Chemistry Lab

PHOSPHATE

The determination of dissolved phosphate, particularly in rapidly deposited organic-carbon-rich sediments, is important in the shipboard analytical program. Phosphate concentrations may vary considerably, and it is therefore advisable to obtain a preliminary idea of the concentration ranges to be expected. This can most easily be accomplished by taking samples in the region of maximum alkalinities, especially if these maximum values occur within 50 to 100 mbsf. Typically if alkalinities attain more than 30 mM, dissolved phosphate concentrations can attain more than 100 mM. Thus, only very small sample aliquots will be needed to establish the concentration range.

The method is, in essence, the colorimetric method described by Strickland and Parsons (1968) as modified by Presley (1971) for DSDP pore fluids.

It is important to note that the concentration in the final test solution cannot exceed more than about 10 mM. Thus, for open-ocean (low sedimentation rate, low organic carbon) samples, one might need to do the determination on 2 cm³ of sample (expected range 0-10 mM), but in typical continental-margin settings, where concentrations can exceed 100 or 200 mM, a 0.1 or 0.2 cm³ sample aliquot must be used. As mentioned above, the concentration range must be established prior to running the samples, and it is highly advisable to make standards that cover the range of concentrations to be expected. In this manner, standards and samples will all get the same treatment.

Reagents

AMMONIUM MOLYBDATE SOLUTION: Dissolve 2 g (NH₄)₆Mo₇O₂₄^.4H₂O in 1000 cm³ of nanopure water. The solution is stable indefinitely if stored in a plastic bottle.
SULFURIC ACID SOLUTION: Dilute 10 cm³ of concentrated H₂SO₄ (specific gravity 1.82 g/cm³) to 1000 cm³ with nanopure water.
ASCORBIC ACID SOLUTION: Dissolve 3.5 g ascorbic acid in 1000 cm³ of nanopure water. This solution must be kept refrigerated and should not be stored for more than a week.
POTASSIUM ANTIMONYL-TARTRATE SOLUTION: Dissolve 0.09 g of KSbC₄H₄O₇^.1/2H₂O in 1000 cm³ of nanopure water. This solution is stable for many months.
MIXED REAGENT: Mix together 50 cm³ ammonium molybdate, 125 cm³ sulfuric acid, 50 cm³ ascorbic acid, and 25 cm³ potassium antimonyl-tartrate. Do not store this solution for more than a few hours. It is best to prepare the mixed reagent immediately before making the determinations.
PHOSPHATE STANDARD: Dissolve 1.3614 g KH₂PO₄ in 1000 cm³ of nanopure water. This yields a 0.01 M phosphate standard solution which lasts quite a long time, unless there is evidence for growth of algae or other extraneous biotic material. It should be remembered that the range of expected concentrations should be established first, then the appropriate standards can be prepared and the proper dilution factor chosen.

Procedure

Put the appropriate aliquot of sample or standard in a small glass vial (~5 or 10 cm³ size), e.g., 1 cm³ for "open ocean" sites, 0.1 or even 0.01 cm³ for high organic carbon, high alkalinity sites. In the latter case one ought to add about 1 cm³ of nanopure water, the main thing being that in no case the final concentration of phosphate is more than 10 mM. Add 2 cm³ of mixed reagent. After a few minutes a blue color develops, which remains stable for a few hours. For these reasons it is best to make the readings of the absorbance at 885 nm about half an hour after the addition of the mixed reagent. Use 1 cm cells.

Return

NITRITE

Under normal circumstances the need for determination of the nitrite concentration of ODP samples is limited unless, perhaps, drilling is in areas of very slow sedimentation, where dissolved oxygen may penetrate to considerable depth into the sediment column. In such a case, redox reactions involving nitrate reduction, which often is accompanied by an intermediate production of nitrite, may be detectible at greater depths. Prime candidates for such sites are areas of red clays or extremely slow carbonate deposition. In any case, the method needs to be described because it is used in the method of nitrate determination, in which nitrite is produced by a reductive technique.

The method is based on an adaptation of the method proposed by Strickland and Parsons (1968). In the method, nitrite is allowed to react with sulfanilamide in an acid solution. The resulting diazo-compound reacts with N-(1-naphtyl)-ethylene diamine to form a pink azo dye, whose absorbance is measured at 543 nm.

