INTRODUCTION
As has already been mentioned the production
of a synthetic or artificial substitute for seawater is not a new idea, but is of
increasing importance for several reasons
(1)
Ease of transportation to inland destinations.
(2) Stability of formulation, given adequate batch supervision of the salts used.
(3) The density of the final solution is controllable.
(4) The formula can be varied if required for specific uses.
(5) The sterility of the final solution can be assured.
Many synthetic formulations demonstrated inadequacies in several respects
when comparisons with their modern counterparts were made. Firstly, the statistics for natural
waters from which they were compiled were not sufficiently accurate, especially with regard to their minor components, and, secondly, the purity of their component salts would not have been high enough. Today these problems still exist, but can be carefully controlled, and are ,for most purposes, well within acceptable limits.
**
It is now relatively easy to produce a basic or standard water from which comparisons can be made,or variations for specific purposes can be derived. The first step in this process is to obtain an accurate analysis, or analyses, of the major compounds present in natural seawater. Such a breakdown is given in
Table l, which gives figures for the major nine ions present. The nine ions shown in this table vary little throughout the oceans , a situation not found
when the minor ions are considered later.
Given the information in
Table 1 and knowing the exact chemical composition of the salts to be used the weights of such salts can be calculated. Two problems arise here and have to be considered before proceeding:
(1) THE PURITY OF THE SALTS PROPOSED FOR USE.
Most laboratory and analytical grade chemicals vary in purity between 90% and 99.9%.Technical grade salts are far less pure.
Considering the accuracy required for the correct reproduction
of certain of the minor constituents this purity is critical , endangering the final formulation unless the quantity and quality of the impurities are known. To give an example of this point, if the Sodium chloride (common salt content of the final formulation is added using commercial grade or household salt the arsenic and barium levels (even though these are in minute quantities and can be measured in single figures, in parts per million) can present problems unless accounted for. In addition to these two, sodium chloride of this grade contains as many as twelve other elements in substantial enough quantities to give rise to doubt as to what their final concentrations would be
2 Allowance
for the minor constituents
Of the minor constituents; many exist naturally in critical ratios, any variations of which are alien to marine life. Natural seawater probably contains all known elements and at least 78 of these have now been isolated
(and accurately measured. A list of all recorded minor elements is given in Table 2
below. Of these minor elements some are far more important than others*** . It
has been well known for many years that
silicon, phosphorus, manganese, iron, copper and nitrogen are biologically
significant but many others have recently been identified as having possible biological importance. A list of these 'latecomers', & their better recognised predecessors is given in
Table 3,which also shows their minimum and maximum recorded concentrations for open ocean waters. (In waters near the coast in temperate & sub-tropical regions many of these greatly exceed this range.) Salts added to introduce minor elements also add major ions .
If the above mentioned newly-acquired information is reliable and there is certainly no doubt on this score, It may answer at least in part a number of problems which are as yet unsolved Most synthetic seawaters have at some stage failed in certain respects, eg; in maintaining a specific organism, or
in the breeding and rearing of the higher forms of marine life . Controversy over the relative merits of natural and synthetic
waters has always been rife. More information and study of synthetic substitutes in the light of recent research is badly needed
This introduction is but a simplified explanation of the facts that should be taken into account if an accurate synthetic seawater is to be produced and the natural ratios of component ions preserved . But do they ? Is what is required as exact a copy of the real thing possible using modern techniques and the most modern data ? Even now it is still not possible to produce a product TOTALLY indistinguishable from the real thing The argument for its production is a natural one; we are trying to study marine life,& as such should reproduce its natural environment as closely as possible to obtain valid answers. Arguments against seem to vary between 'if it cannot be done totally, then don't do it at all-try something else ' , and 'its uneconomic'
To a limited extent the latter has some validity - old formulae are still being used reasonably successfully, and solutions of salts not at all resembling seawater will support certain life.
forms for limited durations. A good formula would be expensive to produce. It is not within the scope of this report to favour either of these arguments. `
For the purpose of comparison in the report proper we have formulated a
Standard Synthetic Seawater based on the following
criteria:
(1) The percentage composition of the major ions follows those quoted in Table l. |
(2) The minor components of known or postulated biological importance are based on modern readings taken for open oceanic waters and follow closely the figures given in Table 3. All minor constituents are present in ratios calculated from Table 3. |
(3) Atomic weights used are those published by IUPAC. 1973
(4) Chemicals used are the highest grade commercially sold by Messrs. B. D. H. Maximum limits of impurities are known and have been allowed for in the trace element entry. |
(4) All calculations were computed to seven decimal places excluding; zeros. For convenience some have been reduced on the final print-out. |
All salts quoted in the final formula are anhydrous ,(or dry forms) with the exception of strontium chloride. |
(6) When dissolved in 95 litres of distilled water and topped up to 100 litres at 60 deg. F.the resultant solution has a salinity of 35 parts per thousand after aeration for 1 hr. |
(7) After 1 hour of aeration the resultant solution should: (a) Have a carbon
dioxide content in equilibrium with the atmosphere. |
(b) Show no precipitation. |
(c) Give a pH reading of 8.19 +/- 0.05 |
* Aeration of the distilled water prior to dissolving of the salts can be made in addition to that stated above.
All formulae examined later were tested as to their compliance with specifications
(6) & (7) above in addition to other tests carried out.
It must be noted at this stage that this
'Standard Synthetic Seawater'
,which for lack of a better epithet we shall from now on call the Calypso formula ,is ,in all practicality, exceedingly expensive
to produce.
TABLE
2
. RECORDED MINOR CONSTITUENTS |
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Information from various international sources. |
**Data on regional variations would need to be consulted if total accuracy
was required.For most purposes a good basic water is adequate.
***
The degree of importance is relative to the final use of the discussed
water.
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