Determination of potassium in fertilizers

S. Vicente, E.P. Borges, B.F. Reis, E.A.G. Zagatto, Exploitation of tandem streams for carry-over compensation in flow analysis. I. Turbidimetric determination of potassium in fertilizers, Anal. Chim. Acta 438 (2001) 3. 

Fig. 4.10 Scheme of continuous segmented analyser with continuous filtering system for the determination of potassium in fertilizers. (Reproduced from [5] with permission of Pergamon Press). 

McCracken et al. 164) compared atomic absorption with the tetraphenyl-boron method for determining potassium in 1190 fertilizers, and very close agreement was found between the two methods. Hoover and Reagor 16S) also found good agreement between the two methods, and atomic absorption was far more rapid. They reported that the 7665 A potassium line was more subject to interference than the less sensitive 4044 A line. Temperli and Misteli 166> reported far better results for low concentrations of potassium in soil extracts by atomic absorption spectroscopy than by flame emission spectroscopy. 

Examples of the use of FIA with ISE detection involve the determination of nitrate and total nitrogen in environmental samples [48, 49, 125, 166], potassium, sodium [125], calcium [51] and urea [124] in serum or major nutrients in fertilizers [73]. An interesting combination of an ISFET sensor with the FIA principle [52] is shown in fig. 5.17. This is a simultaneous determination of potassium, calcium and pH in serum during dialysis on an artificial kidney. 

Potassium is analyzed in chemicals that are used in the fertilizer industry and in finished fertilizers by flame photometric methods (44) or volumetric sodium tertraphenylboron methods (45) as approved by the AO AC. Gravimetric determination of potassium as IC PtClknown as the Lindo-Gladding method (46), and the wet-digestion determination of potassium (47) have been declared surplus methods by the AO AC. Other methods used for control purposes and special analyses include atomic absorption spectrophotometry, inductively coupled plasma (icp) emission spectrophotometry, and a radiometric method based on measuring the radioactivity of the minute amount of the isotope present in all potassium compounds (48). 

Water-soluble potassium (expressed as K2O) can be determined by manual or automated flame photometric and tetraphenylboron titration methods for all types of fertilizer samples. Atomic absorption can be used for samples below 5% K2O to maintain the highest degree of precision and accuracy. (Note Tetraphenylboron method for potassium is being used less often because of safety issues associated with formaldehyde.) Methods using ICP-OES for high concentration of potassium are currently being developed. Note Potentiometric measurements can be used to determine potassium in fertilizers but are not official approved methods and do not meet the accuracy and precision requirements as current methods. 

The second reason for acid-digestion is the determination of the total soil elemental content of, e.g. potassium, phosphorus or trace elements. This is seldom done for potassium in normal soil samples, mainly because the total K in soils is of no value as an index to the availability of K to plants, nor is it always of value in tracing the movement or accumulation of applied fertilizer K (Pratt, 1965). The unreactive soil phosphorus is obtained by subtracting the naturally leached reactive phosphorus from the total phosphorus, and a method for determining the latter by extraction with sulphuric acid and potassium persulphate is cited by Turner and FHaygarth (2000). They analysed... 

Mesocarp—fruit ratio is largely genetically determined and is little affected by environmental factors. Fruit—bunch ratio depends mainly on the efficiency of pollination. Oil-mesocarp ratio depends in part on the ripeness of the fruit, since oil is only synthesized during the later stages of fruit development. There is also considerable variation in oil-mesocarp of bunches from the same progeny harvested at different times of the year (23). It has also been shown that application of potassium fertilizer leads to a reduction in the oil-bunch ratio (24) but the increase in fruit yield in response to potassium was more than enough to compensate for the reduced oil-bunch ratio. 

Shortly after postulating the need for fertilization of plants with the basic nutrients nitrogen (N), phosphorus (P), and potassium (K) by Justus von Liebig, it was determined that, if solid fertilizer materials are applied, the availability of these elements depends to a large extent on solubility. In most cases a high solubility is desired requiring large surface area which is synonymous with small particle size. It was also found that, additionally, micronutrients are necessary and, because of the small amount of these trace elements in a fertilizer formulation, they had to be added as fine powders. 

The effectiveness of a nitrogen fertilizer is determined mainly by its mass % N. Rank the following fertilizers in terms of their effectiveness potassium nitrate ammonium nitrate ammonium sulfate urea, CO(NH2)2-... 

Baier J (1992) Fertilizer recommendation as determined by chemical and physical parameters of soil and by K requirement of crops. In Anonymous, ed. Potassium in Fcosystems, 23rd Colloquium of the International Potash Institute Basel, Switzerland. 

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