Reagents

SULFANILAMIDE SOLUTION: Dissolve 5 g of sulfanilamide in a mixture of 50 cm³ concentrated HCl (1.18 g/cm³) and ~300 cm³ of nanopure water. Dilute to 500 cm³. The solution is stable for many months.
N-(1-NAPHTYL)-ETHYLENE DIAMINE DIHYDROCHLORIDE SOLUTION: Dissolve 0.50 g dihydrochloride in 500 cm³ of nanopure water. Store in dark bottle. The solution should be made prior to the determinations. It will be stable for only a short time (a few weeks at best).

Procedure

Add 0.1 cm³ of the sulfanilamide solution to a 2 cm³ sample and allow reaction for 2-8 minutes (make sure this is done in a reasonably consistent manner). Then add 0.1 cm³ of naphtyl ethylene diamine dihydrochloride solution and mix immediately. A nice pink color develops immediately if nitrite is present.

After 10 minutes, but before 2 hours has elapsed, measure at 543 nm in the 1 cm path length, flow-through cell. Use nanopure water as a blank.

Return

NITRATE

Nitrate concentrations, especially in open-ocean sediments with low sedimentation rates, can be useful indicators of diagenetic processes involving organic carbon. For these reasons it will be useful to have this method available on board ship, even though the methodology is relatively laborious and will probably be used only when sampling programs are not too busy. Generally, concentration ranges will be between 0 and 60 mM.

Often a good judgment can be made for the potential use of the nitrate method on the basis of the ammonia measurements. If the latter rise very quickly above 50 mM, it is almost certain that little or no nitrate will be present but rather that the zone of sulfate reduction has been entered.

The method is adopted from Strickland and Parsons (1968) and makes use of the catalytic reduction of nitrate to nitrite, using a cadmium reduction column. A peristaltic pump (of autoanalyzer type) is used to force the samples and standards through the reduction columns. The use of only one channel of the pump is advocated to keep better track of the samples and standards. It should be remembered that each and every one of the columns has its own individual characteristics.

Column Preparation

There are two types of suggested columns:

1. Teflon tubing of 3 mm internal diameter. Put a small amount of glass wool in the bottom of the teflon tube (fine copper wool is supposed to be preferable). Fill about 5 cm length of the tubing with small (0.5 to 2 mm) Cd chips. Put a small amount of glass or copper wool on top of the loosely packed column.

2. A more preferred method is to use < 1 mm (i.d.) tygon tubing and fill this with cadmium wire. A thin piece of copper wire can be used on both sides of the cadmium wire.

Of importance is to note that the columns, after their activation as described below, remain out of contact with air because they will get poisoned. Also avoid contact with hydrogen sulfide-containing solutions. They will produce CdS and finish the columns.

The columns get attached to 1/16-inch-by-1/8-inch tygon tubing which can be used in the peristaltic pump or at the other end. When not in use, keep both ends in water to prevent aeration of the columns. Intake------peristaltic pump------column------outflow.

Activation of the column(s) can be achieved as follows. (If starting with a new, clean Cd column, step 1 may be eliminated.)

1. Pass 5% HCl through the columns for a few minutes, then wash with nanopure water until the effluent has a neutral pH.
2. Pass a 2% copper sulfate solution through the column for a few minutes (10-20 cm³), followed by a wash with dilute ammonium chloride (see below). The column is now ready.

Reagents

CONCENTRATED NH₄Cl: 175 g of NH₄Cl in 500 cm³ of nanopure water. This solution is used for buffering of samples and standards.
DILUTE NH₄Cl: 50 cm³ of concentrated NH₄Cl diluted to 2000 cm³. This solution is used for washing the columns.
SULFANILAMIDE SOLUTION (as in the nitrite method): Dissolve 5 g of sulfanilamide in a mixture of 50 cm³ concentrated HCl (1.18 g/cm³) and ~300 cm³ of nanopure water. Dilute to 500 cm³. The solution is stable for many months.
N-(1-NAPHTYL)-ETHYLENE DIAMINE DIHYDROCHLORIDE SOLUTION (as in the nitrite method): Dissolve 0.50 g N-(1 naphthyl)-ethylene diamine dihydrochloride in 500 cm³ of nanopure water. Store in dark bottle. The solution should be made prior to the determinations. It will be stable for only a short time (a few weeks at best).

Procedure

If a column has not been used recently, it is advisable to run a few standards through it to check the column's activity. If it appears that there may be a problem, it is advisable to make new columns. In any case, columns do not last too long and are usually soon in a state of deterioration when not used regularly.

Before running samples through the column, do a pre-rinse using dilute NH₄Cl.

