AN-SPEC-1052
2025-10
Quantifying sulfuric and phosphoric acids for fertilizer production by Raman spectroscopy
Summary
This document describes a method for the determination of phosphoric and sulfuric acid concentration in the wet process of phosphoric acid production. This method is primarily used in the fertilizer industry and can be applied over dihydrate, hemihydrate, and hemi-di production methods. Phosphoric and sulfuric acids can be quantified in under a minute with only a filtration step for sample preparation.
Samples
In-process phosphoric/sulfuric acid mixes from fertilizer production plants.
Instruments and accessories
| Quantity | Article No |
|---|---|
| 1 | BWT-840001234 i-Raman EX |
| 1 | BWT-840000909 BAC102 probe |
| 1 | BWT-840000287 Vial Holder Adapter |
| 1 | BWT-840001137 Vision Software |
Sample preparation
All samples were filtered either with a syringe filter or with vacuum filtration to remove solid gypsum. Samples were then decanted into 15x26 mm glass vials for analysis.
Raman analysis
The i-Raman EX with 1064 nm excitation wavelength and vial holder attachment for the BAC102 probe were used for measurement to reduce the effects of fluorescence that arise from oxidized metal chromophores.
Samples were measured with 2-15 seconds acquisition time at 330 mW laser power depending on site and approximate concentration of phosphoric acid. All spectra were normalized to acquisition time before modeling. Spectra were treated with an N-point smooth and a 1st derivative with a 17 cm-1 segment and a 1st derivative with a 21 cm-1 segment.
Reference methods
To measure phosphoric acid concentrations, the molybdo vanadate spectrophotometric method was used. In this method, phosphoric acid reacts with molybdate and vanadium to produce vanadomolybdophosphoric acid, which is yellow in color. The color is then determined spectrophotometrically and correlates to the amount of phosphoric acid present. In-depth pre-treatment of the phosphoric acid can be necessary to clean up the sample before analysis. Turbidity of the sample and temperature variations can also lead to inconsistent results.
To test sulfate concentration, manufacturers use thermometric titration in which the sulfuric acid reacts with concentrated sodium hydroxide and a temperature change is recorded. The measured temperature change correlates to acid concentration. Inductively coupled plasma – optical emission spectroscopy is also used to determine sulfate concentration. This method requires a large amount of energy and an argon feedstock to generate plasma. The sulfur line at 182 nm is tracked to determine sulfate concentration. While this method is relatively quick, sample preparation and data analysis can be tricky, requiring trained chemists and technicians to produce consistent results. This method also requires the samples to be transported from the plant to the lab, increasing the time from sampling to analysis.
Results
SEC – Standard error of correlation
SECV – Standard error of cross validation
SEP – Standard error of prediction
Comments
- Samples can range in color from light yellow to dark amber to bright emerald green. This is due to oxidized metal chromophores that are present in the phosphate rock. All of these are acceptable colors for phosphoric acid, but these chromophores make using 1064 nm laser wavelength excitation necessary.
- All spectra were collected at phosphoric acid plants during production, at-line. Measurements were taken after filtration, but did not require transportation to a lab, so there was minimal time between sampling and analysis.
- Models can be further refined by plant, quarry, reactor, concentration range, etc. and still provide tight agreement between SEC, SECV, and SEP.
Conclusion
Raman spectroscopy offers a quick and efficient method for quantifying phosphoric and sulfuric acids during wet process production. With minimal sample preparation, values can be generated in under a minute at-line, with minimal lag time allowing for quicker response time and easier monitoring of reactions.