they use temps up to 250C, but oxygen is also eliminated. In thepresence of oxygen, PEG decomposition from heating is usually seen indarkening, less viscosity, and lower pH. PEG degradation occurs byrandom chain scission. (Brownstein 1982)

* PEG ages more slowly in wood than by itself . Lignin may act as an anti-oxidant. (Grattan 2000)

* PEG is chemically unlikely to crosslink with the constituents of the wood. (Grattan 2000)

* Determining where the PEG goes in the cell structure is usually donewith cobalt thiocyanate staining. The stain inhibits autofluoresence oflignin (PEG does not affect fluorescence) and the stain only bonds withPEG, so if the fluorescence is quenched it shows where the PEG has gone.(Young and Wainwright 1982)

Beginning impregnation with a low percentage of PEG is important toprevent osmotic collapse. (Grattan and e 1987)

Lower mw PEG penetrates the micro-capillaries of the cell wall, whilehigher mw penetrates the lumens, flows through the vascular system(Grattan 1986.)

PEG 1450 is still expected to penetrate the cell wall (Young andWainwright 1982)

* Comparison of Air Dried and Freeze Dried Solutions of PolyethyleneGlycol 3350: They expected all to line the walls and bottom of beakers.Air dried samples had consistency of beeswax and occupied only thebottom the beaker, might have lined walls if they were more porous?Freeze dried samples had a powdery matrix appearance and occupied thefull volume that the frozen water/PEG mix had originally occupied. Forthe freeze-dried, when they were returned to low temperature, higherconcentrations that had been freeze-dried (20% and above) showed someconcentrated PEG solution at the bottom of the beaker that had not beenable to get past plugs of PEG. 5% did not have that, so water must haveescaped. (Jeberien and Bilz 2000)

Does PEG modify the crystal size of water? (Viduka, 2002)

PEG darkens considerably if heated over long periods, and presence ofoxygen seems to enhance the darkening. (Brownstein, discussion 1982)

http://ellencarrlee.wordpress.com/2009/04/08/what-do-we-know-about-peg/