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Chemical Methods for Interstitial Water Analysis aboard JOIDES Resolution

Technical Note 15

Ocean Drilling Program
Texas A&M University

Joris M. Gieskes
Marine Research Division, A-015
Scripps Institution of Oceanography
University of California, San Diego
La Jolla, California 92093
U.S.A.

Toshitaka Gamo
Ocean Research Institute
University of Tokyo
1-15-1 Minamidai, Nakano-ku
Tokyo 164
Japan

Hans Brumsack
Geochemistry Institute
University of Göttingen
Goldschmidtstrasse 1
D-3400 Göttingen
Federal Republic of Germany

August 1991

Because of the size of this technical note, it has been divided into three HTML parts. For printing purposes, we recommend downloading the PDF version. On some computer systems, figures will print with highest clarity by choosing the black-and-white print option.

TABLE OF CONTENTS

INTRODUCTION

Methodologies in the chemistry laboratory aboard JOIDES Resolution have undergone an increasing degree of sophistication during the last few years. During Leg 102 Gieskes and Peretsman (1986) reported on the procedures of the chemistry laboratory at that time. Here we wish to update that 1986 "cookbook" with each method presented in its own section. A copy of this technical note is kept onboard ship in loose-leaf-folder fashion to facilitate continuous updating of methods. We recommend this stapled copy be taken to the ship, with any new procedural techniques being noted, so analyses could be repeated in your laboratory following the cruise.

In addition to the already described methodologies (Gieskes and Peretsman, 1986), this manual also contains a discussion of atomic absorption (AA) methods. For this purpose use has been made of the notes on this subject originated by Hans Brumsack during ODP Leg 127.

We are appreciative of the assistance given to us by Valerie Clark and Scott Chaffey. They helped with questions and worked with us especially on AA and pH methodologies.

In a separate manual, "Wet Chemical Analysis of Sediments for Major Element Composition on JOIDES Resolution" by Joris Gieskes and Toshitaka Gamo, wet chemical methods for the analysis of major constituents of sedimentary rocks are described. That methodology, though perhaps somewhat less accurate than XRF methodology, can serve as an alternate, especially during any potential down times of the XRF. "Cookbooks" for other specialized methodologies are also kept aboard ship and at ODP headquarters. Requests for information regarding methodologies not described in this technical note may be directed to the Manager of Science Operations, Ocean Drilling Program, 1000 Discovery Drive, College Station, TX 77845-9547, U.S.A.

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UNITS

All geochemical data on interstitial waters should be expressed in terms of molar concentration units in an effort to stay close to Standard International Units (SI). This precludes the use of old volumetric units such as liters and milliliters. Although SI has not given us an appropriate expression for molar concentration units, we suggest using the symbol M, which gives continuity with the past. At the same time it is important to realize that seawater salinity is a quantity which has no units. SI has no room for the symbol â, and the Joint Panel on Oceanographic Tables and Standards has eliminated the pretense that salinity gives a value in grams per kilogram (g/kg) of seawater by making salinity a unit-less quantity. Thus salinity, measured with the Goldberg refractometer (standardized with standard seawater), yields only an approximate value for the concentration of dissolved solids.

The use of the following units is suggested: 

                Salinity        no units
                Alkalinity      millimoles/1000cm3 or mM
                Ca2+            millimoles/1000cm3 or mM
                Mg2+            millimoles/1000cm3 or mM
                K+              millimoles/1000cm3 or mM
                Na+             millimoles/1000cm3 or mM
                SO42-            millimoles/1000cm3or mM
                Sr2+            micromoles/1000cm3 or µM
                Mn2+            micromoles/1000cm3or  µM
                Li+             micromoles/1000cm3or  µM
                H4SiO4          micromoles/1000cm3 or  µM
                HPO42-          micromoles/1000cm3 or  µM
                NH4+            micromoles/1000cm3 ( µM)
                                        or millimoles/1000cm3 (mM).
Other constituents can be expressed in appropriate units consistent with those above.

