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Frequently Asked Questions - Laboratory
  • After adding DPD to my water sample, the water flashed pink and then faded to colorless. What could be happening?
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    • Most likely, your chlorine level is above the measurement range of the reagents. You’re seeing the reagents react and then get “bleached out” by the remaining chlorine. Try taking another sample, diluting it with an equal portion of distilled or deionized water and re-testing it. Depending upon the initial concentration of chlorine in your sample, you may have to perform several dilutions to get the chlorine down to a readable level. Remember to take the dilutions into account when reporting your chlorine concentration.
  • How often should I calibrate my pH meter?
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    • When using the meter for reporting purposes, a calibration should be performed before each use. If you’re using the meter several times during the day for process monitoring and not for reporting, you can probably get away with a single calibration performed at the beginning of the day.
  • I’ve replaced the membrane and the fill solution on my DO meter and I’m still having problems getting steady, reliable readings. Any suggestions?
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    • You’re off to a good start. Replacing the membrane and fill solution is always the first step to try when you’re having trouble with your DO meter. The fill solution loses its ability to carry oxygen over time and the membrane can become clogged or coated. Once you’ve replaced both of these items and you’re still having problems, the problem is most likely with the probe’s gold cathode or silver anode. For correct probe operation, the gold cathode must be textured properly. It can become tarnished or plated with silver after extended use. The gold cathode can be cleaned by using ultra-fine fine sandpaper or a piece of cardboard. Using the sandpaper or cardboard, wet sand the gold with a twisting motion about 3 times or until all silver deposits are removed and the gold appears to have a matte finish. Be careful not to sand off all the gold - it’s usually a very thin layer! Wipe the gold cathode thoroughly with a wet paper towel before putting on a new membrane cap.
      Usually after a year or so, a thick layer of AgCl also builds up on the silver anode. The anode must be cleaned to remove this layer and restore performance and sensitivity. To clean the anode chemically, remove the membrane cap and soak the entire anode section in a 14% ammonium hydroxide solution for 3 to 4 minutes. Rinse thouroughly in cool tap water followed by a rinsing with distilled or deionized water. The anode should then be wiped with a wet paper towel to remove the residual layer from the anode. The anode can also be cleaned mechanically by sanding off the dark layer from the silver anode with 400 grit wet/dry sandpaper. Wrap the wet sandpaper around the anode and twist the probe. Rinse the anode with clean water after sanding, followed by wiping thoroughly with a wet paper towel.
  • What method should I use for ammonia testing?
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    • First, always check with your local reporting agency to see what their requirements are and what test method they recommend. That said, we recommend using an ammonia ISE electrode when testing for reporting purposes. The ammonia ISE method is less susceptible to interferences than the traditional Nesslerization method, there is no pre-distillation step, and you don’t have to worry about disposing of mercury-containing Nessler reagents.
      For quick checks or simple process monitoring, using a test kit or handheld colorimeter that utilizes the Nesslerization or salicylate methods without a pre-distillation step may be more convenient than using an ammonia ISE. The test kits and colorimeter won’t be as accurate as an ISE, but it may serve your purposes.
  • How long can I expect my pH probe to last?
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    • Like most things, the life of a pH electrode depends primarily on its suitability for the application and how well it is taken care of. Assuming that you are using the correct electrode for your application (single-junction for potable water and double-junction for wastewater), you can expect a useful life of 6 months to 1.5 years out of your pH electrode IF IT IS PROPERLY TAKEN CARE OF. This means rinsing the probe after each use with distilled or deionized water and storing it in commercial storage solution between uses. Occasional cleaning with commercial cleaning solutions may also be necessary to remove build-up on the bulb and un-clog reference junctions.
  • My colorimeter does not seem to be giving me a correct reading. What things should I try before calling for help?
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    • Verify that the correct sample tube is being used.
      Verify that the same sample tube is being used for both zeroing and sample testing.
      Verify that the tube is clean and free of fingerprints and dirt.
      Verify that the tube is being positioned the same way every time. (Most colorimeters and tubes have an index that must be aligned before taking a reading.)
      Verify that the reagents are not expired.
      Verify that the batteries in the colorimeter are not low.
  • What’s the difference between primary and secondary calibration standards?
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    • Primary standards are required to perform an actual recalibration of a unit, whether it’s a chlorine colorimeter or a turbidimeter. Secondary standards are used as a check to see whether an instrument needs to be recalibrated with primary standards. For instance, the EPA requires that you recalibrate your turbidimeter at least once every 3 months using a primary calibration standard. After you calibrate it using the primary standard, you would place the secondary standard into the measuring chamber, take a measurement and write the measured value on the outside of the secondary standard vial. Over the course of the next few months, you would periodically place the secondary standard in the chamber and take a reading. Because secondary standards are designed to maintain a given value, the reading should match the initial reading written on the vial or at least be within the manufacturer’s specified tolerance range. If the reading is outside that range, the turbidimeter itself has drifted. You would then perform another calibration using a primary standard.