See
Recognizing a Compromised Reagent from the Taylor website. I list below the chemicals found in various kits, grouped roughly by shelf life based on their content.
Indicator Dyes and FAS-DPD (most sensitive; replace at least every 2 years if kept in cool dark place; if not cool, replace every year)
R-0004/R-0014 Phenol red pH indicator dye (different concentrations for sample size); also contains a mix of proprietary chlorine neutralizers (not thiosfulfate)
R-0600 Orthotolidine (OTO), chlorine indicator dye
R-0870 N, N-diethyl-p-phenylenediamine (DPD), powdered indicator dye plus phosphate buffer, EDTA and organic acid
R-0871 Ferrous ammonium sulfate (FAS), titrant reagent; usually modified for stability as with Ferrous Ethylenediammonium Sulfate (FEAS)
R-0008 Bromocresol green and Methyl red, Total Alkalinity (TA) indicator combination dye
R-0011L 1-(2-Hydroxy-1-naphthylazo)-2-naphthol-4-sulfonic acid sodium salt (Calcon), Calcium Hardness (CH) indicator dye
Indeterminate shelf-life -- play it safe and replace every 2 years
R-0003 Potassium iodide, DPD Reagent #3 used for combined chlorine (CC) test (or Total Chlorine test in some kits) - sunlight plus oxygen oxidizes it
R-0007 Sodium thiosulfate, chlorine neutralizer
R-0012 Ethylenediaminetetraacetic acid (EDTA), Calcium Hardness (CH) titrating reagent -- published shelf-life 18-48 months
R-0013 Melamine, Cyanuric Acid (CYA) reagent; usually has pH buffers to force low pH; forms melamine cyanurate in the test
Acids/Bases (least sensitive; essentially no degradation)
R-0005 Hydrochloric acid, acid demand reagent
R-0006 Sodium hydroxide, base demand reagent
R-0009 Sulfuric acid, for TA titration
R-0010 Sodium hydroxide, Calcium Hardness (CH) buffer (precipitates magnesium hydroxide to test only calcium hardness)