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STANDARDS

The primary standard for many of the methods discussed in this technical note remains IAPSO (International Association for the Physical Sciences of the Ocean) standard seawater, which has the following major-element concentrations: 
                        Alkalinity              2.325 mM
                        Calcium                 10.55 mM
                        Magnesium               54.0 mM
                        Potassium               10.44 mM
                        Strontium               87  µM
                        Sulfate                 28.9 mM
                        Chloride                559 mM
                        Sodium                  480 mM
                        Lithium                 27  µM
For many of the titrations described below, standardization with standard seawater will, in principle, be sufficient. When concentrations in the pore fluids exceed those in standard seawater to a great degree, it will be of value to develop a set of secondary standards that will cover the range of concentrations to be expected. A good example of this is the estimation of alkalinity. A small error in the estimations of the acidity of the ~0.1 M HCl used in the titration with standard seawater (say 3%) can lead to an error of 0.15 mM at 5 mM, but an error of 3 mM at 100 mM. Similarly, higher calcium standards can also serve to double check accuracy in case high calcium concentrations are encountered.

Gieskes and Peretsman (1986) suggested a method for preparing such a set of standards. Below we repeat the recipe in a revised fashion. This preparation can easily be achieved on board ship, and final concentrations can be checked in the shore laboratories of interested shipboard investigators. The suggested procedure can be found in Table 1 of this section; approximate concentrations are reported in Table 2. 

            Table 1. Volume (cm3) of stock solutions A-D
                      used in preparing artificial JOIDES standards.

---------------------------------------------------------------------------------------------------
       Standard         A               B               C               D
---------------------------------------------------------------------------------------------------
        J-1             230             2.5             15              2.5
        J-2             233             5.0             10              2.0
        J-3             236             7.5             5               1.5
        J-4             236.5           10.0            2.5             1.0
        J-5             235.8           12.5            1.2             0.5
        J-6             235             15.0            0.0             0.0
---------------------------------------------------------------------------------------------------

A: 2000 cm3 of ~0.5 M NaCl (~29 g of NaCl in 1000 cm3)
B: ~1 M CaCl2 (~11 g CaC12 in 100 cm3)
C: ~1 M MgSO4 (~25 g MgSO4.7H2O in 100 cm3)
D: ~1 M KCl (~7.5 g KCl in 100 cm3)



Table 2. Composition of standards (mM)
---------------------------------------------------------------------------------------------------------
Standard        Ca2+           Mg2+           SO42-           K+                Cl-
---------------------------------------------------------------------------------------------------------
IAPSO           10.55           54.0            28.9            10.44           559
J-1             10.0            60.0            60.0            10.0            490
J-2             20.0            40.0            40.0            8.0             514
J-3             30.0            20.0            20.0            6.0             538
J-4             40.0            10.0            10.0            4.0             558
J-5             50.0            4.8             4.58            2.0             574
J-6             60.0            0.0             0.0             0.0             590
Accuracy        0.5%            1-2%            1%              2%              0.4%
-----------------------------------------------------------------------------------------------------------
For Ca and Mg, the standards can be checked by using the "super Ca-Mg method" described in the section on calcium.

Standards are calculated on the basis of precise molarities from the previous page. They will change if amounts in Table 1 are slightly different; hence they require double checking in shore-based laboratories.

For the other constituents, again it is important to adopt standards that bracket the range of concentrations to be expected, which, especially for silica, phosphate, and ammonium, can constitute a considerable range. In this manner, even if considerable dilutions will be necessary, the samples and standards will go through a similar treatment.

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CHLORIDE

The determination of the chloride concentration is one of the most important determinations in the interstitial-water program on board JOIDES Resolution. This is the case not only because of the importance of determining the downhole concentration gradient of dissolved chloride but also because the concentration of chloride, being the major anionic constituent, can be used to determine the concentration of dissolved sodium by charge balance calculation, sodium being the most important cationic constituent under most circumstances.

