pH testing with pH test strips.
At first glance the number of different types of pH test strips may seem confusing. So here are a few examples of how we use pH test strips in the lab (and brewery).
pH indicator papers are good where multiple tests need to be made quickly but accuracy only needs to be approximate. Usually the steps are 1 pH unit. In the lab we use them for quickly finding out if a sample is acidic or alkaline. Sometimes a reacted solution needs to be neutralized so it can be read on a photometer. Several small lengths can be cut off the reel in readiness. The reels are usually long (5 m) so it won't matter if it takes a while to find around pH 7 or if we overshoot. We use scissors and tweezers to cut and hold the pieces so there is no contamination from fingers. The colours in these papers tend to run a little so the colours are sometimes hard to read. Best to only moisten the paper.
Duotest papers also come in a 5 m reel but they have some advantages over indicator paper. Two different indicators are used and they are separated on the strip by an impervious barrier that stops the colours mingling with each other. This gives better accuracy. Duotest come in different pH ranges with accuracy down to 1/3 pH unit. When we go brewing we use Duotest 3.5 - 6.8. These are accurate enough to tell us if the mash is around the pH 6.3 mark. These same range Duotest papers can be used to test the safety of foods that use acidity to reduce bacterial growth. They will work in moist semi solids like rice or in preserves so long as the colour of the test solution doesn't interfer with the colours in the test papers. Where this is likely to be a problem choose Pehanon test strips.
pH-Fix pH indicator strips use 3 or 4 different indicators. These are in separate bands along the stick so with more than 1 colour to compare, getting an accurate reading is easier. pH-Fix are designed to be used in a variety of samples including those that can make the indicator run. For weakly buffered samples the sticks can be held in the sample for longer to get a good reaction - with no mixing of indicators. The picture shows pH-Fix being used to test a quite alkaline solution. Probably not one that you would want to get your fingers in. This is where pH-Fix are handy as the indicator pads are at a safe distance down the little flexible strip.
See the range of pH papers and strips at pH test strips.