5.17
Aula: Gravimetria de Precipitação
A gravimetria de precipitação baseia-se na conversão de um analito em um precipitado pouco solúvel, que é separado por filtração e pesado. Um precipit…
Na gravimetria por precipitação, o analito reage com um precipitante para produzir um precipitado que é isolado e pesado.
O precipitado deve ser de baixa solubilidade, alta pureza, composição conhecida e facilmente separável da mistura de reação.
Por exemplo, na estimativa do níquel, o precipitante é a dimetilglioxima etanólica, que produz um precipitado vermelho de bis (dimetilglioximato) de níquel.
O pH da reação é manipulado para garantir a precipitação completa.
Em seguida, o precipitado é digerido para aumentar o tamanho das partículas, seguido de filtração usando um funil de filtragem pré-pesado.
Este precipitado é lavado com água fria, seco em estufa e deixado esfriar em um exsicador.
Em seguida, obtém-se a massa do precipitado, após o que são aplicadas relações estequiométricas para calcular o teor de níquel.
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Q1: What are the key requirements for an ideal precipitate in gravimetric analysis?
An ideal precipitate must be pure, insoluble, and of known composition to ensure accurate results. It should be easily separable from the reaction mixture and have low solubility to prevent loss during filtration and washing. These properties enable reliable mass determination and stoichiometric calculations for quantifying the analyte.
Q2: How does digestion improve the quality of a precipitate in gravimetry?
Digestion involves heating the precipitate over a hot water bath for several hours, allowing impurities occluded during initial formation to dissolve and the precipitate to slowly reform. This process removes contaminants and increases particle size, resulting in a purer, more easily filterable precipitate that yields more accurate mass measurements.
Q3: Why is pH control important during precipitation gravimetry?
Manipulating reaction pH ensures complete precipitation of the analyte from solution, preventing loss of target material. In nickel gravimetry, dilute ammonia is added dropwise until precipitation occurs, with slight excess ensuring all nickel ions convert to the red nickel bis(dimethylglyoximate) precipitate needed for accurate quantification.
Q4: What is the role of washing and drying in the gravimetric procedure?
After filtration, the precipitate is washed with cold water to remove soluble impurities and residual reagents. Drying in an oven removes moisture, and cooling in a desiccator prevents reabsorption of atmospheric water. These steps ensure that the final mass reflects only the pure, dry precipitate for accurate analyte calculation.
Q5: How is the analyte concentration determined from the precipitate mass?
After obtaining the dried precipitate mass, stoichiometric mole relationships are applied to calculate the original analyte content. In nickel gravimetry, the mass of nickel dimethylglyoximate is converted to moles, then to moles of nickel using the known 1:1 stoichiometry, allowing calculation of nickel concentration in the original sample.
Q6: Why is ethanolic dimethylglyoxime used as the precipitant for nickel determination?
Ethanolic dimethylglyoxime selectively reacts with nickel ions to form a red precipitate of nickel bis(dimethylglyoximate) that is sparingly soluble and easily filtered. This organic precipitating agent produces a pure, stable product of known composition, making it ideal for accurate gravimetric analysis of nickel in samples.
Q7: What is the purpose of cooling the precipitate in a desiccator before weighing?
Cooling in a desiccator prevents the dried precipitate from reabsorbing moisture from the air, which would artificially increase its mass and introduce error into calculations. The desiccator maintains a dry environment, ensuring the measured mass accurately represents only the pure, anhydrous precipitate needed for reliable analyte quantification.