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TEXT BY MARLIN ATKINSON AND CRAIG BINGMAN
The Composition Of Several Synthetic Seawater Mixes
Many marine aquarists use synthetic seawater mixtures in their aquaria. The relative merits of the various mixes are often debated by aquarists and expounded in advertisements, yet there have been few published analyses of these products. Although every naturally occurring element can be found in natural seawater, private aquarists typically have access to test methods for determining only inorganic nutrients and a few major and minor ions. This paper presents a detailed and comprehensive analysis of eight commercial formulations of synthetic seawater available in North America. A refereed version is published in the Journal of Aquariculture and Aquatic Science 8(2):39-43.
Methods
Samples of commercial synthetic seawater mixes were purchased from That Fish Place and shipped directly to the Hawaii Institute of Marine Biology. The containers were opened, manually mixed and sub-sampled 10 times. The subsamples were placed in ziplock bags and sealed. About a week later, these samples were analyzed. (Follow this link for an explanation of the methods used in this study.)
The salinity of near-surface seawater in the tropics is approximately 35 parts per thousand (ppt), so 35 gram samples were dissolved in highly purified water, brought to one liter and analyzed. The elements sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), strontium (Sr) and boron (B) were determined with a Perkin Elmer Atomic Absorption Spectrometer. The anions chloride (Cl-) and sulfate (SO42-) were determined by ion chromatography, which would also detect Br- and F- if present. The concentrations of lithium (Li), silicon (Si), molybdenum (Mo), barium (Ba), vanadium (V), nickel (Ni), chromium (Cr), aluminum (Al), copper (Cu), zinc (Zn), manganese (Mn), iron (Fe), cadmium (Cd), lead (Pb), cobalt (Co), silver (Ag) and titanium (Ti) were measured by Inductively Coupled Plasma (ICP) spectroscopy.
TableI
MajorConservative Elements
CATIONS ANIONS
Salinity
(ppt) Na Mg Ca K Sr Cl- SO4- BO3 HCO3-
CO3-- Molar Mass
(grams per mole)
Seawater 35 470 53 10.3 10.2 0.09 550 28 0.42 1.90 22.9898
Instant Ocean 29.65 462 52 9.0 9.4 0.19 521 23 0.44 1.90 24.305
Tropic Marin 32.64 442 46 8.9 9.1 0.08 497 21 0.36 1.10 40.08
HW Marine Mix 29.40 467 53 9.0 10.1 0.15 538 28 0.41 2.10 39.098
Reef Crystals 28.91 461 50 9.3 9.5 0.08 520 27 0.65 0.75 87.62
Red Sea Salt 30.07 472 55 9.0 9.9 0.10 537 25 0.54 1.08 35.453
Kent 28.85 460 57 10.4 10.1 0.10 531 24 0.54 2.52 32.06(S)
Coralife 28.39 464 63 10.1 9.3 0.08 566 15 1.26 .32 10.81(B)
SeaChem 29.54 504 37 10.1 10.7 0.21 516 37 4.90 .12 12.011(C)
all measured in millimoles per kilogram
To convert from millimolar to ppm, multiply the concentration in millimolar by the atomic mass. For example: The concentration of calcium in seawater is 10.3 milligrams per killigram. Ca++ is 10.3(40.08) = 413 ppm.
Total inorganic carbon was determined by acidifying a sample, then measuring CO2 with an OI model 700 Analytical TOC Analyzer. Total alkalinity was measured by a two-point addition of one hundredth normal hydrochloric acid (N HCl) to bring a 20 milliliter sample to a pH of 3.0 to 3.8. pH was measured with a Sensorex S-100C pH probe and an Orion EA-940 ion-analyzer. The dissociation constants used to calculate borate alkalinity and the speciation of inorganic carbon in seawater were taken from Stumm and Morgan (1981).
A Technicon II autoanalyzer was used to determine the concentrations of the inorganic nutrients phosphate (PO43-), nitrate (NO3-, ammonium ion (NH4+) and silicate (SiO4), using slightly modified Technicon Industrial methods (Walsh 1989). Total nitrogen and phosphorus were measured by oxidizing samples with ultraviolet (UV) light and peroxide, and then measuring inorganic nutrients as above. The difference between inorganic and total nutrients is generally considered to be “organic nutrients” and is reported as dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP).
All results were adjusted to 35 ppt salinity, a temperature of 25 degrees Celsius (77 degrees Fahrenheit) and a density of 1.023 kilograms per liter (kg/L) for comparison. The errors in analysis were estimated from duplicate samples. The values from these duplicates were averaged and then rounded off to the average error of these duplicate analyses. The values are reported in terms of milli- or micromoles per kilogram of solution. To convert these values to parts per million (ppm) multiply the concentration in millimolar by the molar mass of the element. Multiplying the values in micromoles per kilogram by the molar mass gives parts per billion (ppb). To convert ppm to milligrams per liter (mg/L), multiply the value in ppm by the density, 1.023. Micrograms per liter can be derived from ppb through the same process.
