Teste de Tollens

teste de Tollens para aldeído: positivo à esquerda e negativo à direita

O teste de Tollens[1] serve diferenciar aldeídos de cetonas e diferencia os açúcares redutores. O nitrato de prata adicionado reage com o hidróxido de amônio e precipita hidróxido de prata. A adição de mais de hidróxido de amônio dissolve o precipitado pela formação do íon complexo diamin prata.

AgNO3 + NH4 OH → AgOH ↓ + NH4 NO3

AgOH + 2NH4 OH → [Ag(NH3 ) 2 ]OH + 2H2 O

Este complexo atua como redutor para aldeídos, que por sua vez são oxidados a ácido carboxílico/ carboxilato. A prata metálica, sob condições de repouso e pH adequados, se deposita nas paredes do tubo de ensaio e forma um espelho de prata. Caso o pH não esteja básico o suficiente ou o sistema for agitado, pode-se formar apenas um precipitado cinza, que também caracteriza a presença de açúcares redutores.


Adicione 0,01 g ou 0,1 mL de amostra e adicione 1 mL do reagente de Tollens recentemente preparado (veja preparação a seguir) em um tubo de ensaio bem limpo. Coloque o tubo em um banho de água aquecido durante 2 minutos. A positividade do teste é dada pela formação de um espelho de prata nas paredes do tubo ou pela formação de um precipitado escuro.


Reagente de Tollens: em um tubo de ensaio colocar 2 mL de uma solução de AgNO3 5% e adicionar 1 gota de uma solução diluída de NaOH 10%. Juntar gota a gota uma solução de NH4 OH 2%, agitando até o total desaparecimento do precipitado de óxido de prata.

Evite o excesso de hidróxido de amônio. O reagente de Tollens não deve ser armazenado porque se decompõe, precipitando uma substância explosiva.[2] Anote suas observações sobre a formação de produtos, tempo de reação e aparência do produto.

Referências

  1. ↑ Tollens, B. (1882). "Ueber ammon-alkalische Silberlösung als Reagens auf Aldehyd" [On an ammonical alkaline silver solution as a reagent for aldehydes]. Berichte der Deutschen Chemischen Gesellschaft (in German). 15 (2): 1635–1639. doi:10.1002/cber.18820150243
  2. ↑ Vogel, Arthur Israel. (1996). Vogel's qualitative inorganic analysis. 7th ed. ed. Harlow: Longman. OCLC 34410809 

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