Orthophosphate at concentrations greater than 25 ppm will precipitate as silver phosphate to cause positive interference. This can be prevented by diluting orthophosphate concentrations below 25 ppm with DI water. Bromide, iodide, and cyanide at all levels titrate as equivalent chloride concentrations. Sulfide, thiosulfate, and sulfite interfere but can be removed by treatment with hydrogen peroxide. Quats may interfere; to prevent add 10 drops of R-0884. | Chloride |
Iron > 10 ppm may cause negative interference. Ortho- and polyphosphate may cause positive interference; to prevent, test for interfering agent, dilute sample with DI water as necessary, and retest. Positive interference caused by EDTA at all levels. Filming amines and quats may cause negative interference; to prevent, add 3 drops of R-0884. | Phosphonate |
Metal ions may cause interference; to prevent, add titrant containing EDTA to sample before buffer and indicator, then test as normal making sure to count drops of titrant added initially in total required to reach endpoint. If interference still occurs, dilute sample with DI water as necessary and retest. Quats may cause interference. | Hardness |
Iron may cause positive interference; to prevent, dilute sample with DI water as necessary and retest. Sulfide and tannin may cause negative interference; to prevent, dilute sample with DI water as necessary and retest. | Silica |
High halogen level may change indicator reaction from green/red to blue/yellow; to prevent, add thiosulfate prior to testing. Filming amines, quats, and biguanides may interfere and cause the endpoint to look purple; to prevent, add 10 drops of R-0884 prior to testing. | Alkalinity |
Test should be run on clean, clear sample; filter out colloidal color and turbidity before beginning test procedure. High concentrations of HEDP (over 30 ppm) may cause positive interference; to prevent, test for interfering phosphonate, dilute sample with DI water as necessary, and retest. Thiocarbamate-based biocides may cause interference; to prevent, do not test while feeding thiocarbamates into water system. Sodium nitrite over 800 ppm may cause negative interference; to eliminate, dilute sample with distilled, deionized, or molybdenum- and nitrite-free water then retest. | Molybdenum |
Glycol may cause interference; to prevent, use "CAN" test method (cerium oxidation of nitrite) on glycol-treated systems. Quats may cause interference by forming a precipitate. | Nitrite |
Erythorbic acid, sulfide and ferrous iron may cause positive interference; to prevent, test for suspected interfering agent, dilute sample with DI water as necessary, and retest. Copper and nitrite may cause negative interference; to prevent, test for suspected interfering agent, dilute sample with DI water as necessary, and retest. Delay between taking sample and testing may cause negative interference; to prevent, perform test immediately after sample is taken. Quats may cause interference by forming a precipitate and causing a yellow endpoint. | Sulfite |