Phenol Red is a crystalline compound that is stable in air and holds its shape when formed into tablets. It is usually white in color, although if mixed into a solution then it will be either yellow or red, based on the pH of that solution. As such, phenol red is water soluble and has a weak acidity rating. Although this compound used to be part of a test to determine kidney function, its main purpose these days is to check the pH balance of pool water. You can find phenol red tablets at most pool supply stores as a means to ensure that your water is properly treated. When dissolved, the tablet will produce coloration that will tell you how acidic or alkaline your water is. Your ideal pH is around 7.2, which will result in an orange hue. If the water is too acidic, then the phenol will turn bright red. Although this compound is stable in air and sold without restriction, it does pose a couple of health threats. First of all, if ingested phenol red is poisonous and can cause vomiting and diarrhea. If the compound gets into your eyes, it can also be dangerous, so avoid touching your eyes after handling phenol red. For some people, touching this compound directly will result in mild skin irritation, but that’s not always the case. Regardless, it’s recommended that you avoid direct contact as oils on your skin can affect the results of your pH test. Finally, phenol red is somewhat flammable, so it’s important that you store in a cool, dry area and keep it away from fire or hot surfaces. Phenol red is not expressly toxic nor does it have a significant environmental impact. It can be disposed of without any extra precautions being taken.