Especially during many of the upcoming legs, in areas of subduction zones and gas-hydrate occurrences, dissolved chloride concentrations can vary considerably, as has been well established during Legs 66, 67, 84, 110, and 112 (inter alia). In other cases (e.g., Leg 86) it has been demonstrated that very small but distinguishable increases in dissolved chloride attest to past salinity changes associated with glacial periods. Thus emphasis on accuracy is of the utmost importance. For these reasons the Mohr titration with silver nitrate using the indicator potassium chromate/potassium di-chromate is still the preferred method, unless specialized or more accurate methods are employed by individual investigators. This method, of course, includes not only dissolved chloride (558 mM in standard seawater) but also bromide (0.86 mM in standard seawater) and dissolved iodide (at most~ 0.2 M in standard seawater). In extreme cases, bromide may rise to 2 mM and iodide to 2 mM. Any determination of chloride, therefore, should be corrected for this in principle, but on a practical basis the corrections will always be less than 1%, i.e., at a level of 2 times the accuracy of the chloride determination.

Reagents

SILVER NITRATE:

Make a 0.1 M AgNO3 (~17 g/1000 cm3) solution in nanopure water (deionized Barnstead water, equivalent to double-distilled water or milli-Q water). INDICATOR: Dissolve 4.2 g A.R. potassium chromate and 0.7 g potassium di-chromate in 100 cm3 nanopure water.

Procedure

Pipette 0.1 cm3 of sample into a 10-cm3 glass beaker and add about 5 cm3 of nanopure water. Add 0.1 cm3 of indicator solution. Under vigorous stirring (magnetic stirrer) and with the METROHM burette tip immersed, titrate until a faintly reddish-brown color (silver chromate) is observed and stays permanent. It is important to stir vigorously in order to keep the AgCl precipitate from coagulating. The latter can trap chloride ions and thus prevent the end point from being reached. Coagulation seems to increase just before reaching the endpoint, so that especially at the end of the titration, stirring becomes very important. In addition, it is also important to proceed very slowly in the second half of the titration, to avoid entrapment of chloride ions in the flocs of silver chloride. Another precaution needs to be taken when room temperatures are variable. Eppendorf pipettes are precise but not accurate, and volumes depend on the room temperature. Frequent calibration with standard seawater will overcome this problem. The color change is somewhat subjective, but each individual investigator can easily reach a precision of better than 0.3%.

Standardization is made with IAPSO standard seawater (Cl = 19.376 g/kg or as specified on the bottle; at this chlorinity the sum of chloride and bromide is equal to 559 mM).

When calculating the chloride concentration of the unknown, one should bear in mind that in actuality (Cl + Br) are being measured. In the absence of data on bromide, however, one can assume a bromide concentration of [("titration chloride"/559) x 0.86] and correct for bromide. At seawater concentrations the error will be less than 0.15%, i.e., less than the estimated precision of the method.

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CALCIUM

Calcium is normally determined on board JOIDES Resolution by a mini-version of the titration method of Tsunogai et al. (1968). In this method, ethylene-bis-(oxyethylenenitrilo)-tetra-acetic acid (EGTA) is used as a titrant, and 2,2'-ethane-diylidine-dinitrillo-diphenol (GHA) is used as an indicator. The calcium-GHA complex is extracted quantitatively into a layer of n-butanol, which enhances the color and makes the endpoint detection much easier.

One small problem with this method lies in the presence of magnesium in many samples. After the addition of the borate buffer, this causes a precipitation of Mg(OH)2 , which co-precipitates a small amount of the calcium (and strontium) present. Below, two variants of the method are described, one in which this co-precipitation is prevented (the "super method") and one in which a corrective calculation is carried out (routine method). The former method requires several titrations and thus is much more sample-consuming than the routine technique. With the corrective method, sufficient accuracy (~2%) is obtained to serve for most geochemical purposes. The routine method requires a "magnesium" titration (next section), but any work on Ca usually requires work on Mg also.

Reagents

    • EGTA STOCK SOLUTION: 3.8 g EGTA are dissolved in 30 cm3 of 1M NaOH (4 g/100cm3) and diluted to 100 cm3. This yields a 0.1 M EGTA solution. From this a 10 mM EGTA solution can be made by appropriate dilution.
    • BORATE BUFFER: 5 g of borax (Na2B4O7.10H2O) and 15 g of sodium hydroxide are dissolved in 250 cm3 nanopure water.
    • INDICATOR: 40 mg of GHA are dissolved in 100 cm3 of ethanol. Should be made fresh each day that titrations are carried out.

