Knowing How to Look for the Trouble Makes it Easier to Find
To find the concentration of an analyte in a solution, scientists turn to titration. Once a drop-by-drop-by-drop method, this can now be handled automatically with a titrator, but getting an accurate answer still takes some skill. Part of the needed skill depends on what is being tested. Nonetheless, some general information applies to virtually any titration situation.
“Titration issues are typically either systematic or random,” says Matthew Eby, application and technical support manager for the Mettler-Toledo NA division in Columbus, Ohio. “Systematic errors generate the same results, although incorrect, every time; random errors generate no results or various incorrect results without consistency.” Since systematic errors generate the same problems every time, they are the easiest to figure out. To see whether a problem is systematic, says Eby, “It is always advised before troubleshooting to run the sample or method with the issue multiple times to look for trends and to see the repeatability of the issue before attempting to address the problem.”
Scientists in research and industry use titration in so many ways that variety alone creates a challenge. “Depending on the type of reaction, whether it’s an acid-based, a redox, a complexometric, or a nonaqueous titration, using the right electrode can make all the difference,” says Lori Carey, product manager for titration at Metrohm USA in Riverview, Florida. “Metrohm has fine-tuned the electrode design for even the most difficult samples, and referencing our Metrohm titration monograph is very helpful as an overview for many titrations.”
In search of the source
The goal of titration is finding the end point, but that can be elusive. “There are many different reasons why an end point may not be obvious,” Carey says. “There are a few simple questions you can ask yourself to identify the cause.”
The questions, says Carey, include, is the correct titrant at the proper concentration being used? Is the electrode fully submerged in the sample? Is the sample completely dissolved, and is the analyte of interest available for titration? Are the increments in the titration method set properly? And did the titration just stop too soon?
Other experts also see users struggling at times with the titration end point. According to Alicia Guardado, technical adviser at Hach Company in Loveland, Colorado, “For titrations in which the user is looking for a visual end point, the most common issue is probably going to be overshooting the end point.” She adds, “This is usually caused by not completely mixing in each drop of titrant or dispensing drops that are too large.” Even for titrations that require a pH-change end point, Guardado points out, it can easily be overshot if the pH probe being used does not have a fast enough response.
Related Article: Adding More Automation and Raising Accuracy in Titration
Also, when titration runs into trouble, history impacts the troubleshooting. “If a titration method and procedure have been used for some time with success,” Eby says, “then that is the last thing one would want to modify; the fluctuations are most likely due to hardware, reagent, or sample inconsistencies.”
Even the sample can contribute to the problem. “With sample fluctuations, users should try to pretreat samples in a way that minimizes effects of interference, temperature, pH, etc., so that the results are consistent,” Guardado says. “Things like pH adjustment to a specified pH, dilution, and filtration are common pretreatment steps, and these will vary largely depending on the type of titration being performed.”
Keep up the care
“The best way to deal with any fluctuations in system performance or sample results is to make sure you are taking care of your titration system,” Carey says. “Making sure the electrode is stored in the correct solution and calibrated properly will help reduce fluctuations.” In addition, she says, “Making sure that your dosing burettes are properly vented to the molecular sieve or ascarite will help protect the titrant concentration from degrading.”
Diminished reproducibility can really destroy results over time. “When reproducibility starts to fade, it’s time to verify a few things,” Carey says. For one thing, inspect the electrode for wear or blockage. “Two key components of every electrode system are the measuring membrane and the reference diaphragm,” she says. “If either of these areas becomes slightly blocked or scratched, the electrode won’t be able to respond properly and will give irregular results.”
Eby agrees that the electrode can often be the source of errors, and he points readers to Mettler-Toledo’s GTP Sensor Use and Maintenance on-demand webinar (www.mt.com/gtp-sensor). He adds, “Seeing [whether] a new sensor addresses the issue is the quickest way to troubleshoot the sensor, and operators must remember that electrodes are consumable items and will eventually wear out.”
The problem, though, might not be the device. “The other most common source of issues is the titrant or solvents used,” Eby says. “Changing titrant strength due to varying temperature or contamination, improper storage and protection of titrants, or contamination of solvents are the typical issues regarding reagents.” To find the source of this kind of problem, the user should exchange the reagents one by one and see whether that solves the problem.
It’s also possible that the system that purifies the water used in titration needs maintenance.
To keep your automated titration running smoothly and accurately, Carey offers some valuable advice. “It’s always good to refresh your mind with how the electrode system works, because understanding the key functioning areas of an electrode will often lead you to the proper troubleshooting techniques,” she says. “Additionally, review the different modes of titration and how changes in titration parameters affect the shape of your titration curve.” To develop new titration knowledge or refresh it, Metrohm hosts several Titration Bootcamp courses. Carey says, “I would always recommend [these courses] to anyone working in the field of titration, regardless of their years of experience.”
So keeping up titration knowledge and the condition of the equipment go a long way toward preventing problems. Even an automated titrator needs care and skill to ensure ongoing accuracy in the results. The level of skill needed, however, depends on the task at hand.
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For additional resources on Troubleshooting a Titrator, including useful articles and a list of manufacturers, visit http://www.labManager.com/titrators