Recebido em 4/11/05; aceito em 14/6/06; publicado na web em 10/1/07 TITRIMETRIC METHOD FOR PHOSPHITE DETERMINATION IN AGRICULTURAL CHEMICAL SAMPLES. A new titrimetric method for the determination of phosphite in fertilizer samples, based on reaction of H 3 PO 3 with standard iodine solution in neutral media, is proposed. Diluted samples containing ca. 0.4% m/v P 2 O 5 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 Na 2 HPO 4 / NaH 2 PO 4 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 Na 2 HPO 4 and 176 mg NaH 2 PO 4 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.Keywords: phosphite; titrimetry; iodine. INTRODUÇÃOO uso recente de produtos à base de fosfito nas atividades agríco-las brasileiras tem crescido significativamente em função da busca por aumento na produtividade e na qualidade dos produtos finais [1][2][3] . Entre as principais vantagens da utilização de fosfito na agricultura merecem destaque a absorção mais rápida de fósforo pela planta em comparação com produtos à base de fosfato, o baixo custo relativo da matéria-prima, o prolongamento do tempo de conservação do produto após a colheita, e por fim, a ação dupla do fosfito, ou seja, além de fertilizante, ele atua como fungicida 2,4 . Nesse contexto, o desenvolvimento e a aplicação de novos métodos analíticos para determinação de fosfito em insumos agroindustriais são importantes não apenas do ponto de vista acadêmico, mas também do social e econômico: dispondo-se de um controle de qualidade eficiente, há grandes chances de se gerar mais confiança em toda a cadeia produtiva e consumidora.A espectrofotometria e a gravimetria são as técnicas analíticas mais utilizadas para determinação de fósforo 5,6 . Dos métodos espectrofotométricos, destacam-se o do Azul de Molibdênio e o do Amarelo de Molibdênio 7 . Para aplicá-los na determinação qualitativa ou quantitativa de fosfito, o pré-requisito essencial é a conversão prévia dos íons PO 3 3-em PO 4 3-, para que ocorram as respectivas reações indicadoras principais.Em relação à gravimetria, um dos métodos descritos na literatura 6 baseia-se na precipitação de fosfato sob a forma de Mg(NH 4 )PO 4 . 6H 2 O ou de pirofosfato de magnésio -Mg 2 P 2 O 7 . Devese salientar que a precipitação de fosfato como sal duplo de magnésio e amônio exige também a conversão prévia de fosfito a fosfato empre...
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)6Mo7O24.4H2O, and 40 mg of ascorbic acid per measurement.
A sensor based on graphite electrode modified with palladium-platinum-palladium film is proposed for phosphite determination by flow-injection amperometry. The modified electrode was prepared by a sequential cathodic deposition of Pd, Pt and Pd on a graphite electrode from 0.5% m/v PdCl 2 þ 28% m/v NH 4 OH and 2% m/v H 2 PtCl 6 þ 10% v/v H 2 SO 4 solutions. After suitable conditioning, the electrode showed catalytic activity for phosphite oxidation when 0.15 V was applied. The proposed system handles approximately 50 samples per hour (0.01 -0.05 mol L À1 Na 2 HPO 3 ; R 2 ¼ 0.9997), consuming ca. 70 mL of sample per determination. The limit of detection and amperometric sensibility were 5 Â 10 À4 mol L À1 and 1.5 mA L mol À1, respectively. The proposed method was applied to analysis of fertilizer samples without pre-treatment. Results are in agreement with those obtained by spectrophotometry and titrimetry at 95% confidence level and good recoveries (96 -109%) of spiked samples were found. Relative standard deviation (n ¼ 12) of a 0.01 mol L À1 Na 2 HPO 3 sample was 2%. The useful lifetime of modified electrode was around 220 determinations. For routine purposes it means that this electrode can be continuously used for 5 hours.
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