      • Routine Method

      Transfer, preferably by means of an Eppendorf or equivalent microburette, a 0.5 cm3 sample (can be 0.4 cm3 or even 0.2 cm3) into a 10 cm3 beaker and add about 2 to 3 cm3 of nanopure water. While stirring (magnetic stirrer), add 0.5 cm3 of (0.04%) GHA solution and 0.5 cm3 of buffer solution. Stir for about 3 minutes, unless you have quite high calcium values, in which case you must start titrating immediately after adding the buffer (the color tends to fade after 5 or 6 minutes). Add 2 cm3 butanol when the reddish color starts to diminish. When the color becomes even less reddish, stop stirring and wait for the butanol to separate. Examine the color and start stirring again while adding a small amount of the 10 mM EGTA titrant. When red color fails to reappear, the titration is finished.

      Below, in the description of the magnesium method, we will describe how corrections can be made for the presence of magnesium in the sample.

      Super Method

      One can in principle bypass the magnesium interference by complexing most of the Ca2+. This is achieved by adding more than 98% of the needed EGTA titrant prior to the buffer addition. Once the Ca2+ is complexed it will not go back into solution, thus avoiding co-precipitation with magnesium hydroxide.

      The first titration utilizes the routine method, which yields the approximate amount of EGTA titrant necessary for the complexation of calcium. Subsequently, this amount of EGTA is added to the second aliquot prior to the GHA and buffer addition. The titration is repeated, and the next best estimate of the necessary amount of EGTA is made. Usually one more titration will suffice to obtain the real Ca concentration.

      The correction formulas presented in the section on magnesium have been devised on the basis of a comparison of the routine method and the super method using a set of substandards as described in the standards section.

      Standardization

      Standardization is achieved by titration of IAPSO standard seawater. It should, however, be remembered that standardization of the routine method should use the same procedure as the routine method, and the super method should use the super procedure.

      Corrections for strontium usually are trivial and can be ignored for most practical purposes. For calcium concentration close to seawater, there is no real problem because standardization includes the IAPSO strontium.

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      MAGNESIUM

      In order to obtain the value of dissolved Mg2+, a titration is carried out for the total alkaline earths, i.e., Ca2+, Mg2+, and Sr2+ (other contributors being trivial) and the values for Ca2+ and Sr2+ are then subtracted. The formulas given below are used for the routine Ca2+ titration. If the super Ca method is used, subtraction of super Ca2+ will suffice (Sr2+ corrections usually are small, since Sr2+ concentrations usually so not exceed 1 mM, except perhaps in evaporite situations).

      Reagents

        • EDTA (di-sodium Ethylenediamine-Tetraacetate): Dissolve ~15 g of EDTA (sodium salt) in 1000 cm3 of nanopure water to yield a ~0.03 M solution. Add 1 cm3 of a 50 mM MgCl2 (0.65 g MgCl2.2H2O/100 cm3) to the EDTA. This ensures that the Eriochrome- Black-T endpoint will be detectable at zero magnesium concentrations.
        • BUFFER: 67.5 g of NH4Cl are added to 570 cm3 of NH4OH, the final volume being made up to 1000 cm3 with nanopure water.
        • INDICATOR: 0.05 g of Eriochrome-Black-T are dissolved in 50 cm3 of 80% ethanol solution. A fresh batch should be made before each site or every other day.

      Procedure

      To a 0.5 cm3sample aliquot, add 5 cm3 of nanopure water. Add 1 cm3 of ammonia buffer and 0.1 cm3 indicator solution. Start stirring (magnetic stirrer). The color change from reddish to blue can best be observed at the edges of the bottom of the beaker glass, especially where an extra glow of light occurs. Reproducibility of the method is ~0.5%, and accuracy ~1%.

      As usual, standard seawater (Ca + Mg + Sr = 64.64 mM) is used as the primary standard.

      Calculations

      As mentioned in the previous section, the routine method for calcium requires a correction for interference by magnesium. Gieskes and Peretsman (1986) suggested two simple formulas for the calculation of Ca2+ and Mg2+, following the procedure of Gieskes and Lawrence (1976). In part, these formulas are based on titrations carried out on the standards described in the standards section.

