Turbidity - Clarifying Low-Level Measurements
In the first part of this article, in the last issue, we discussed tube selection, stray light reduction and molecular scattering.
In the first part of this article, in the last issue, we discussed tube selection, stray light reduction and molecular scattering. Here, we’ll discuss negative results, sample handling and calibration.
A useful feature for evaluating low-level turbidity is a turbidimeter that displays negative results. Meters will occasionally make a reading that’s negative and should be displayed as a negative number. This situation is more likely to happen when measuring low-level turbidity. Natural variations in all measurements, instrument and non-instrument related, can lead to a negative result. Thus, some turbidimeters are designed to round up a negative number to 0.00 NTU, since a result of less than 0.00 NTU is theoretically impossible. But in practice, these results are actually quite meaningful. If a meter consistently gives a negative result, there’s a problem. The problem could be operator technique or error. It could also indicate a problem with the low turbidity/turbidity-free water used for a blank or a problem with the calibration. If the meter rounds the negative result to 0.00 NTU, the user won’t be alerted to a potential problem.
Variability in geometry of the glassware and the technique used to orient tubes in the light chamber are main causes for inconsistent results. Slight variations in wall thickness and diameter of the tubes may lead to slight variations in test results. To eliminate this error, tubes should be placed in the chamber with the same orientation each time. Many turbidity tubes have indexing lines for this. Even better than an indexing line are orientation devices that physically force a tube to be positioned in the same orientation every time. For improved accuracy and precision, especially at low concentrations, some form of orientation device is highly recommended.
Before adding a water sample for turbidity measurement, the tube should be carefully rinsed three to five times with solution, whether it’s a blank or a test sample. The tube should then be capped immediately to prevent dust or other contaminants from entering. Be sure not to spill water on the outside of the tube. If there’s a spill, immediately wipe the tube clean before dry deposits can form.
For low-level turbidity, the tube should be allowed to sit for several minutes to degas after adding the sample. Tiny invisible bubbles of dissolved gasses can cause a positive interference in results. Degassing with a vacuum can lead to contamination if extreme care isn’t taken. A sonic bath can be used to degas a sample, if the tube was rigorously cleaned beforehand. If the tube wasn’t scrupulously clean, a sonic bath can dislodge particles from the tube walls and increase turbidity. After the sample is degassed, the tube should be inverted gently to suspend any particles that may have settled out. The tube should then be allowed to sit and come to a quiescent state before taking a reading on the turbidimeter.
Low-level readings should always be verified that they’re stable by taking two or three readings on a sample or blank. Unstable readings can result from convection currents in the sample due to mixing. A few large particles passing through the detection area of the tube can also cause unstable readings. To compensate, some turbidity meters have a signal averaging option that allows the meter to average multiple readings on an unstable sample. Being able to observe the reading during this averaging process lets the user see if the sample is providing a stable measurement.
Studies indicate low-level turbidity analyses are frequently affected by user techniques. Since there’s a learning curve to making good turbidity measurements, meticulous care and practice in measurement techniques are required for high accuracy and precision in such cases.
Nephelometers usually have a factory calibration for the full range when purchased. Regulatory authorities often require calibrations in these meters be verified and adjusted. Turbidity standards are required for calibration/verification of meters. Most meters are supplied with turbidity standards, such as AMCO, SDVB (styrene divinyl benzene), formazin or “stabilized formazin,” as primary standards. Primary standards are approved by the EPA for calibration of turbidimeters. Other types of secondary standards also are available but can only be used to verify a calibration.
Analysts should only use standards recommended by the nephelometer manufacturer. Formazin is a universal standard but dilutions are highly unstable. AMCO standards are more stable and easy to use, but must be uniquely formulated to individual meter designs. An AMCO standard designed for one turbidimeter cannot be reliably used with a different type of meter, even if these meters are from the same manufacturer.
“Stabilized formazin” is stable but requires special mixing instructions that must be carefully followed. The refractive index of low-level “stabilized formazin” standards is very different from low-level formazin standards and from most ultrapure low-level turbidity water. The refractive index of low-level formazin standards and most ultrapure low-level turbidity water, however, is very similar. Differences in refractive indices can lead to very different results. A meter calibrated with “stabilized formazin” can’t be verified with formazin standards, at low-levels. At high turbidity levels, the refractive index differences become small and both stabilized and un-stabilized formazin should give the same results.
There’s a linear relationship between scattered light at 90° and turbidity of water below 40 NTU. Two approaches exist to calibrate turbidimeters for low-level testing. One uses intermediate level turbidity standards to establish the linear relationship between 40 NTU and ultrapure water. The meter is calibrated at the intermediate level and then extrapolated backwards to measure samples below 1 NTU. A problem with this method is any errors made in establishing the intermediate calibration point are magnified when measuring a sample farther and farther from that point. At very low turbidity levels, these errors could become a significant part of the measurement.
The second approach to calibration is to establish linearity in the turbidity range of the samples to be measured. This is a common approach to calibrating scientific instruments. If the samples to be measured are below 1 NTU, linearity is established by calibrating with a 1 NTU standard. Stable 1 NTU standards are available for turbidimeters calibrated by both approaches. The same careful measurement techniques, for making good low-level turbidity readings discussed previously, are required for low-level calibration. Once these techniques are learned, they become part of the process of making quality readings of all types, whether they’re sample, calibration or verification readings.
Turbidimeters that use a blank have an advantage in establishing a linear calibration with two points in the range of samples to be tested. For low-level turbidity, a turbidimeter can be calibrated with ultrapure water and a 1 NTU standard. Use of good technique, ultrapure low turbidity water and a stable 1 NTU standard are critical for this calibration. The result is a very well established calibration for the range of 0-1 NTU. Blanking the meter before reading samples always ensures the ultrapure low turbidity water reading is referenced every time a sample is determined. This helps maintain a good low-level calibration. It has been shown that turbidimeters that use some sort of blanking procedure, or a procedure to set a predetermined low reading such as 0.02 NTU, read lower than meters that don’t use this procedure. The difference increases over time most likely because the base line response drifts upward on meters not repeatedly calibrated with an ultrapure low turbidity standard.
Reliable low-level turbidity measurements are a critical water quality measurement for regulatory compliance and monitoring filtration efficacy. To obtain dependable results, careful attention to measurement technique is critical. Maintaining tubes with good optical qualities, minimizing stray light and careful sample handling are all important factors. A well designed instrument is only effective if it’s accurately calibrated. Finally, referencing an ultrapure low-level turbidity standard, such as a blank, while testing samples will help preserve and verify a good low-level calibration testing program.
About the Author: James W. Egan Jr., Ph.D., is R&D chemist for LaMotte Company. Based in Chestertown, MD, LaMotte sells and manufactures equipment for testing water, soil and air. It services the pool & spa, environmental science education, aquaculture & aquarium, industrial wastewater, drinking water, soil testing, and home science markets. Contact: 800-344-3100 or www.lamotte.com