There is no one answer to your question, as it varies tremendously depending on kit type, and even within the same kit type between makers (who may use very different test protocols to test for the same parameter.)
Whenever you buy a new type of test kit, you should ALWAYS obtain the appropriate MSDSs (Material Safety Data Sheets) for the contents of that kit. These either come in the kit itself, are available on the website of the manufacturer, or will be mailed to you by writing or calling the maker. By US law, makers cannot refuse to provide you with MSDSs for the hazardous portions of a kit (although they may not always be willing to tell you precisely what is in the kit if it is of a proprietary nature).
Whenever I work with test kits, I wear disposable latex gloves and protective eye glasses with side shields, and I work on a non-porous countertop protected with newsprint or paper towels. Never eat or smoke while testing, and wash your hands with soap and water immediately after. Dilute any spills with lots of fresh water, and at the end flush the contents of the test vial down a bathroom sink (not the kitchen sink) with lots of water, or flush it down the toilet. Keep test kits locked up away from children. Have the local poison control center phone number handy for accidents, especially if you have small children or cat/dog type pets, and retain all the instructions and other paperwork that come with your kits. If you follow those simple commonsense rules you'll never have a problem.
Finally, I'll give a quick run down of the most common kits and hazards.
pH - These kits contain mostly harmless dye indicators, many of which were originally discovered by being extracted from plants and were used to dye fabrics before synthetic dyes were discovered. Most such dyes are fairly harmless, but they can stain skin and clothing, and can cause mild irritation if gotten in the eyes. Never touch or ingest them, or play stupid pranks with them! Some pH kits have the indicators pre-mixed with dilute acids so they can use acid-range indicators in the alkaline-range of saltwater tanks, so be especially careful not to get these in the eyes. The acid is dilute enough that accidental skin contact can just be washed off with water, or a paste of water and baking soda followed by lots of water. If you use a pH meter, remember that the pH 10 calibration fluid is a strongly base liquid that can irritate skin and eyes, and spills should be cleaned up or washed off with white distilled vinegar followed by water. In case of accidental eye contact, just flush with LOTS of plain water.
gH/dKH - Hardness test kit reagents are just a combination of a dilute acid and a pH indictor dye, so the same precautions apply as for the pH kits above.
Calcium/Magnesium - The drops in these kits are Sodium Hydroxide. While quite dilute, it CAN burn eyes and skin and it WILL eat holes in clothing. Wash off/clean this up with distilled white vinegar followed by LOTS of water. Flush eyes with LOTS of plain water only. The colored indicators in the kit are plant dyes like the pH indicators, and the same cautions apply. The clear liquid titrant is Ethylene Di-Amine Tetra Acetic Acid, in an aqueous solution. Though it is technically a mild acid, it is harmless enough that it is used to clean out your root canal just before the dentist fills it in with gutta-percha. Avoid touching or eating it anyway, and keep it out of your eyes. Clean up spills with lots of plain water.
Ammonia/Nitrite/Nitrate - These test kits should be used with the most caution of any of the common reef testing kits. There are only two common test protocols for testing for these compounds that I am aware of, and they are both very nasty. The common reef kits use Nessler's Reagent, which is made from Mercuric Iodide (HgI2). If your Ammonia/Nitrite/Nitrate test kit turns some shade of orange or yellow to indicate its reading, it uses this method. Avoid ANY contact with the reagents in these test kits, and flush the used sample down the toilet when done with it, flushing at least twice. (The competing test protocol is worse, since it uses Cyanide.)
These are the common reef test kit hazards, but there are others, some quite unexpected, such as the use of the heavy-metal Cadmium in some Nitrite and low-range Nitrate kits, and use of Orthotolidine (a carcinogenic) in some older chlorine and ozone kits.
At any rate, make it a practice to follow the cautions I list above and you should be fine.
(By the way, if anyone out there knows of a simple, non-toxic test protocol for Ammonia/Nitrite/Nitrate, please post it.)
[ October 10, 2001: Message edited by: BReefCase ]
