Sanitizer that is too weak does not work and too strong is unsafe. These answers cover the correct concentrations for chlorine, quat, and iodine, and how to verify them. Primary source: U.S. Food and Drug Administration.
The technical answers here reflect these published codes and standards, which your local authority (AHJ) adopts, amends, and enforces — educational, not engineering or legal advice. Confirm the current requirements for your jurisdiction.
For food-contact surfaces, a chlorine (bleach) solution is commonly used at about 50 to 100 ppm. FDA Food Code 4-501.114 sets the governing parameters, tying the minimum concentration to water temperature and pH, along with a required contact time. Below the minimum the solution will not reliably sanitize; well above the range it can be unsafe and corrosive. Always confirm the strength with chlorine test strips rather than eyeballing the pour, since bleach loses potency over time and dilutes as it is used.
Quaternary ammonium (quat) concentration follows the EPA-registered product label, because FDA Food Code 4-501.114 defers to the label's use directions for quats, along with minimum temperature and water-hardness limits. Many quat products are used around 200 ppm, but the label is authoritative, so check the specific product you stock. Use quat test strips matched to your product to confirm the concentration, and note that hard water beyond the label's limit can reduce quat effectiveness even at the correct ppm reading.
Iodine sanitizers are used at a lower concentration than chlorine, generally in the range specified by FDA Food Code 4-501.114, commonly cited as about 12.5 to 25 ppm, with a minimum temperature of 68 degrees F and a pH at or below 5.0. Iodine solutions turn amber, and a solution that has lost color has usually lost strength. As with any sanitizer, verify with the correct iodine test strips and follow the product label, since conditions like pH and temperature affect efficacy.
Concentration is what makes a sanitizer work. Too weak, and it fails to reduce pathogens to a safe level, leaving surfaces effectively unsanitized even though they look clean. Too strong, and it can leave unsafe chemical residue on food-contact surfaces and corrode equipment. FDA Food Code 4-501.114 specifies concentration alongside temperature, pH, and contact time because all of those factors together determine whether sanitizing actually occurs. Hitting the right concentration, at the right temperature, for the right contact time is the whole point.
Use test strips matched to your specific sanitizer type: chlorine strips for chlorine, quat strips for quat, and iodine strips for iodine, since a strip for one chemical will not correctly read another. FDA Food Code 4-302.14 requires having such a test device on hand. Dip the strip in the mixed solution and compare it to the color chart. Test when you make up the solution and periodically during service, because concentration drops as the sanitizer is used, evaporates, or is diluted by wet items.
Remake it whenever a test strip shows the concentration has fallen below the required range, and proactively during a busy shift, because sanitizer weakens as soiled items dilute and deplete it. Do not wait for a fixed clock interval alone; the test strip is the real trigger. Wiping-cloth buckets in particular lose strength quickly as cloths add soil, so change them frequently. Keeping fresh solution at the correct FDA Food Code 4-501.114 concentration is what ensures surfaces are actually being sanitized.