Thermometric Titration for Multiparameter Analysis in Fertilizers

Estimates show that without the usage of fertilizers, around half of the world’s population could not be supported1. Thus, in the modern world, agriculture at significant scale without fertilizers is no longer possible.

In order to grow a sufficient amount of produce for close to 8 billion people, in addition to domesticated animals and industrial uses, fertilizers of different nutrient compositions are available to cater to the unique requirements of different soil types.

Information on the fertilizer’s composition (e.g., total nitrogen, phosphorus, and potassium) is given to choose the ideal fertilizer for a specific soil. Conventionally these constituents are established either gravimetrically (e.g., potassium, phosphorus, or sulfate)2–4 or with ICP-OES (e.g., phosphorus or potassium)5.

These techniques either require expensive instrumentation with high running costs (ICP-OES) or have the disadvantages of long analysis times combined with laborious sample preparation (gravimetry).

This article describes how thermometric titration is an inexpensive and quick alternative technique to supply information on the content of various nutrients in different fertilizers.

Fertilizer Composition for Different Needs

Known as macronutrients, the main nutrients in fertilizers are phosphorus, nitrogen, and potassium. Other nutrients are micronutrients (e.g., boron) or secondary macronutrients (e.g., sulfur).

In particular macronutrients are vital for plants as they are required for development of seeds and fruits, leaf growth, and for water transport within the plant. Fertilizers are usually classified by their nutrient composition.

Further to single nutrient or straight fertilizers (e.g., ammonium nitrate or single superphosphate), multi-nutrient fertilizers consisting of two or more nutrients are common, for example mono- and di-ammonium phosphate (MAP and DAP) or NPK (nitrogen-phosphate-potassium) fertilizers.

 

In addition to classification according to their nutrients, fertilizers may also be categorized into inorganic mineral fertilizers and organic fertilizers2. As their name implies, organic fertilizers are made up of organic matter derived from plants and/or animals, such as dung. In contrast, with the exception of urea, inorganic fertilizers do not contain carbon-based materials. This article will mainly focus on inorganic fertilizers.

Conventional Determination of Nutrient Composition

Aside from the fertilizer composition, it is crucial to know about the nutrient content. Too much fertilizer might be distributed to plants without this information, leading to environmental pollution and undesirable fertilizer burns.

Therefore, fertilizer producers are required to specify the amount of nutrients within their products, and for the standardized determination of these nutrients, various norms from EN, ISO, and AOAC exist. Yet, some of the proposed analysis methods are very time-consuming, or require expensive equipment.

For instance, potassium, phosphorus, and sulfur are normally established gravimetrically or by using ICP-OES. Meanwhile, Kjeldahl digestion is typically needed to establish nitrogen components with a subsequent acid-base titration.

Thermometric titration supplies an inexpensive alternative solution for the analysis of sulfur, ammoniacal nitrogen, potassium, phosphorus, and urea without any time-consuming steps. The general principle of thermometric titration is outlined before elaborating on the analysis of these specific nutrients by thermometric titration.

The Principle of Thermometric Titration

Titration is an established analysis technique, where the content of a species (analyte) is determined by adding a reagent solution (titrant) which reacts stoichiometrically with the analyte.

The analyte content can be reliably determined by establishing the required titrant volume for the reaction. The required titrant volume can be established by utilizing a suitable indication technique.

In order to indicate the endpoint of the titration, thermometric titration employs the principle of the reaction enthalpy. Titrant and analyte react with each other either endothermically (decrease in temperature) or exothermically (increase in temperature).

When the titrant is added at a constant rate, the temperature in the titration vessel also increases or decreases at a constant rate. Heat is no longer consumed or produced when the endpoint of the titration is reached, and a sharp break in the temperature curve can be seen which indicates the reaction endpoint (Fig. 1)3.

Idealized titration curve for an exothermic titration reaction. In this example, as long as analyte is present, the temperature increases with the titrant addition. When all analyte is consumed, the temperature decreases again as the solution equilibrates with the atmospheric temperature and/or due to the dilution of the solution with titrant. This temperature decrease results in an exothermic endpoint.

Figure 1. Idealized titration curve for an exothermic titration reaction. In this example, as long as analyte is present, the temperature increases with the titrant addition. When all analyte is consumed, the temperature decreases again as the solution equilibrates with the atmospheric temperature and/or due to the dilution of the solution with titrant. This temperature decrease results in an exothermic endpoint.

Thermometric titrations utilize a thermistor which can detect the smallest temperature change, to show the endpoint of a titration (Fig. 2). These sensors can measure temperature differences which are less than 0.001 °C, and enable the collection of a measuring point every 0.3 seconds.

The Metrohm Thermoprobe is capable of measuring temperature changes of less than 0.001 °C, and allows the collection of a measuring point every 0.3 seconds

Figure 2. The Metrohm Thermoprobe is capable of measuring temperature changes of less than 0.001 °C, and allows the collection of a measuring point every 0.3 seconds.

