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Phosphonate Testing Tutorial

Taylor offers two drop-count titration tests for phosphonate as shown in the chart below. Both kits employ the thorium nitrate method, but one uses xylenol orange (XO) as the phosphonate indicator and the other uses chrome azurol S (CAS). They also differ in the pH indicator used, in the acid provided for adjusting pH, in the treatment blends that can be monitored, and in the drop equivalencies for a particular antiscalant. Specifically, the XO kit K-1583 is designed for use with ATMP, Na5ATMP, HEDP, K6HDTMP, DTPMP, and Na5DTPMP. The CAS kit K-1584 is appropriate for testing ATMP, Na5ATMP, HEDP, plus HPA and PBTC. With either, analysts are instructed to run a blank before testing the treated water.  

Treatment Blend K-1583
with XO Indicator
with CAS Indicator
ATMP 1 drop = 1.0 ppm 1 drop = 0.8 ppm
Na5ATMP 1 drop = 1.3 ppm 1 drop = 1.0 ppm
HEDP 1 drop = 0.9 ppm 1 drop = 0.8 ppm
K6HDTMP 1 drop = 1.2 ppm n/a
DTPMP 1 drop = 1.45 ppm n/a
Na5DTPMP 1 drop = 1.7 ppm n/a
HPA n/a 1 drop = 0.6 ppm
PBTC n/a 1 drop = 0.8 ppm
  uses pH test paper #9315 uses pH indicator strip #6051


Accurate results are greatly dependent on the sample’s pH. A very limited pH range is specified in the instructions. You will likely need to adjust the sample’s pH with the acid provided before continuing with the phosphonate determination.

The XO kit K-1583 includes a 200-count vial of pH test paper #9315 for making the pH adjustment. The #9315 test paper has gradations of color from purple to peach at eight points in the range 1.8–3.8. The paper is dipped into the sample after the addition of sulfuric acid and all color zones are immersed for three seconds. The developed color in the indicator zone (the unnumbered square between 2.7 and 3.0) is then compared to the color scale on the strip for the pH reading.


If necessary, more acid is added and another reading is made with a fresh test paper until the pH target of 2.6–3.0 is reached. (NOTE: When testing this same system water on a repeated basis, you can expect to use about the same number of drops of sulfuric acid to reach the proper pH level. This should help expedite the process.

Then thorium nitrate is added to the XO solution until it changes from yellow to purple-pink. The phosphonate reading is made by counting the number of drops of titrant used, subtracting the number of drops used when the blank was run, and multiplying the result by the chosen drop equivalency.

Left, #9315 graded test paper in K-1583; right, #6051 ColorpHast indicator strips in K-1584.


The CAS kit K-1584 contains 100 ColorpHast™ strips (#6051) to adjust pH. A pad affixed to the end of a plastic strip is impregnated with a reagent which will turn a shade from orange to reddish brown when swirled in sample water to which hydrochloric acid has been added. The final color that develops on the pad is compared to the color standards on the strip container. If necessary, the procedure is repeated with the same strip until a pH of 3.6–3.9 is obtained. (These values are adjacent on the color scale.) Once the proper pH is achieved, the sample containing CAS is titrated with thorium nitrate until the solution’s color changes from peach to purple. The phosphonate level is determined by counting the number of drops of thorium nitrate used, subtracting for the blank, and multiplying the result by the chosen drop equivalence.

It should be noted that fluoride will interfere in the K-1584 phosphonate test but the K-1583 contains a fluoride masking agent. Because of the chemistry used in the K-1584 method, no fluoride masking agent can be used— fluoride will remain a positive interference. Both tests are affected by the presence of iron, orthophosphate, and polyphosphate. If unusual test results cause you to doubt their validity, test for these interfering agents.

NOTE: Taylor’s K-8014 colorimeter method incorporates a UV digestion of the water sample, which will provide the most accurate results for phosphonate testing.

Please call Taylor’s help line, 800-837-8548, if you have any questions about our phosphonate kits.