      Denoting total alkaline earths by Dt and the "routine" calcium titration value by Cat, we obtain the corrected Mg and Ca values as follows:

      Mgcorr = (Dt - 0.94Cat)/1.01;
      Cacorr = 0.94Cat + 0.01Mgcorr.

      Eventually, when data are available on dissolved Sr2+, minor corrections can be made for this component, as follows:

      Cafinal = Cacorr - 0.8Sr + 0.08;
      Mgfinal = Mgcorr + 0.2Sr.

      However, as pointed out before, this correction is usually superfluous. Furthermore, seawater strontium is usually taken into the calcium titration standardization through the use of standard seawater.

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      pmH

      For the pH measurement, the standard operation has been based on the use of the NBS (National Bureau of Standards) buffers. These buffers are designed as low ionic strength solutions made of potassium hydrogen phthalate (pH = 4.008 at 25o C), mixtures of mono-hydrogen and di-hydrogen phosphates (pH = 6.865 and 7.413 at 25o C, respectively), and 0.01 m borax (pH = 9.18 at 25o C). One problem with these buffers has always been that they have given the impression to the casual user that the thermodynamic activity of the hydrogen ion is actually being measured in any solution, independent of what the ionic strength might be. Bates and Culberson (1977) point out the fallacy of this concept, especially in marine systems. For these reasons a subcommittee of the Joint Panel on Tables and Standards (JPOTS) has advocated the adoption of a new pH scale, utilizing buffers designed specifically for media of ionic strengths similar to that of seawater.

      The newly proposed pH scale relies on the determination of the "free" hydrogen ion concentration scale (Bates and Culberson, 1977), in which mH is the concentration of the free hydrogen ions in mol/kg-H2O and where we denote -logmH with the symbol pmH.

      For the purpose of standardizing pH electrode systems in terms of this new pH scale, Bates and Culberson (1977), Khoo et al. (1977), and Bates and Calais (1981) developed two buffers, which have been specifically designed for use in seawater and seawater-like solutions. These buffers are described in Table 1.

      It should be noted that Bates and Calais (1981) propose the use of a specially prepared artificial seawater, taking great care in the preparation of the NaCl. During Leg 131 we did not have pure NaCl available, and for this reason we made the buffer solutions from Nankai Trough bottom water (S = 34.68), which has an approximate alkalinity of 2.5 mM, i.e., considerably less than that introduced in the preparation of the buffer. Thus we consider the effect of the small bicarbonate contribution to be small enough to be neglected. Surface water will be equally useful.

      In order to study the effects of salinity and of the composition of artificial seawater, BIS and TRIS buffer solutions were also made in Nankai Trough bottom seawater diluted to S = 30, as well as in artificial seawater with 476, 558, 637 mM NaCl and 47.8, 55.9, 63.1 mM MgSO4. The latter solutions were prepared as artificial seawater "equivalents" for salinities 30, 35, and 40. Results are reported in Table 2. From this table it is apparent that the solution chemical effects are not large and that pmH values can be obtained with a precision of better than 0.02 pmH units.

      Standardization with pH buffers based on the NBS scale yielded an electrode slope of 57.64 mV/pH unit. Our solutions with TRIS/BIS on the other hand yielded a slope of 59.38 mV/pmH unit. The latter slope is much closer to the theoretical value of 59.15 mV/pmH unit.

      A comparison with NBS standard (pH = 7.413), using a slope of 59.15 mV/pH unit, yielded a systematic difference between the two pH scales of 0.114 + 0.013, pmH values being systematically lower. This is mostly the result of the difference in concept: the pmH scale is a concentration scale whereas the NBS scale yields activities, with an albeit ill-defined activity coefficient.

      We recommend the adoption of the TRIS/BIS standards with the caution that the values should be reported as pmH, not pH. The occasional user may think that a tremendous change has been made, but what really has happened is that the pH measurements are now based on a more honest concept of concentration, rather than on a so-called thermodynamic entity.

