Water is essential in the production of an H&E stain. Water is not only used for rinsing and bluing but as a diluent for many of your working stains and reagents. The quality of the water you use can ultimately impact the quality of your stain. So, what kind of water should you be using? Is regular tap water safe to use or should you always be using deionized or DI water?
As anyone who drinks tap water knows, the quality of tap water can vary greatly depending on the location, and even the season. Tap water contains elements such as iron, sulfur, and chlorine which can affect the pH of the water. In order for water to be used as a bluing agent in an H&E stain, the pH of the water must be greater than or equal to 8, in other words it must be an alkaline solution. If you have a lot of iron or sulfur in your tap water, your pH is not going to be high enough to get the right bluing, so distilled or deionized water is going to be a better option. If you are going to use tap water, run the tap water for a minute and then check the pH to make sure it is usable.
In most cases, tap water is okay to use for an H&E stain, but DI will produce more consistent results and reduce the amount of potential contaminant on your slide, which is always a good thing, especially if something goes wrong and you need to identify the source of a problem.
Being aware of the quality of your water, either tap, or DI (checking your filtration system there is important too) will ultimately help you prepare for the results of your stain and make the adjustments as needed to get the best result.