Use 1 cm³ of sample or standard and add 4 cm³ of nanopure water. Add 0.1 cm³ of concentrated NH₄Cl solution as a buffer. Run the buffered sample through the column at a speed of ~3-5 cm³/minute. Collect the last 4 cm³. For the actual analysis of the nitrite, only 2 cm³ is used, as described above in the nitrite method.

After all the samples have been run through the column, a wash with dilute NH₄Cl is advisable for the duration of at least several minutes. Also make sure that the intake tube and the outlet tube remain submersed in water in order to prevent any inadvertent contamination of the reduction column.

Standards are made from a stock solution of 10 mM KNO₃. The range of the standards should be 0-60 mM. It is best to prepare the standards in synthetic seawater: 30 g NaCl, 10 g MgSO₄^.7H₂O, 0.05 g NaHCO₃ in 1000 cm³ of nanopure water.

Return

ATOMIC ABSORPTION METHODS

See Technical Note 29: Analysis of Major and Trace Elements in Rocks, Sediments, and Interstitial Waters by Inductively Coupled Plasma–Atomic Emission Spectrometry (ICP-AES).

Return

REFERENCES

Bates, R. G., and Calais, J. G., 1981. Thermodynamics of the dissociation of Bis.H+ in sea water from 5 to 40C. J. Solution Chem., 10:269-279.

Bates, R. G., and Culberson, C. H., 1977. Hydrogen ions and the thermodynamic state of marine systems. In Anderson, N. R., and Malahoff, A. (Eds.), The Fate of Fossil Fuel CO₂ in the Oceans: New York (Plenum Press), 45-61.

Brumsack, H., Notes on Atomic Absorption, Leg 127 (in press).

Gieskes, J. M., and Lawrence, J. R., 1976. Interstitial water studies, Leg 35. In Hollister, C. D., Craddock, C., et al., Init. Repts. DSDP, 35: Washington, DC (U.S. Govt. Printing Office), 407-424.

Gieskes, J. M., and Rogers, C., 1973. Alkalinity determination in interstitial waters of marine sediments. J. Sed. Petrol., 43:272-277.

Gieskes, J. M., and Peretsman, G., 1986. Water chemistry procedures aboard JOIDES Resolution--some comments. ODP Technical Note 5: College Station, TX (Ocean Drilling Program).

Gieskes, J. M., Elderfield, H., Lawrence, J. R., Johnson, J., Meyers, B., and Campbell, A. C., 1982. Geochemistry of interstitial waters and sediments, Leg 64, Gulf of California. In Curray, J. R., Moore, D. G., et al., Init. Repts. DSDP, 64(2): Washington, DC (U.S. Govt. Printing Office), 675-694.

Grinstead, R. R., and Snider, S., 1967. Modification of the Curcumin method for low level boron determinations. Analyst, 92:532-533.

Kastner, M., Elderfield, H., et al., 1990. Diagenesis and interstitial water chemistry at the Peruvian Margin--major constituents and strontium isotopes. In Suess, E., von Huene, R., et al., Proc. ODP, Sci. Results, 112: College Station, TX (Ocean Drilling Program), 413-440.

Khoo, K. H., Ramette, R. W., Culberson, C. H., and Bates, R. G., 1977. Determination of hydrogen ion concentrations in sea water from 5 to 40C: standard potentials at salinities from 20 to 45%. Analytical Chem., 49:29-33.

Kremling, K., 1983. Determination of major constituents: bromide. In Grasshoff, K., Ehrhard, M., and Kremling, K. (Eds.), Methods of Sea Water Analysis. New York (Springer Verlag).

Pedersen, T. F., 1979. The geochemistry of sediments of the Panama Basin, eastern equatorial Pacific Ocean. Ph.D. Thesis, University of Edinburgh.

Presley, B. J., 1971. Techniques for analyzing interstitial water samples. Appendix Part 1: Determination of selected minor and major inorganic constituents. In Winterer, E. L., et al., Init. Repts. DSDP, 7(2): Washington, DC (U.S. Govt. Printing Office), 1749-1755.

Solorzano, L., 1969. Determination of ammonia in natural waters by phenol-hypochlorite method. Limnol. Oceanogr., 14:799-801.

Strickland, J.D.H., and Parsons, T. R., 1968. A manual for sea water analysis. Bull. Fish. Res. Bd. Canada, 167.

Tsunogai, S., Nishimura, M., and Nakaya, S., 1968. Complexometric titration of calcium in the presence of large amounts of magnesium. Talanta, 15:385-390.

Weiss, J., 1986. Handbook of Ion Chromatography. DIONEX Corp.

Return to Part I

Return to Part II