Compared to other sensors employed in titration, they can be utilized to establish various analytes and are nearly maintenance-free. The applicability of thermometric titration for the analysis of various nutrients in fertilizers will be demonstrated in the next section.

Applicability of Thermometric Titration for Fertilizers

Phosphorus

Usually, phosphorus in fertilizer is present as phosphate. As it is a macronutrient, it is not only found in multi-nutrient fertilizers (e.g., MAP, DAP, and NPK-fertilizers), but also as straight fertilizer in the form of superphosphates. Historically, gravimetric analysis is used to measure total phosphorus content.

Alternatively, spectrophotometric analysis or ICP-OES may be utilized for the determination. These techniques all need time-consuming sample preparation steps or regular calibrations.

Thermometric titration system consisting of a Metrohm 859 Titrotherm equipped with a Thermoprobe for the indication and two 800 Dosinos for the titrant and addition of auxiliary solution. The system is controlled via the Metrohm tiamoTM software.

Figure 3. Thermometric titration system consisting of a Metrohm 859 Titrotherm equipped with a Thermoprobe for the indication and two 800 Dosinos for the titrant and addition of auxiliary solution. The system is controlled via the Metrohm tiamoTM software.

The thermometric titration is based on the exothermic formation of insoluble struvite (MgNH4PO4 · 6 H2O) in alkaline media according to the following reaction equation:

For the titration, magnesium nitrate is employed as the titrant in an alkaline NH3/NH4Cl buffer solution. So, the titration is an adoption of a classical gravimetric procedure. Yet, by using thermometric titration, results for the phosphate content can be collected within five minutes without any further steps like washing, filtering, and drying.

Exothermic titration curve of the phosphate determination in an NPK fertilizer by precipitation with magnesium ions in the presence of an ammonia/ammonium chloride buffer (blue = titration curve, pink = second derivative showing the endpoint).

Figure 4. Exothermic titration curve of the phosphate determination in an NPK fertilizer by precipitation with magnesium ions in the presence of an ammonia/ammonium chloride buffer (blue = titration curve, pink = second derivative showing the endpoint). 

The ratio of H2PO4- and HPO42- is another crucial parameter in the phosphoric acid manufacturing process. It is usually not possible to distinguish these two species in a potentiometric acid-base titration because of the leveling effects of water.

Yet, thermometric titration is not affected by this problem. So, it is possible to simply establish the ratio by employing sodium hydroxide as titrant. If phosphoric acid is titrated, it is possible to observe all three endpoints (Fig. 5).

Exothermic titration curve of phosphoric acid titrated with NaOH. Here, all three  deprotonation steps can be clearly distinguished (blue = titration curve, red = second derivative showing the endpoints).

Figure 5. Exothermic titration curve of phosphoric acid titrated with NaOH. Here, all three  deprotonation steps can be clearly distinguished (blue = titration curve, red = second derivative showing the endpoints).

Potassium

As it plays a key role in water regulation as well as plant growth, potassium is a primary macronutrient for plants. It is available as a straight fertilizer in the form of potash for this reason, in addition to in multi-nutrient fertilizers like potassium dihydrogen phosphate (MKP) or NPK fertilizers.

Traditionally, potassium is established gravimetrically, but more recently, flame photometry or ICP-OES is utilized. Potassium can also be determined by titration using the precipitation reaction of potassium with sodium tetraphenyl borate (STPB).

A reliable determination of potassium using thermometric titration supplies a result in around five minutes. This titration technique is already incorporated into the Chinese recommended professional standard HG/T 2321 on the analysis of fertilizer grade potassium dihydrogen phosphate6 because of this advantage, in addition to the low costs per analysis and minimal sample preparation.

If the fertilizer contains ammonium, prior to the titration this must be removed as ammonia, as otherwise it would  precipitate concurrently with STPB and so interfere with the determination.

Exothermic titration curve of the potassium determination in potash by precipitation with STPB (blue = titration curve, pink = second derivative showing the endpoint).

Figure 6. Exothermic titration curve of the potassium determination in potash by precipitation with STPB (blue = titration curve, pink = second derivative showing the endpoint). 

Sulfate

Sulfur is vital for chloroplast growth and function and is a secondary macronutrient for plants. In fertilizers, sulfur is typically supplied in the form of sulfate. It is also present during the wet phosphoric acid production process.

For an optimal production process, the sulfuric acid content within phosphoric acid, in addition to that within final products (e.g., DAP, TSP, MAP,and NPK fertilizers) should be known.

Exothermic titration curve of the sulfate determination in a NPK fertilizer spiked with sulfuric acid for enhanced method sensitivity (blue = titration curve, pink = second derivative showing the endpoint).

Figure 7. Exothermic titration curve of the sulfate determination in a NPK fertilizer spiked with sulfuric acid for enhanced method sensitivity (blue = titration curve, pink = second derivative showing the endpoint). 

Conventionally, sulfate content is established gravimetrically by precipitation with barium. The same reaction principle is utilized for the thermometric titration and results can be collected within three minutes, with only minimal sample preparation.