Craig Bingman
Graphs illustrating results of the assay.
Results and discussion
Table I shows the experimentally determined salinity of the various samples, which were prepared by dissolving 35 grams of each sample in water, to give a final volume of 1 liter. The experimental salinity reported was determined by simple summation of all of the experimentally determined concentrations of the major and minor ions in the samples. The observed salinity of all the samples was between 2 to 6 ppt lower than 35 ppt, because all of the mixtures contained substantial water of hydration.
Table I also gives the concentrations of major and minor ions, after normalization of all results to 35 ppt salinity. For comparitive purposes, the table also gives typical ionic concentrations for tropical ocean surface water (Nozaki 1994).
As illustrated in Figure 1 to 7, most of the salts resemble seawater in terms of their major ion content. A few generalizations can be made. The sample of Coralife salt assayed was significantly higher than natural seawater in magnesium content and lower than natural seawater in sulfate content. The sample of SeaChem salt was significantly lower in magnesium than natural seawater. Moreover, the sulfate concentration of the SeaChem salt was significantly higher than natural seawater. The molar concentration of magnesium and sulfate concentration of the SeaChem salt were equal, which may result from the use of epsom salts (magnesium sulfate heptahydrate) as the sole source of both magnesium and sulfate in this formulation.
TABLE II
Buffer System Components
Prior to Equilibration with Atmospheric CO2
TCO2 CO2
(x10-3) HCO3- CO3- CA BA TA pH
Seawater 1.90 9 1.66 0.23 1.9 — 2.3 8.25
Instant Ocean 1.90 8 1.65 0.24 1.90 — 2.3 8.35
Tropic Marin 1.10 0.9 0.70 0.40 1.10 — 1.5 8.90
HW Marine Mix 2.10 6 1.74 0.35 2.10 — 3.1 8.49
Reef Crystals 0.75 0.2 0.34 0.41 0.75 — 3.2 9.28
Red Sea Salt 1.08 2 0.81 0.26 1.08 — 1.6 8.69
Kent 2.52 20 2.32 0.18 2.52 — 2.7 8.08
Coralife 0.32 0.1 0.16 0.16 0.32 — 1.5 9.17
SeaChem 0.12 0.1 0.084 0.036 0.12 — 2.2 8.81
Buffer System Components
After Equilibration with Atmospheric CO2
(pCO2 = 350 microatmospheres)
TCO2 CO2
(x10-3) HCO3- CO3- CA BA TA pH Aragonite
saturation
Seawater 1.94 11 1.73 0.20 2.13 0.10 2.23 8.18 2.06
Instant Ocean 1.99 11 1.78 0.19 2.17 0.10 2.27 8.21 1.71
Tropic Marin 1.55 11 1.41 0.13 1.67 0.072 1.74 8.10 1.16
HW Marine Mix 2.27 11 2.02 0.24 2.50 0.11 2.61 8.26 2.16
Reef Crystals 1.53 11 1.40 0.12 1.64 0.12 1.76 8.11 1.12
Red Sea Salt 1.42 11 1.31 0.10 1.52 0.099 1.62 8.08 0.90
Kent 2.38 11 2.10 0.27 2.64 0.15 2.79 8.30 2.81
Coralife 1.26 11 1.17 0.08 1.33 0.21 1.54 8.04 0.81
SeaChem 1.82 11 1.65 0.16 1.97 1.07 2.2 8.17 1.62
TCO2 = total inorganic carbon
CO2 = CO2 + H2CO3
HCO3- = bicarbonate ion milliequivalents per liter (mEq/L)
CO3- = carbonate ion mEq/L
BA = borate alkalinity mEq/L
TA = total alkalinity mEq/L
Aragonite saturation based on a solubility product of 0.89 x 10-6 at 25 degrees Celsius (77 degrees Fahrenheit)
The buffer system of natural seawater is dominated by bicarbonate, carbonate and borate ions. Table II shows the components of the buffer system in synthetic seawater mixes immediately after mixing, and the calculated speciation of buffer components after equilibration with air containing 350 microatmospheres of carbon dioxide. Although the borate concentration of most salts (see Figure 8) closely resembled natural seawater, the SeaChem and, to a lesser extent, the Coralife, salts had significantly higher than natural seawater concentrations of boron. The buffer system of these salts is fundamentally different than seawater.
The total inorganic carbon content of the salts ranged over a factor of 20 (see Figure 9). The initial pH (see Figure 10) of the mixed synthetic seawater often deviated substantially from 8.25. Salts mixing to a high initial pH value were low in total inorganic carbon. Total alkalinity values ranged from 1.5 to 3.2 milliequivalents per liter (mEq/L).
