Almost any water miscible, anhydrous fluid can be used as a dehydrant providing that it does not damage the tissue proteins and is also miscible with the fluids to be used subsequently. Cost may also be a factor.
All of the commonly used fluids are inflammable and suitable precautions must be taken. There should be fire fighting equipment readily at hand, and it should be placed in an area that is accessible should a fire happen. Equipment mounted on a wall just outside the room used for processing is a convenient place. The equipment should include a fire blanket and fire extinguishers suitable for use on a solvent fire. The advice of a fire marshall should be taken with regards to the type of equipment and its placement. If there is a fire that is beyond the capabilities of simple equipment, leave the area immediately and call the fire department. Do not take chances with your life.
In some jurisdictions it is possible to purchase recycling equipment for these reagents. These are fundamentally stills and the fluid is repurified by distillation. In the case of ethanol it is customary to consider the distillate as having 96% purity due to contamination by the water removed from the tissues during use. It can be used for most purposes, but may contain unidentified contaminants from the tissue and other sources, depending on what use was made of the ethanol initially.
The most convenient anhydrous fluid available at reasonable cost, and for most technologists the dehydrant of choice, is ethanol, a common industrial chemical manufactured in bulk. It is available almost completely water free as absolute ethanol, although the slightly less expensive 96% ethanol may be used for the initial few changes.
We tend not to think of ethanol as a toxic chemical but it certainly is. Ingesting relatively small amounts of pure ethanol can kill. It should be remembered that there are two sources of industrial ethanol. One source is from fermented sugars, often from maize, which is then concentrated by distillation. However, the maximum purity obtained by distillation is about 96%, and to obtain absolute ethanol requires the addition of benzene to the ethanol before re-distilling, traces of which remain in the absolute ethanol. Benzene is a known carcinogen. The other source of ethanol is chemical manufacturing from natural gas or oil sources, with trace amounts of who knows what.
Some jurisdictions restrict the availability of pure ethanol as it is the main constituent of liquors and a common recreational drink. From concern about this for religious and/or taxation reasons, restrictions are sometimes placed on its availability. In those cases industrial methylated spirits is often used as a substitute. This is ethanol to which some methanol and perhaps some other chemicals and dyes have been added to make it unpalatable. It may be referred to as denatured alcohol when materials other than methanol have been added.
Methanol is poisonous and the amount in methylated spirits is sufficient to cause significant harm if ingested. Methylated spirits is quite satisfactory for dehydration of tissue blocks. It also is usually available as both 96% and absolute spirits, and they may be substituted freely for ethanol.
Isopropanol (as it is usually spelled) is more common in histology laboratories, although for most practical histological purposes the two propanols are interchangeable with each other. Isopropanol has been quite commonly used when ethanol or methylateed spirits cannot be used for some reason, and it is just about as effective a dehydrant. The author was trained in a laboratory that used isopropanol as the routine processing dehydrant, and it was completely satisfactory at reasonable cost. It is not drinkable, but it is of relatively low toxicity.
Acetone is quite inflammable, more so than the other dehydrants given, and is really unsuitable for routine dehydration. It has an intrusive odour and evaporates rapidly, which increases the fire risk unless the containers are tightly capped. It is an effective dehydrant and some technologists have used a final acetone bath following ethanols to ensure complete dehydration. This is not necessary if enough changes of a safer dehydrant are used.
I should be noted that some automatic tissue processors have seals or gaskets which may be damaged by acetone, particularly if used with heat. This must be investigated before using it.
Methanol is sometimes used as a dehydrant, although it is more likely to be for a few blocks rather than a large hospital sized daily workload. It evaporates faster than ethanol and is more inflammable. These factors, along with its extra cost, make it a poor choice for routine dehydration.
Methanol is very poisonous, as little as 10 mL can cause blindness, and only a little more causes death. This is due to liver enzymes which metabolise alcohols. In the case of methanol these enzymes produce formaldehyde which then kills cells. The treatment is to give ethanol, as the enzymes preferentially metabolise this alcohol, allowing time for the methanol to be excreted in the urine.
occasionally the use of fluids such as 2-ethoxy-ethanol, or something similar, is encountered. These fluids are usually more expensive than the common fluids and are usually being used for a specific purpose.