Comparative estimation of nitrogen in urea and its derivatives using TKN, CHNS and a portable refractometer
The use of hand-held refractometer-based detectors has attracted considerable attention due to their small size, low cost, portability, lack of skilled personnel, and real-time detection. In this device, a few drops of liquid suspension containing urea can be placed on the prism, and as a result, the reading is displayed on the screen in 3 s. This portable instrument operates on the principle of refractive index. When a liquid sample is placed on the surface of the prism, light is transmitted through the solution, while some of the light is reflected and detected by photodiodes. As a result, a shadow line is created whose position is closely related to the refractive index of the solution. Therefore, the refractive index or another unit of measurement according to the refractive index is correlated by internal software after the position of the shadow lines is determined using the instrument. Thus, the final liquid concentration can be viewed on the refractometer display. This device is easy to calibrate by simply mounting a few drops of liquid standard or distilled water on the surface of the prism.
In order to determine whether the refractometer as an alternative method to TKN and CHNS for the estimation of N% for conventional urea as well as nano-urea fertilizers, the three techniques were systematically compared in terms of linearity and on the basis of other performance such as cost, sample throughput capabilities, environmental acceptance and automation. The calculation of N% in different concentrations of urea and nano-urea was analyzed by TKN, CHNS and refractometer and then compared. Table 1 shows the descriptive statistics of the data used to compare the N% analysis using different methods (where a duplicate was taken for each sample; n = 2).
In addition, their comparison was evaluated in terms of correlations of results obtained by a different technique. Figure 1a-d shows the linear fit between estimated nitrogen (%) and various urea concentrations ranging from 0.25-10% for TKN, CHNS, refractometer and the theoretical value of expected N%.
Interestingly, the refractometer was able to detect the lower limit of urea at 0.25% (0.11% N) and displayed 0.092% N. Moreover, it was observed that the N% detected by TKN exhibited the straight line with lowest R2 (0.98879) compared to other techniques. While the CHNS and the refractometer detected the different concentrations of urea as their R2 is close to the theoretical value of N%. In addition, the estimation of N% in nanourea was carried out using all techniques to evaluate the performance of the instrument in terms of linearity. A linear fit between the measured N (%) and different nanourea concentrations ranging from 1 to 10% for TKN, CHNS and refractometer can be seen in Figs. 2a–d.
These results indicated that the refractometer-based apparatus showed R2 = 0.99935 with an intercept of −0.04667 ± 0.02455 and a slope of 0.46667 ± 0.00396. It can be concluded that the linear fit for the estimation of N% in nanourea by refractometer is closer to the theoretical value of N% compared to other methods. Additionally, given the importance of DEF solution in diesel engines for the prevention of air pollution, higher concentrations of urea up to 40% were used for analysis. A linear fit between nitrogen (%) and different urea concentrations ranging from 0.25 to 40% for TKN, CHNS, refractometer and theoretical N% was displayed in Figs. 3a–d. On these concentrations, the refractometer-based device produced remarkable results in terms of R2 (0.99918) in comparison with other techniques, indicating that the refractometer can also be used for DEF solution analysis.
Table 2 presents the different techniques for measuring urea and nano-urea samples containing a concentration of up to 10% and their respective values such as R2intercept and slope extracted by linear fit.
In order to further evaluate, the measured N% of different concentrations of urea and nano-urea (1, 5 and 10%) were extracted by all methods and compared, as shown in Figure 4a,b.
It has been observed that the estimate of N% by TKN is inconsistent with the theoretical value of N%. The reason is that this technique only detects ammonium nitrogen as well as organic constituents such as amino acids, nucleic acids and proteins in the sample. However, it is not possible to measure other forms of nitrogen present in nitrites and nitrates using the TKN technique.8. Moreover, the results based on the refractometer are closer to the theoretical value of N% compared to the other techniques, as shown in Fig. 4. Figure 4c displays the analysis of N% in DEF solution using different methods such as CHNS, TKN, and refractometer and then compared their results with the theoretical value of N%. In this analysis, TKN, CHNS, and a refractometer-based device showed N content of 15.27, 14.01, and 14.72% in DEF, respectively. It was observed that the N% was deviated by approximately +6.29% in TKN, −2.14% in CHNS, and −1.53% in refractometer with the theoretical value of N% in DEF. Therefore, the refractometer-based device showed the closest value to the theoretical N% in case of DEF.
Measurement time and automation: when analyzing a large number of samples in laboratories, these characteristics are important. For example, TKN can only analyze 8 samples (including two replicates of each sample and two blanks) using an 8-tube digestion block and a distillation-titration unit in 4 h. In the case of CHNS, approximately 13 to 17 analyzes can be performed in the same period. In contrast, a refractometer can analyze about 170-180 samples (including sampling, testing and prism surface washing) in 4 h. Thus, this device allows rapid detection of N% for urea samples.
Regarding automation capabilities, TKN has a few manual steps (e.g. inserting reagents into the digestion tube, diluting chemicals after digestion, and positioning the digestion tube in the distillation system ). Additionally, sample analysis by CHNS has some progress in terms of inserting samples via an autosampler. On the other hand, one can easily check the results by pressing the button of a refractometer after depositing a few drops of liquid samples.
Environmental and Safety Perspective: The use of hazardous acids (sulphuric acid, sodium hydroxide) and catalyst-based heavy metals is a big concern when using TKN. Additionally, there are a small number of uses of heavy metals during sample analysis by CHNS. On the contrary, the refractometer does not require any hazardous chemicals or toxic elements while analyzing the sample. Surprisingly, DI water is only used for washing the prism surface after measurement.
Cost: Both instruments (TKN and CHNS) are expensive. The price of sample analysis includes their fixed (instrument cost), variable (glassware, standard chemicals, other chemicals, electricity, water consumption and maintenance cost) and labor cost. work of a technician. On the other hand, a handheld refractometer is quite cheap compared to both instruments. The operation of the device is simple and no other cost is involved. For all these reasons, the detection of urea based on portable refractometer could be a possible technique in the field of fertilizer industries.