Industrial Testing Interferences
In my last post, I talked about how chlorine can be a common interference when testing water in pools and spas. There are some common interferences to deal with when testing industrial waters as well. Although chlorine can be one of these, it’s rare since chlorine levels are generally kept below 1 ppm — well below the 10 ppm where chlorine would become a problem.
We can, however, experience problems from copper or iron that may be in the make-up water or due to corrosion in a system. Either way, metals may affect many of the tests we perform in the field. Have you ever done a hardness test and found it difficult to reach an endpoint that holds its color? The sample turns from red to blue and then back to red again. You keep adding drops of titrant, but the sample keeps turning back to red. Now you have added so many drops, and you know the hardness level cannot be that high. What’s going on? The sample probably contains a high level of iron and/or copper. The EDTA chelant used as a titrant loves metals. It prefers binding to the metals more than the calcium and magnesium, so the sample keeps going back to red. How do we correct for this? Try adding two or three drops of titrant to the sample before adding any buffer and indicator. Those first drops of titrant tie up the metals. Then go ahead and perform the test as directed in the instructions. Don’t forget to add those two or three drops to the total drops of titrant used to reach the endpoint. This time, you should get a crisp blue endpoint.
Interferences are also common when performing organophosphate (OP) tests. Organophosphates, such as HEDP and ATMP, are generally used in cooling water programs and must be dosed at proper levels to prevent scale formation. Underdosing leads to poor results, while overdosing leads to poor economics. In the OP drop test (not the UV digestion method) fluoride is a positive interference. Fluoride is added to many municipal drinking water systems at a level which will cause an interference — and even more so as it cycles up in the cooling water system. What’s going on? Fluoride will lead you to believe there is more OP in the water than actually exists. How do we correct for this? Add a fluoride masking agent to the sample before titration. The masking agent will tie up the fluoride so you are only titrating the OP; otherwise, you may be underdosing the chemical, which could lead to poor results.
A final example of an interference occasionally encountered is when performing an “O” alkalinity test on a boiler water sample using the “2P-M” method. Sometimes your testing shows a negative value for “O” alkalinity, which can’t be, especially when the boiler water pH is around 11. What’s going on? The polymer in your treatment chemical may be the culprit. High levels of polymer will titrate as “M” or “total” alkalinity. Your value for “P” alkalinity is correct, but the “M” or “total” reading is higher than it should be. How do we correct for this? Titrate for “O” alkalinity directly using the barium chloride method. Barium chloride is added to the sample water before the phenolphthalein reagent to react with the carbonate alkalinity in the sample. All that is left is hydroxide alkalinity, or “OH.” Now you are titrating straight for the “O” alkalinity.
Test kit instructions usually list interferences and how to correct for them. You can also contact the manufacturer for more information or to discuss a particular concern.
