V. Conclusões Gerais
1.1 Técnicas de análise
1.2.6. Polarografia
A polarografia é um caso particular de voltametria, cuja resposta é originada pela combinação do transporte de massa por difusão e convecção. Na polarografia linear é utilizado um elétrodo de mercúrio gotejante, DME (“dropping mercury electrode”) e é efetuado um varrimento linear do potencial. Uma representação da corrente em função da diferença de potencial demonstra as oscilações de corrente originadas pelas gotas de mercúrio que caem do capilar. As concentrações de sulfuretos inorgânicos dissolvidos nas amostras de água são, em geral, determinadas com um elétrodo de mercúrio, aproveitando a facilidade de oxidação do mercúrio devido à precipitação do sulfureto.
128 Ricardo M.P.Mendes Usando as técnicas de polarografia diferencial com impulsos (DPP) e de voltametria de redissolução catódica (DPCSV), determina-se o teor em sulfuretos ficando o precipitado adsorvido à superfície da gota de mercúrio num passo prévio, consoante a concentração da amostra (Madureira, 1997).
Na voltametria diferencial de redissolução catódica, o analito é primeiro adsorvido no minielétrodo durante um determinado período de tempo, formando um composto insolúvel, a um potencial mais positivo do que o valor de equilíbrio elétrodo/composto insolúvel/anião em solução.
A análise da solução contendo HS-, fez-se por Polarografia Diferencial por Impulsos (DPP), onde se utilizou um elétrodo getejante de mercúrio (DME), onde ocorrem as seguintes reações:
Os iões HS- reagem com o mercúrio do elétrodo formando sulfureto de mercúrio que é adsorvido a este. A deposição do sulfureto é feita durante 60s a um potencial constante de -400 mV.
HS- + Hg HgS (precipita) + 2e-
É então feito um varrimento catódico entre -400 e 900 mV com uma velocidade de 5 mV s-1. Nesta fase, o sulfureto de mercúrio é “redissolvido” passando o ião sulfureto à solução e regenerando o mercúrio do elétrodo.
2e- + HgS (precipitado) Hg + S2-
Tal como na técnica de DPP, os iões HS- reagem com o mercúrio do elétrodo, formando o sulfureto de mercúrio, que é adsorvido a este. Numa segunda fase o sulfureto de mercúrio é redissolvido, regenerando-se o mercúrio do elétrodo e o ião sulfureto passa à solução (Canário, 2000).
Ricardo.M.P.Mendes 129
Glossário
Meios Anóxicos Meios com maior potencial redutor e elevado teor em espécies reduzidas.
Meios Óxicos Meios com maior potencial oxidante e elevado teor em espécies oxidadas.
Meios Subóxicos Meios compreendidos entre os meios óxicos e anóxicos com propriedades e espécies intermédias dos dois meios.
Sapal De uma forma geral, é como uma pradaria de halófitas em sedimentos aluviais na fronteira de corpos de água salgada cujo o nível varia com as marés. Estuário Localização geral das zonas onde os rios encontram o mar, e onde a água dos rios se mistura com a água do mar.
Rizosfera Camada sedimentar que é diretamente influenciada pelas raízes das plantas, sendo importante na determinação de processos na mobilidade de metais e a sua disponibilidade para reações físico-químicas.
Corer Peça metálica com uma extremidade em forma semicilíndrica utilizada para retirar amostras de sedimentos
Core Amostras de sedimentos obtidas através do corer, em forma cilíndrica.
Tortuosidade Medida de difusão das águas intersticiais entre os poros do sedimentos.
130 Ricardo M.P.Mendes
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