      The additional advantage, of course, is that the alkalinity electrodes will not further undergo the shock treatment of large salinity changes during calibration, as they will remain always in solutions of ionic strengths similar to those of seawater.

      New standards for pmH

      Two new standards are recommended for use in pmH measurements on board JOIDES Resolution: TRIS and BIS standards and their hydrochlorides:
    • TRIS = tris(hydroxymethyl)amino methane or (2-amino-2-(hydroxymethyl)-1,3-propanediol).
    • BIS = bis(hydroxymethyl)methylamino methane or (2-amino-2-methyl-1,3-propanediol).
Tris, Tris.HCl, and Bis are obtainable commercially (e.g., from Sigma Chemical Co., St. Louis, MO 63178). Bis.HCl is not commercially available because of the very hygroscopic nature of Bis.HCl. Bis.HCl must be crystallized from a concentrated solution of Bis that has been neutralized with purified HCl. This is produced by heating deionized water in a glass beaker then adding Bis while stirring. When the solution becomes saturated (precipitation is observed) add HCl (Seastar brand in the Teflon bottle in acid cabinet) until neutral (pH 7 tested with pH paper). Cool solution (a gel will form at the bottom) then decant water. Place in a warm oven until dried (it will form a smooth surface and need to be scraped from the container). Remove the chemical immediately to a vaccuum desiccator to cool. This chemical must be kept in a desiccator and weighed quickly (take first weight) when making up the buffer. It should be redried in the oven then cooled in the desiccator each time prior to weighing.

Bates and Calais (1981) propose a special recipe for artificial seawater. We found, however, that Nankai Trough bottom water (S = 34.89) suffices for this purpose. Surface water will be equally useful.

The following two standards can be produced by dissolution in 1000 g of seawater (1023 cm3): 

A:      0.02 moles (2.423 g) Tris;              B:      0.02 moles (2.103 g) Bis;
        0.02 moles (3.152 g) Tris.HCl.                  0.02 moles (2.832 g) Bis.HCl.

Table 1A. pmH values for Standard A (Tris buffer)

Temp.  C           5      10        15        20        25        30       35          40
-----------------------------------------------------------------------------------------------------------
Salinity
        30      8.798    8.635     8.479     8.330     8.187     8.050     7.917     7.789
        35      8.812    8.649     8.493     8.343     8.200     8.062     7.929     7.801
        40      8.826    8.663     8.507     8.357     8.214     8.076     7.943     7.815
-----------------------------------------------------------------------------------------------------------

Table 1B. pmH values for Standard B (Bis buffer)

Temp.  C          5        10        15        20       25        30        35        40
-----------------------------------------------------------------------------------------------------------
Salinity
        20      9.509    9.341     9.178     9.022     8.873     8.729     8.588     8.453
        35      9.574    9.404     9.241     9.084     8.934     8.788     8.647     8.512
        45      9.599    9.430     9.267     9.110     8.960     8.814     8.673     8.538
-----------------------------------------------------------------------------------------------------------

 Table 2. pmH comparison in seawater and artificial seawater (based on standardization in S = 35 seawater).

Solution          Tris/Tris HCl           Bis/Bis HCl
                EMF             pmH             EMF             pmH
                mV                              mV
-----------------------------------------------------------------------------------------------------------
S = 35          -68.58          8.200           -112.17 8.934   Reference Nankai
                                                                        Trough bottom water

S = 30          -68.55          8.20            -112.55 8.94    dilute Nankai
                                (8.19)                          (8.92)

"S = 30"(a)     -67.45          8.18            -114.90 8.98(b)
                                (8.19)                          (8.92)

"S = 35"(a)     -69.55          8.22            -112.47 8.93
                                (8.20)                          (8.93)

"S = 40"(a)     -68.68          8.22            -112.81 8.94
                                (8.21)                          (8.95)
------------------------------------------------------------------------------------------------------------

(a) = Values in brackets are pmH values for synthetic seawater buffers (Table 1).

(b) = This solution probably affected by erroneous Bis.HCl weighing.

         In almost all cases deviations from expected values are within 0.02 pmH.
N.B. In the near future the JPOTS committee on carbon dioxide standards will recommend a final set of buffers as well as the final choice of pH scale.

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