The Truth About Phosphonate in Industrial Water Testing

One of the questions I’m asked most frequently is: “Why don’t I get a consistent result with my phosphonate drop test?”

To discuss this topic, I would like to start with the end in mind: “What are the repercussions of inconsistent results?” (These apply to all testing of “the actives” of treatment chemicals.)

• Having lower phosphonate levels than the test measures will lead to undertreatment of the system. If you measure 8 ppm and really have 1 ppm, the treatment program may not work. Additionally, in this scenario, if you report 8 ppm of treatment chemical and your competitor tests the water correctly and reports 1 ppm, then you will not look like the water specialist you claim to be.
• Having higher phosphonate levels than needed will lead to wasted chemical and reduced contract profitability. This goes to the water treaters who say, “I don’t know the actual levels, so I’ll just feed more chemical.” The system might be protected from failure, but you may not be protected from the person who watches your account gross margins. If your customer is “paying by the drum,” then you would be open to a competitor again pointing out the obvious.
• What if “I don’t check my actives; I use a tracer (molybdenum, fluorescence, etc.) to make sure I feed the correct amount of chemical”? That assumes all systems have the same amount of stresses. Heat, time (HTI), oxidizers, suspended solids, and saturation indices are just some of the stresses to a treatment chemical, like a scale inhibitor. Your tracer may indicate you are under control, but you never know that unless you confirm your actual active chemical levels.

Inconsistent results can be caused by a myriad of factors, and there are many potential answers to the initial question. To begin to answer that question, let’s look at the criteria that lead to proper phosphonate test results:

1. A proper sample is collected
2. The proper sample size is used
3. The reagent is added as prescribed and the reagent is fresh

Okay, these are on the tester. So, what about the test itself?

4. The chlorine must be eliminated from the sample (usually using thiosulfate)

5. The sample pH must be adjusted to the desired level, such as 2.6 to 3.0

6. The indicator is added, and the titration must be taken to the proper endpoint

7. Remember to subtract from your result the number of drops for your “blank” (this may be time to reread those directions!)

The chlorine destruction is not difficult, but the pH adjustment can be. A popular method uses “just one tablet” to neutralize chlorine, adjust pH, and add the indicator. When samples are collected across different territories, you cannot use one set buffer amount to correct for pH. In Pennsylvania alone, we have such varying alkalinity levels in our makeup water that a single tablet could not buffer two different samples to the same pH level. Proceeding with a phosphonate test without adjusting pH will cause result inconsistencies.

What about possible interferences?

Fluoride: Fluoride is a positive interference to the phosphonate titration tests. Fluoride is added to many municipal drinking water systems for dental health. If the system’s makeup water source contains fluoride, then it must either be masked or its presence added into the titration results. I see methods which do not mention this interference, but it does exist. Not correcting for fluoride will cause a much higher amount of titrant to be added, leading the tester to think there is more inhibitor in the system than there is.

If in testing you find expected results from water systems using well water or surface waters for makeup but higher levels (or greatly reduced chemical usage rates) from municipal water systems, then not accounting for fluoride in the municipal waters may be the cause.

Other interferences:

• Orthophosphate and polyphosphate at all concentrations cause a positive interference
• Iron at higher levels (>5 ppm) cause a negative interference

I am all for simplicity, but simplicity in testing for phosphonate in cooling water may lead you to the wrong conclusions. You should look for phosphonate drop tests that (1) confirm correct pH levels in the sample before titrating, and (2) use a fluoride masking agent.

When in doubt, go back to the full UV digestion procedure and cross-check your results. The digestion method has only nitrite (>200 ppm) as an interference.