Studies on calcium content in sea water: I. chelatometric determination of calcium in sea water

Studies on calcium content in sea water: I. chelatometric determination of calcium in sea water

Kenji Kato^*

Faculty of Fisheries, Hokkaido University - Japan. Oceanographic Institute, University of São Paulo - Brazil

SYNOPSIS

The chelatometry - volumetric titration whith a chelate compound such as EDTA-2Na (disodium ethylenediaminetetracetate) - was applied to the determination of calcium in sea water. Sea water contains in general, approximately three times more magnesium than calcium. It is true that the chelatometry has presently given the possibility" of selective analysis of calcium in the coexistence of magnesium, but an abundant amount of coexisting magnesium, as in the case of sea water, is likely to cause an unfavorable negative-error of the titre in titrating calcium with EDTA using murexide indicator in a strongly alkaline medium.

The author firstly examined how the negative error of the titre was affected by the probable conditions of the analysis; i.e., the quantity of coexisting magnenium, the hydrogen-ion concentration of the medium, and the addition of a masking reagent, as sucrose or sodium carbonate.

These chemical examinations about the analytical conditions led to a procedure of analysis, by means of which it is possible to determine calcium in sea water easily, in a few minutes, with fairly high precision. As this method is very simple and gives an accuracy of ± 0.4% using only 10 ml of the water sample, it is recommended as a procedure to determine calcium in oceanographical observations aboard, in which a lot of water samples have to be analysed in a comparatively short period of time.

RESUMO

Empregamos a complexometria - titulação volumétrica com um composto complexométrico como o EDTA-2Na (disodium ethylenediaminetetracetate) para a determinação do cálcio na água do mar. A complexometria nos dá, atualmente, a possibilidade de fazer uma análise seletiva do cálcio, na presença do magnésio; mas uma quantidade grande de magnésio, como no caso da água do mar, irá provavelmente, ocasionar um erro negativo desfavorável no título, quando o cálcio for titulado com EDTA, empregando-se o indicador murexida num meio altamente alcalino, com pH acima de 12.

O autor procurou, primeiramente, saber de quanto seria afetado o erro negativo do título, por possíveis condições de análise, tais como a quantidade presente de magnésio, a concentração hidrogênio-iônica do meio e a adição de alguns reagentes mascarantes recomendáveis, isto é, sucrose e carbonato de sódio.

Esses estudos químicos sobre as condições analíticas levaram a um novo processo de análise, por meio do qual se poderá determinar, facilmente, o cálcio na água do mar, em poucos minutos, e com uma precisão razoavelmente alta. Como esse método é muito simples e dá uma boa precisão (±0,4% de erro padrão), com uma amostra de somente 10 ml de água, seria muito interessante que a determinação do cálcio fosse uma das análises de rotina nos estudos oceanográficos, quando há necessidade de se analisar grande número de amostras de água em um espaço de tempo relativamente curto.

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ACKNOWLEDGEMENTS

The author is much indebted to Prof. W. BESNARD, Director of the Oceanographic Institute, University of São Paulo, for his untiring encouragement and instructive suggestions in the present investigation. Thanks are also due to Messrs. I. EMILSSON and M. FURUYA for furnishing the author with facilities for this work.

The present investigation was rendered possible through the support given by a fellowship grant from the National Research Council, Brazil, to which the author expresses his sincere thanks.

GRIPENBERG, S. 1937b. The calcium content of Baltic water. Ibid., vol. 12, n.º 3, p. 293-304, figs. tabs.

(Received for publication in 1959)

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*

At present in the Faculty of Fisheries, Prefectural University of Mie, Tsu, Japan.

Publication Dates

• Publication in this collection
  15 June 2012
• Date of issue
  1966

History

• Received
  1959