Sea water is alkaline mainly due to the presence of appreciable quantities of carbonate, bicarbonate and borate ions. These react with the H+ from the ionization of water leaving an excess of OH\u27 present…

Robinson, Rex J.

West, Lloyd E.

English

Yale University

      P.O. BOX 208118 | NEW HAVEN CT 06520-8118 USA | PEABODY.YALE. EDU   JOURNAL OF MARINE RESEARCH The Journal of Marine Research, one of the oldest journals in American marine science, published important peer-reviewed original research on a broad array of topics in physical, biological, and chemical oceanography vital to the academic oceanographic community in the long and rich tradition of the Sears Foundation for Marine Research at Yale University.  An archive of all issues from 1937 to 2021 (Volume 1–79) are available through EliScholar,  a digital platform for scholarly publishing provided by Yale University Library at  https://elischolar.library.yale.edu/.  Requests for permission to clear rights for use of this content should be directed to the authors, their estates, or other representatives. The Journal of Marine Research has no contact information beyond the affiliations listed in the published articles. We ask that you provide attribution to the Journal of Marine Research.  Yale University provides access to these materials for educational and research purposes only. Copyright or other proprietary rights to content contained in this document may be held by individuals or entities other than, or in addition to, Yale University. You are solely responsible for determining the ownership of the copyright, and for obtaining permission for your intended use. Yale University makes no warranty that your distribution, reproduction, or other use of these materials will not infringe the rights of third parties.  This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. https://creativecommons.org/licenses/by-nc-sa/4.0/   POTENTIOMETRIC ANALYSIS OF SEA WATER II. DETERMINATION OF TITRATION ALKALINITY LLOYD E. WEST1 AND REX J. ROBINSON Oceanographic and Chemical Laboratories University of Washington, Seattle, Washington Sea water is alkaline mainly due to the presence of appreciable quantities of carbonate, bicarbonate and borate ions. These react with the H+ from the ionization of water leaving an excess of OH' present. H20 =::; H+ + OH' HC03 ' + H + =::; H2COa C03" + H+ =::; HCOa' H2B03 ' + H + =::; HaBOa The OH' together with the uncombined anions of weak acids may be titrated with H +. The number of milliequivalents of H + needed to neutralize one liter of sea water is referred to as titration alkalinity. The major portion of the titration alkalinity is due to the effect of the carbonate and bicarbonate and only a small portion due to the borate. At the hydrogen ion concentration normally present in sea water about 92% of the total carbon dioxide is present as bicarbon-ate ion, about 8% as carbonate ion and less than 1% as dissolved carbon dioxide and carbonic acid. Titration alkalinity is determined experimentally by treating 100 ml. of sea water with 5 ml. of 0.07 N HCl to convert the borate to undissociated boric acid, the carbonate and bicarbonate to carbonic acid and then by refluxing for five minutes to decompose the carbonic acid and expel the carbon dioxide. After cooling, the excess HCl is titrated with 0.025 N carbonate-free NaOH. Either methyl orange or methyl red is used as the indicator so that the titration will be stopped after the HCl has been titrated but before the boric acid ionizes and is titrated. Usually the HCl is standardized against sodium carbonate in a synthetic sea water to simulate the actual ex-1 Present address, Department of Chemistry, Oregon State College, Corvallis, Oregon. ( 38) 1941] JOURNAL OF MARINE RESEARCH 39 perimental condi~ion~ as nearly as possible and thus compensate for error~ of determmatwn. Actually some difficulty is experienced in locatmg the exact end point of the titration using either methyl red or m.eth~l or.ange ~s the indicator. It was thought that a potentio-metnc tltratwn m1ght eliminate this difficulty. The potentiometric lo90 C/) ""I. 70 a:: 1&.1 a.. 2 <( 0 a:: (,) lo50 -2 1.30 .>ENDPOINT 9o35 MLo 9o0 9o5 M L. 0.2661 N Figure 5. A Titration Alkalinity Curve by Indirect Titration. apparatus, using bimetallic electrodes, previously described by the authors (1) was tried for this purpose and proved to be very satis-factory as the following description of these experiments will indicate. Potentiometric titrations were made in a manner similar to the above described . procedure, excepting no indicator was needed for determination of the end point. Twenty-five ml. of 0.01812 N HCl was added and the excess titrated with 0.02661 N base. A tungsten electrode served as the indicating electrode and natural graphite as 40 SEARS FOUNDATION [IV, 1 the reference electrode. The end point was very clearly marked as illustrated by Figure 5. The time required for titration _was about the same as when indicators were used. Table I contams results of analyses of a sea water sample made in. such a manner as .well as analyses using indicators for the detectiOn of the end pomt. In Table I are also shown results in which the titration alkalinity was determined by direct addition of 0.01812 N acid with no back titration. TABLE I TITRATION ALKALINITY OF A SEA WATER SAMPLE Methyl Orange 2.25 2 .15 2.18 2.21 Ave. 2.20 Back Titration with NaOH M ethyl Red 2.07 2.03 2 .05 Direct Titration with HCl p owntiometric 2.07 2 .01 2 .04 2 .04 2 .16 2 .17 2 .17 2 .16 Ave. 2.17 Approximately 0.1 milliequivalent of H+ is needed to change the H+ concentration from the end point of methyl red to that of methyl orange, considering them to be at pH of 5 and 4 respectively. If this correction is made the results using the two different indicators agree very well. In the potentiometric back titration the most abrupt change in potential would occur as near neutrality as possible but before the H 3B03 would begin to ionize, i. e. at a H+ concentration essentially at the end point of methyl red. The results confirm this conclusion. In the direct titration the end point would occur after the conversion of HC03' to H2C03 which is near the methyl orange end point. This is also confirmed experimentally. For this reason the indirect titration should yield results more in keeping with the definition of titration alkalinity. As mentioned previously the end point with the tungsten and graphite electrodes was quite easily as-certained. In as much as this is an acid-base titration other elec-trodes would prove as satisfactory. Others have been used in sample acid-base titrations. In the indirect titration a buret electrode would 1941] JOURNAL OF MARINE RESEARCH 41 offer the advantage of preventing absorption of carbon dioxide from the air during titration. A glass electrode would undoubtedly also prove satisfactory. A simple hydrogen electrode could also be used, but would yield variable results. The bubbling hydrogen would wash out carbon dioxide which would not affect the sharpness of the end point but would be intermediate between the methyl red and methyl orange end points. It is felt that the selection of the end point by potentiometric titration increases the accuracy of this titration. REFERENCE 1. WEsT, LLOYD E. and REX J. RoBINSON. 1940. A Vacuum Tube Potentiometer, Application to Potentiometric Pre-cipitation Titrations. Ind. Eng. Chern., Anal. Ed., 12, 476. 

Potentiometric analysis of sea water II. Determination of titration alkalinity

https://elischolar.library.yale.edu/cgi/viewcontent.cgi?article=1580&context=journal_of_marine_research

Potentiometric analysis of sea water II. Determination of titration alkalinity

Abstract

Similar works

Full text

Available Versions

Yale University