The samples can be spiked with a standard sulfuric acid solution in order to increase the sensitivity of the technique, which is then considered when calculating the result.

As they can form insoluble calcium sulfate, calcium (e.g., from CAN fertilizers) can interfere with this determination. Therefore, it must either be precipitated with excess oxalate or removed prior to the titration using cationic exchange resins.

Ammoniacal Nitrogen and Urea

Nitrogen is the third significant macronutrient needed by plants, and represents by far the largest product group of fertilizers. Nitrogen in fertilizers can be present in various forms including nitrate, ammonium, or urea.

Usually, ammonia is determined after alkaline distillation by acid-base back-titration, while other nitrogen species are usually first converted to ammonia by digestion prior to analysis. A different method is employed to determine ammonia content for the thermometric titration.

Ammonium ions react exothermically with hypochlorite in a redox reaction. This reaction is catalyzed further in the presence of bromide ions in slightly alkaline solution, where the more reactive species hypobromite is formed, which then reacts with ammonium to form nitrogen.

This titration method enables results without any prior distillation step after an analysis time of two minutes. Urea has the highest nitrogen concentration among straight fertilizers that contain nitrogen. Urea is generated from carbon dioxide and ammonia. The production process is affected by the amount of ammonia present.

Thus, during the process, it is crucial to know the ratio of ammonia and urea present7. It is possible to not only establish ammoniacal nitrogen in the sample, but also urea as well when using the aforementioned titration approach for ammoniacal nitrogen.

This is possible because urea also reacts with hypobromite, but with a slower reaction rate. The separation of ammonium and urea content in a single titration becomes possible when using an appropriate dosing rate of titrant. The fertilizer can be spiked with some urea for a better separation of both endpoints.

Exothermic titration curve of the ammoniacal nitrogen and urea determination in a NPK fertilizer. The first endpoint corresponds to the ammonia and the second to urea (blue = titration curve, pink = second derivative showing the endpoint).

Figure 8. Exothermic titration curve of the ammoniacal nitrogen and urea determination in a NPK fertilizer. The first endpoint corresponds to the ammonia and the second to urea (blue = titration curve, pink = second derivative showing the endpoint).

Thermometric Titration: The Ideal Solution to Analyze Fertilizers

Thermometric titration is an inexpensive alternative technique to analyze the key substances within fertilizers, or during their production processes. In addition, time-consuming sample preparation and analysis steps can be removed, enabling analysts to gather results within minutes.

In turn, this enables plant operators to optimize process settings faster. This greatly enhances the efficiency of the production plants, while at the same time liberating lab technicians and chemists and to perform other tasks.

References and Further Reading

  1. Our World in Data. World population with and without synthetic nitrogen fertilizers. https://ourworldindata.org/grapher/world-population-with-and-without-fertilizer?time=1900..2015 (Accessed March 23, 2020).
  2. ISO 6598:1985 Fertilizers – Determination of phosphorus content — Quinoline phosphomolybdate gravimetric method. Geneva : International Organization for Standardization, 1985. https://www.iso.org/standard/13009.html
  3. ISO 17319:2015 Fertilizers and soil conditioners – Determination of water-soluble potassium content — Potassium tetraphenylborate gravimetric method. Geneva : International Organization of Standardization, 2015. https://www.iso.org/standard/59569.html
  4. ISO 10084:1992 Solid fertilizers – Determination of mineral-acid-soluble sulfate content — Gravimetric method. Geneva : International Organization for Standardization, 1992. https://www.iso.org/standard/18056.html
  5. ISO/CD 20917 Determination of Available Phosphorous and Soluble Potassium Extracted with Neutral Ammonium Citrate and Quantified by ICP-OES. Geneva : International Organization for Standardization, 2020. https://www.iso.org/standard/69457.html
  6. Fertilizers Europe. Types of fertilizer. https://www.fertilizerseurope.com/fertilizers-in-europe/types-of-fertilizer/ (Accessed May 7, 2020).
  7. Smith, T. Practical thermometric titrimetry; Metrohm Monograph: Herisau , Switzerland, 2006. https://www.metrohm.com/en/documents/80365003
  8. Fertilizers Europe. Booklet - BAT Production of phosphoric acid. https://www.fertilizerseurope.com/wp-content/uploads/2019/08/Booklet_4_final.pdf (Accessed May 7, 2020).
  9. Bache, S. Rapid phos acid process monitoring. Fertilizer International. Issue 486, September-October 2018.
  10. HG/T 2321-2016 Fertilizer grade potassium dihydrogen phosphate. Beijing : The Standardization Administration of the People's Republic of China, 2016.
  11. Fertilizers Europe. Booklet - BAT Production of urea and urea ammonium nitrate. https://www.fertilizerseurope.com/wp-content/uploads/2019/08/Booklet_5_final.pdf (Accessed May 7, 2020).
  12. Fertilizers Europe. How fertilizers are made? https://www.fertilizerseurope.com/fertilizers-in-europe/how-fertilizers-are-made/ (Accessed May 7, 2020).

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.

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