Automated Sulfate Analysis by MTB Colorimetric Analysis
- High Throughput – Rapid sample analysis up to 60 samples per hour
- Low Cost per Analysis – Less lab tech time, very cheap consumables and reagents
- Smaller bench space – The single-channel unit occupies less than one foot square off table space and stands one foot tall
- Compliant with Standard Methods 4500 - avoid the use of toxic reagents
- Sulfate, methyl thymol blue, colorimetric, FIA, flow injection, automation, SM 4500-SO4(2-), throughput
Introduction and Principle
Many flow injection methods start with lightly-colored reagents mixing with a sample to yield a strongly-colored product. This is not the case with the sulfate MTB analysis. Initially, a solution of a stable dark blue complex of barium and methyl thymol blue (MTB) enters the FIA manifold. Once a sample containing sulfate mixes with this dark blue stream under ethanoic conditions, free sulfate ions are able to react with barium complexed MTB to yield BaSO4 and MTB. This newly freed MTB species has a gray color, which our unit’s spectrometer reads. Our flame spectrometer reads the signal
460 nm and for added quality control, uses a reference wavelength of 700 nm.
This is not the whole story. The barium MTB is also susceptible to reactions with other metallic cations such as calcium and magnesium . To remove these and other cations that would interfere with the chemistry, our unit includes an ion exchange column, effectively removing these interferences. Since calcium is one of the main interferences, we recommend running a QC check with a mid-range sulfate standard in a matrix containing a typical level of CaCO3. If this standard yields a lesser response than the same standard in DI water, the IC column is ready for regeneration or replacement of the resin.
One thing to note is that the resin will release some of the cations in specific cases. For this reason, samples with a pH less than 2 must be neutralized . Furthermore, highly ionic solutions will release the calcium from the cation exchange column. One study found a sample containing an excess of 2000 mg/l would scrub any accumulation of calcium from the column. Aside from highly ionic or acidic samples, there are a few select anions that interfere with the reaction. The good news is that these are seldom found in quantities that would produce a major interference in water samples. These include tannic acid, sulfide, sulfite and ortho-phosphate . Compared to other methods, the automated sulfate analysis is superior in a few ways. First, its throughput is unrivaled by other techniques including ion chromatography and the gravimetric method. Furthermore, the consumables are much less expensive than either of the mentioned methods. Finally, it has been shown to have better precision and sensitivity than the gravimetric method .that would produce a major interference in water samples. These include tannic acid, sulfide, sulfite and ortho-phosphate .
- Carrier (C): Water
- Diluent Solution (D): 30 mg SO4/L
- Reagent 1 (R1): 0.6mM Methylthymol Blue
- Reagent 2 (R2): 0.18M NaOH Solution
- C: 0.8 mL/min
- D: 0.7 mL/min
- R1: 1.8 mL/min
- R2: 0.5 mL/min
- 420 μL
- Primary Wavelength: 460nm
- Reference Wavelength: 700nm
|Detection Limit||0.5 mg SO4(2-)/L|
|Range Upper Limit||25 mg SO4(2-)/L|
|Sample Throughput||60 samples/hr|
Table 1: Method performance parameters
 Adamski J.M., Villard S.P., “Application of the Methylthymol Blue Sulfate Method to Water and Wastewater Analysis”. Anal. Chem. 47 (1975) 1191-1194.
 Andrew D. Eaton et al., “Standard Methods for the Examination of Water and Wastewater.” Washington, DC: American Public Health Association, (1998) 4-193 – 4-194.
 “Method for Sulfate Determination by Barium Chloride – MTB Reagent, Version 1.1.” FIAlab Instruments,
Inc., 2018. 1 - 8.