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| Frequently Asked Questions - Laboratory |
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- After adding DPD to my water sample, the water flashed pink and then faded to colorless. What could be happening?
View answer | | 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. | |
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- How often should I calibrate my pH meter?
View answer | | 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. | |
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- 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?
View answer | | 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. | |
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- What method should I use for ammonia testing?
View answer | | 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. | |
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- How long can I expect my pH probe to last?
View answer | | 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. | |
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- My colorimeter does not seem to be giving me a correct reading. What things should I try before calling for help?
View answer | - 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.
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- What’s the difference between primary and secondary calibration standards?
View answer | | 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. | |
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- After adding DPD to my water sample, the water flashed pink and then faded to colorless. What could be happening?
View answer | | The chlorine level is most likely above the measurement range of the reagents. The reagents react and then become “bleached out” by the remaining chlorine. Try re-sampling and diluting the sample with an equal portion of distilled or de-ionized water before analyzing it again. Depending upon the initial concentration of chlorine in your sample, several dilutions may be required to permit the sample to be read within the readable range of the reagents. Chlorine is a gas dissolved in water, so dilution of samples, requires expediency and prompt analysis. Remember to multiply the reading obtained on the diluted sample by the dilution factor when reporting your chlorine concentrations. | |
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- How often should I calibrate my pH meter?
View answer | | Calibration of pH meters should occur at least once per shift, more often if it is suspected that the meter has drifted. Many regulators require documentation of the calibration slope in a logbook. If the sample being analyzed is out of the range of the buffers used for calibration, the meter must be recalibrated with buffers that bracket the pH of the sample. | |
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- I have replaced the membrane and the fill solution on my DO meter and I am still having problems getting steady, reliable readings. Any suggestions?
View answer | | Replacing the membrane and fill solution is always the first step to try when there are difficulties with a DO meter. The fill solution loses its ability to carry oxygen over time and the membrane can become clogged or coated. After both of these items have been replaced, if the problem persists, most likely the probe’s gold cathode or silver anode require attention. | | | The silver anode will develop a layer of AgCl (Silver Chloride). 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 electrode section in a 3% ammonium hydroxide solution overnight. (More concentrated solutions of ammonium hydroxide may be used for much shorter periods of time, however if soaked too long in these more concentrated solutions, the probe will be damaged.) Rinse the electrode thoroughly in cool tap water followed by rinsing with distilled or de-ionized water. The anode should be wiped with a wet paper towel to remove the residual layer from the anode. The anode may also be cleaned mechanically by gently sanding off the dark layer from the silver anode with 400 grit wet/dry sandpaper (or fine grade steel wool). Wrap the wet sandpaper around the anode and twist the probe. Rinse the anode with clean water after sanding, then wipe thoroughly with a wet paper towel. Soaking in 3% ammonium hydroxide will also remove many green or black deposits from the gold cathode. | | | The gold cathode may become tarnished or plated with silver after extended use. This tarnish may be removed using ultra-fine sandpaper (or fine grade steel wool), included in a manufacturer DO maintenance kit. Using the sandpaper, wet sand the gold with a twisting motion about 3 times until all silver deposits are removed and the gold appears to have a matte finish. Use care not to sand off all the gold – it’s usually a very thin layer! Wipe the gold cathode thoroughly with a wet paper towel and install a new membrane. If manufacturer ultra-fine sandpaper is not available, a piece of cardboard used in its place may accomplish the same results. | |
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- What method should I use for ammonia testing?
View answer | | Initially verify ammonia analysis requirements with your local reporting agency. EPA requires a distillation step before analysis with a colorimeter or test kit. Local regulators may or may not require this step. For reporting purposes ammonia ISE (Ion Specific Electrode) is usually the preferred method. The ammonia ISE method is less susceptible to interferences than the traditional Nesslerization method, pre-distillation is not required and disposal of mercury-containing Nessler reagents is not an issue. | | | 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 provide the analytical result required. | |
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- How long can I expect my pH probe to last?
View answer | | The life of a pH electrode depends primarily on its suitability for the application and the maintenance performed on the probe. Assuming that the correct electrode is being used for the application (single-junction for potable water and double-junction for wastewater) a useful life of 6 months to 1.5 year may be expected if the electrode is PROPERLY MAINTAINED. | | | Proper care of a pH electrode includes rinsing the probe after each use with distilled or de-ionized water and storing the electrode in commercial storage solution between uses. If commercial storage solution is not available, pH 4 buffer is an acceptable substitute until storage solution may be obtained. NEVER store an electrode in distilled or de-ionized water as the electrolyte from the probe will leach out; sealed probes will be destroyed in a very short period of time. | | | Refillable and flushable electrodes require the fill solution to be changed on a regular basis with the correct fill solution. Generally fill solutions changed weekly should suffice. However, if the meter is not calibrating properly or slope from the calibration is out of range, the first step would be to change the fill solution. Occasional cleaning with commercial cleaning solutions may also be necessary to remove build-up on the bulb or to attempt to un-clog the reference junction. | |
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- My colorimeter does not seem to be giving me a correct reading. What things should I try before calling for help?
View answer | - 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, dirt and color coating the interior of the tube from previous tests.
- Verify that the tube is being positioned the same way every time. (Most colorimeters and tubes have an index (arrow or diamond on the tube and colorimeter) 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.
- Verify the meter with a known standard.
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- What is the difference between primary and secondary calibration standards?
View answer | | A primary standard is used to calibrate an instrument response to the concentration of the analyte (parameter) being measured. | | | A secondary standard is a standard that the manufacturer or independent testing laboratory has certified will provide instrument calibration results equivalent to the results obtained when the instrument is calibrated with a primary standard. A secondary standard is used for routinely monitoring calibration verification. The secondary standard should be monitored for deterioration using a primary standard. Secondary standards should be discarded and replaced when they have reached their expiration date or the standard is determined to be deteriorated when compared with a primary standard. | |
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