It is believed that the saturation state of seawater relative to calcium carbonate can affect skeleton and test formation in organisms. While it is not surprising that most of the salts were somewhat less saturated with respect to aragonite than seawater, it is noteworthy that two of the salts, Coralife and Red Sea salts were slightly undersaturated with respect to aragonite. It is also worth noting that apparent calcification rates in reef aquaria are sufficiently rapid that the starting saturation state is relatively unimportant: calcium and carbonate alkalinity must be maintained by some means.
The nutrient concentration of the salts was variable (see Table III). The inorganic phosphate content varied from 1.20 micromolar in Tropic Marin to 0.05 micromolar in Instant Ocean (see Figure 11). Organic phosphorus contents were low and similar to seawater values.
There was more scatter in the values of dissolved organic nitrogen. Coralife had the highest content at 11.2 micromolar, although this is only slightly higher than the 10 micromolar dissolved organic nitrogen typically found at the sea surface in the tropics. Total organic carbon contents of the salts were all lower than natural seawater and similar to each other.
TABLE III
Nutrients (micromoles per kilogram)
PO4
NO3:N NH4:N SiO3:Si
(*color) SiO3:Si
(**ICP) DOP DON:N TOC:C
Seawater 0.20 0.20 0.20 5.0 5.0 0.20 10.0 50.0
Instant Ocean 0.05 1.00 10.2 4.2 16.0 0.10 2.9 29.0
Tropic Marin 1.20 2.20 0.55 3.2 14.0 0 5.5 32.0
HW Marine Mix 0.46 1.63 9.2 11.5 29.0 0.2 8.2 29.0
Reef Crystals 0.32 5.0 7.8 5.9 46.0 0.2 6.3 28.0
Red Sea Salt 0.37 0.79 5.2 4.5 17.0 0.1 1.9 29.0
Kent 0.16 2.05 11.9 4.1 37.0 0.1 2.4 28.0
Coralife 0.95 6.30 8.4 2.7 18.0 0.2 11.2 28.0
SeaChem 0.57 18.4 0.7 11.3 23.0 0.1 3.1 22.0
Molar mass 30.9738 14.0067 14.0067 28.086 28.086 30.9738 14.0067 12.011
*color = colorimetric analysis
**ICP = Inductively Coupled Plasma spectroscopy)
The variation in the concentration of ammonia in the salts is notable. Figure 12 shows that two salts were notably low in ammonia: Tropic Marin at 0.55 micromoles per kilogram and SeaChem at 0.7. All the other salts were substantially higher in ammonia than tropical ocean surface water, ranging from 5.2 to 11.9 micromoles per kilogram. These concentrations of ammonia will not be toxic to fish or invertebrates and would not be an issue at all when performing a modest partial water exchange in an established reef tank. It would be a wise to aerate freshly mixed synthetic seawater to allow equilibration with atmospheric gases, and to bring it to the temperature of the tank before doing a water exchange. An inert holding container with a heater and an airstone would be sufficient.
The “total” silicon concentrations of all of the salts were higher than natural seawater (see Figure 13) and in some cases there was a substantial difference between the “colorimetric” or “reactive” silicon present in the samples and the total silicon as determined by atomic absorption. This discrepancy is usually ascribed to polymerized forms of silicate that are relatively unreactive to the reagents used in colorimetric analysis (Greenberg et al. 1992).
TABLE IV
Trace Elements (micromoles per kilogram)
Lithium Molybdenum Barium Vanadium Nickel Chromium Aluminum Copper
Seawater 20 0.1 0.04 0.04 0.004 0.003 0.002 0.001
Instant Ocean 54 1.8 0.085 2.9 1.7 7.5 240 1.8
Tropic Marin 29 2.5 0.32 2.8 1.7 7.6 230 1.9
HW Marine Mix 36 3.3 0.71 3.4 2.3 8.3 250 3.0
Reef Crystals 62 2.4 0.27 3.5 2.1 8.8 250 2.4
Red Sea Salt 44 2.8 0.70 3.4 1.9 8.3 240 2.3
Kent 62 2.8 0.39 3.7 1.9 8.9 290 2.6
Coralife 1793 2.7 0.37 3.8 2.2 9.7 270 2.8
SeaChem 117 2.6 0.89 2.9 1.7 7.7 270 2.4
Zinc Manganese Iron Cadmium Lead Cobalt Silver Titanium
Seawater 0.001 0.0004 0.0001 0.0001 0.00006 0.00005 0.00001 0.00001
Instant Ocean 0.50 1.2 0.24 0.24 2.1 1.3 2.3 0.67
Tropic Marin 0.55 0.7 0.24 0.24 2.3 1.3 2.7 0.62
HW Marine Mix 0.75 1.2 0.34 0.34 3.2 1.8 3.6 0.73
Reef Crystals 0.60 1.0 0.27 0.27 2.6 1.6 4.3 0.79
Red Sea Salt 0.60 1.6 0.27 0.27 2.7 1.5 3.7 0.83
Kent 0.60 1.4 0.27 0.30 2.6 1.6 4.0 1.04
Coralife 0.90 0.9 0.30 0.30 2.9 1.7 3.8 0.97
SeaChem 0 1.7 7.7 0.26 2.5 1.4 3.9 0.85
The concentration of “trace” elements is given in Table IV. Figure 14 shows there is one significant outlier in Li content, Coralife salt. This sample had a lithium concentration 90 times that found in seawater. SeaChem was next highest at five times natural seawater concentration, and other salts were one and a half to three times natural seawater lithium.
