Desenvolvimento de métodos analíticos para a determinação da concentração de fosfito em fertilizantes

Métodos titrimétricos (convencional e biamperométrico) e amperométricos (manual e em fluxo) são propostos para determinar a concentração de fosfito em fertilizantes. Os métodos titrimétricos foram baseados na reação entre iodo e fosfito em meio de Na2HPO4/NaH2PO4 (pH = 6,8) a 70 °C. Alternativamente, pode ser feita a titulação convencional por retorno à temperatura ambiente, onde o excesso de iodo é titulado com tiossulfato. O consumo médio de reagentes corresponde a 200 mg KI, 127 mg I2, 174 mg Na2HPO4 e 176 mg NaH2PO4 por determinação. Os métodos amperométricos empregaram três eletrodos contendo: disco de 0,2 cm de raio de grafite quimicamente modificado com Pt e Pd (eletrodo de trabalho), um disco de platina de 0,2 cm2 (eletrodo auxiliar) e um eletrodo de Ag/AgCl,KClsat. (eletrodo de referência). No método manual, correlações lineares entre corrente de pico e concentração de fosfito foram obtidas no intervalo 0,01 - 0,04 mol L-1 H3PO3, apresentando sensibilidade de 30,54 mA L mol-1 H3PO3 com coeficiente de correlação linear R= 0,995. Na amperometria em fluxo, as correlações lineares entre corrente de pico e concentração de fosfito foram obtidas no intervalo 0,009 - 0,045 mol L-1 H3PO3, apresentando sensibilidade amperométrica de 2,42 mA L mol-1 H3PO3 com coeficiente de correlação linear R= 0,9997. Os resultados da análise de amostras comerciais, feitas em triplicata, para os quatro métodos foram concordantes com os resultados obtidos por espectrofotometria e com os valores nominais dos rótulos no nível de 95% de confiança.Classical and biamperometric titrimetry, and manual and flow amperometric methods were proposed for phosphite determination in liquid fertilizer. The titrimetric methods were based on reaction of H3PO3 with standard iodine solution in Na2HPO4/ NaH2PO4 (pH = 6.8) media at 70°C. Alternatively, back titration is also feasible, where an excess of titrant at room temperature, is titrated with thiosulfate. The titration methods consumed about 200 mg KI, 127 mg I2, 174 mg Na2HPO4 and 176 mg by NaH2PO4 per determination. The manual and flow amperometric methods employed three electrodes containing: a disk of 0.2 cm in radius of graphite chemically modified with Pt and Pd (working electrode), a platinum disk of 0.2 cm2 (counter electrode) and an Ag / AgCl, KClsat. electrode (reference electrode). In the amperometric method, linearity between anodic current peak and phosphite concentration were obtained in the range 0.01- 0.04 mol L-1 H3PO3, the amperometric sensibility was 30.54 mA L mol-1 H3PO3 with good linear correlation coefficient (R=0.995). In flow-injection amperometry, linear correlation between anodic current peak and phosphite concentration were obtained in the range 0.009 – 0.045 mol L-1 H3PO3, the amperometric sensibility was 2.42 mA L mol-1 H3PO3 with good linear correlation coefficient (R=0.9997). The results of commercial samples analysed by four methods were in agreement with those obtained by spectrophotometry and the nominal values of the labels at 95% confidence level.Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

Método titrimétrico para determinar fosfito em amostras agroindustriais

A new titrimetric method for the determination of phosphite in fertilizer samples, based on reaction of H3PO3 with standard iodine solution in neutral media, is proposed. Diluted samples containing ca. 0.4% m/v P2O5 are heated and titrated with 0.05 mol L-1 iodine standard until the solution becomes faint yellow. Back titration is also feasible: a slight excess of titrant is added followed by starch indicator and titration is completed taking as the end point the change in color from blue to colorless. The influence of chemical composition and pH of buffers, temperature and foreign species on waiting time and end-point detection were investigated. For the Na2HPO4/NaH2PO4 buffer (pH 6.8) at 70 °C, the titration time was 10 min, corresponding to about 127 mg iodine, 200 mg KI and 174 mg Na2HPO4 and 176 mg NaH2PO4 consumed per determination. Accuracy was checked for phosphite determination in seven fertilizer samples. Results obtained by the proposed procedure were in agreement with those obtained by spectrophotometry at 95% confidence level. The R.S.D. (n=10) for direct and back titration was 0.4% and 1.3% respectively.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

Phosphite determination in fertilizers after online sequential sample preparation in a flow injection system

A flow injection spectrophotometric system is proposed for phosphite determination in fertilizers by the molybdenum blue method after the processing of each sample two times on-line without and with an oxidizing step. The flow system was designed to add sulfuric acid or permanganate solutions alternately into the system by simply displacing the injector-commutator from one resting position to another, allowing the determination of phosphate and total phosphate, respectively. The concentration of phosphite is obtained then by difference between the two measurents. The influence of flow rates, sample volume, and dimension of flow line connecting the injector-commutator to the main analytical channel was evaluated. The proposed method was applied to phosphite determination in commercial liquid fertilizers. Results obtained with the proposed FIA system were not statistically different from those obtained by titrimetry at the 95% confidence level. In addition, recoveries within 94 and 100% of spiked fertilizers were found. The relative standard deviation (n = 12) related to the phosphite-converted-phosphate peak alone was <= 3.5% for 800 mg L-1 P (phoshite) solution. Precision due to the differences of total phosphate and phosphate was 1.1% for 10 mg L-1 P (phosphate) + 3000 mg L-1 P (phosphite) solution. The sampling rate was calculated as 15 determinations per hour, and the reagent consumption was about 6.3 mg of KMnO4, 200 mg of (NH4)(6)Mo7O24 center dot 4H(2)O, and 40 mg of ascorbic acid per measurement

Evaluation of Lines of Phosphorus and Potassium by High-Resolution Continuum Source Flame Atomic Absorption Spectrometry for Liquid Fertilizer Analysis

The usefulness of the side pixel registration approach and molecular absorption was investigated for the determination of phosphorus (P) and potassium (K) in liquid fertilizers by high-resolution continuum source flame atomic absorption spectrometry.The phosphorus oxide (PO) lines at 246.400 nm, 247.620 nm, 247.780 nm, 324.616 nm, and 327.040 nm were evaluated. The influence of side pixel registration to extend the linear working range was also investigated by measuring at wings (404.402 nm, 404.406 nm, 404.410 nm, 404.418 nm, 404.422 nm and 404.426 nm) of the secondary line for K at 404.414 nm. For the lines 327.040 mm and 404.422 nm, analytical curves in the 100-5000 mg L-1 and 50-1000 mg L-1 ranges were obtained with linear correlations better than 0.999 and 0.995, respectively.The proposed method was applied to six liquid fertilizer samples. The found concentrations for K and P were in agreement with those obtained by inductively coupled plasma optical emission spectrometry (ICP-OES), spectrophotometry, and flame atomic emission spectrometry at the 95% confidence level.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq