Refractometers come in all sizes, shapes, capabilities, and prices. Some readers may recognize refractometry as a “high school” analysis method or a specialized technique for measuring specific analytes like sugar in water. Increasingly, lab and process engineers employ refractometry to provide quick measures of key quality attributes that under normal circumstances would require much lengthier assays on significantly more expensive equipment.
Anton Paar (Ashland, VA) launched a new line of refractometers, the Abbemat 300 and Abbemat 500, in 2011. As of this writing, 20 months later, real-world data suggest that Abbemat models are living up to their promise.
Refractive index measurements for the Abbemat 300 and Abbemat 500 are accurate to ±0.0001 and ±0.00002 nD, respectively. Their precision, and design for harsh environments, makes these models suitable for quality control as well as routine analysis. Both models sport hermetically sealed stainless steel casings that are impervious to most spills or foreign materials. But can the instrument withstand the ravages of one of the most corrosive substances in the world?
An Anton Paar customer—a manufacturer of hydrofluoric acid (HF)—recently tested Abbemat’s robustness in a harsh manufacturing environment. Nicknamed “devil’s work,” HF is one of the nastiest substances known. Used as an etchant and starting material for organofluorine compounds, HF is a severe poison that can eat through skin and flesh down to the bone, causing very severe, painful wounds that heal extremely slowly.
Assaying HF concentrations is traditionally done by a lengthy titration measurement that takes three hours and exposes operators and equipment to highly corrosive acid. Moreover the assay is cumbersome, as the titrator must be kept in a fume hood some distance away from the controls.
Abbemat was up to the task. The system employed by the “HF customer” consisted of the refractometer, sampler, and pump from Funk, a custom PTFE polymer sample cell, and custom control software from Anton Paar’s OptoTec subsidiary. Conducting a “longtime prism test” confirmed that the yttrium-aluminum-garnet prism employed by Abbemat refractometers resists serious acids like HF. The PTFE sampler is also acid-resistant.
What’s significant about this test is that a refractometer has successfully replaced a longer, more tedious titration assay involving highly corrosive chemicals. A refractometry run takes between five and ten minutes, compared to several hours to set up, run, and clean a titrator.
According to Alex White, a product specialist at Anton Paar, refractometers are now routinely used in the pharmaceutical industry for quality control and characterization of new substances. “
In many instances refractometry replaces much more expensive equipment such as gas chromatography and mass spectrometry. I would assume that hydrochloric acid concentrations might also be more easily measured with a refractometer than through titration,” White says.
Refractometry’s accessibility and accuracy is improving in-line analysis in process environments as well. Traditionally, manufacturers sampled their process and submitted vials to a lab behind the scenes, which returned results hours or days later— often when it was too late to remediate components that were out of spec.
Digicom Electronics (Oakland, CA), which manufactures printed circuit boards, uses a proprietary chemical mixture to clean their finished products. Highly efficient removal of metal ions significantly reduces circuit board failures.
A critical aspect of the cleaning process involves ensuring that the “green” cleaning solution itself is not contaminated by picking up metals. General Manager Mo Ohady says that a refractometer is the secret to maintaining the proper composition of his solution. “We use it to check the stability of the mix in the tank to see that it’s not compromised,” he says. As someone more familiar with electronics than chemical analysis, he calls his in-line analysis tool “a new, better mousetrap for testing solutions.”
Bring the lab to the plant
“For many applications, refractometers hit a sort of ‘sweet spot’ of precision and convenience,” says Noah Radford, sr. product specialist at ATAGO USA (Bellevue, WA). Refractometers may be even more precise and empirical than methods they replace. “Think reagent strips, which rely on color-matching and have a very narrow time frame of validity.”
For still other situations, refractometers become the “quick and dirty” or “mobile” alternative to more bench-bound methods that might be more accurate but require more time, more energy, and hazardous reagents and/or cannot take place near the product line (such as titration systems for harsh acids).
In these instances, refractometers may not be the “golden” or AOAC-approved test method, but they can supplement them or screen for situations where the full method should be used. Many users save more precise, validated methods for quarterly tests sent to regulatory bodies, for spot checking samples, or to validate standards used in other measurements. Refractometry is reserved for more day-to-day internal QC checks, for which they are more suited thanks to their speed and simplicity.
Often, there is a small but predictable discrepancy between readings achieved through different methods. “Continuous emissions monitoring and vacuum ovens typically generate percent solids values that are higher than a refractometer’s brix measurement,” Radford notes. “Users need to be cognizant of this and apply corrections when necessary.”
Radford relates a success story, from the bioethanol industry, where by using refractometry his client was able to automate a test for percent solids on their digestion/fermentation line. This allowed them to monitor and control the rate of addition of an expensive enzyme into the feedstock. “Mass flow and hourly percent solids checks by hand were not enough,” Radford relates.
The customer first tested the efficacy of several refractometers that they could wheel around on carts between lines and checkpoints, plugging them into a PLC to transmit data and communicate with upstream operations. “Eventually, they permanently installed six in-line refractometers. The monitoring improvement improved yield by three percent. “Combined with the savings on the enzyme, they figured they were netting an extra $2,000–$3,000 per batch,” Radford says.
Angelo DePalma is a freelance writer living in Newton, NJ. You can reach him at firstname.lastname@example.org.