All samples analyzed were significantly higher than natural seawater concentrations in the elements Mo, Ba, V, Ni, Cr, Al, Cu, Zn, Mn, Fe, Cd. Pb, Co, Ag and Ti. The variation from salt to salt was much less striking than the excess of all these elements compared to natural seawater.
The halides ions, Br- and F-, were potentially detectable by the ion chromatographic method used, but were below the threshold of detection in all samples. It should be remembered that all salts vary from lot to lot. We have attempted to stress the major features of these formulations.
Acknowledgments: This work was partly funded by the University of Hawaii SeaGrant Program NOAA, NA36RG0507 R/EL-1. We also thank Terry Siegel for reimbursing us for the commercial salt samples used in this report.
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TEXT BY MARLIN ATKINSON AND CRAIG BINGMAN
The Composition Of Several Synthetic Seawater Mixes
Many marine aquarists use synthetic seawater mixtures in their aquaria. The relative merits of the various mixes are often debated by aquarists and expounded in advertisements, yet there have been few published analyses of these products. Although every naturally occurring element can be found in natural seawater, private aquarists typically have access to test methods for determining only inorganic nutrients and a few major and minor ions. This paper presents a detailed and comprehensive analysis of eight commercial formulations of synthetic seawater available in North America. A refereed version is published in the Journal of Aquariculture and Aquatic Science 8(2):39-43.
Methods
Samples of commercial synthetic seawater mixes were purchased from That Fish Place and shipped directly to the Hawaii Institute of Marine Biology. The containers were opened, manually mixed and sub-sampled 10 times. The subsamples were placed in ziplock bags and sealed. About a week later, these samples were analyzed. (Follow this link for an explanation of the methods used in this study.)
The salinity of near-surface seawater in the tropics is approximately 35 parts per thousand (ppt), so 35 gram samples were dissolved in highly purified water, brought to one liter and analyzed. The elements sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), strontium (Sr) and boron (B) were determined with a Perkin Elmer Atomic Absorption Spectrometer. The anions chloride (Cl-) and sulfate (SO42-) were determined by ion chromatography, which would also detect Br- and F- if present. The concentrations of lithium (Li), silicon (Si), molybdenum (Mo), barium (Ba), vanadium (V), nickel (Ni), chromium (Cr), aluminum (Al), copper (Cu), zinc (Zn), manganese (Mn), iron (Fe), cadmium (Cd), lead (Pb), cobalt (Co), silver (Ag) and titanium (Ti) were measured by Inductively Coupled Plasma (ICP) spectroscopy.
TableI
MajorConservative Elements
CATIONS ANIONS
Salinity
(ppt) Na Mg Ca K Sr Cl- SO4- BO3 HCO3-
CO3-- Molar Mass
(grams per mole)
Seawater 35 470 53 10.3 10.2 0.09 550 28 0.42 1.90 22.9898
Instant Ocean 29.65 462 52 9.0 9.4 0.19 521 23 0.44 1.90 24.305
Tropic Marin 32.64 442 46 8.9 9.1 0.08 497 21 0.36 1.10 40.08
HW Marine Mix 29.40 467 53 9.0 10.1 0.15 538 28 0.41 2.10 39.098
Reef Crystals 28.91 461 50 9.3 9.5 0.08 520 27 0.65 0.75 87.62
Red Sea Salt 30.07 472 55 9.0 9.9 0.10 537 25 0.54 1.08 35.453
Kent 28.85 460 57 10.4 10.1 0.10 531 24 0.54 2.52 32.06(S)
Coralife 28.39 464 63 10.1 9.3 0.08 566 15 1.26 .32 10.81(B)
SeaChem 29.54 504 37 10.1 10.7 0.21 516 37 4.90 .12 12.011(C)
all measured in millimoles per kilogram
To convert from millimolar to ppm, multiply the concentration in millimolar by the atomic mass. For example: The concentration of calcium in seawater is 10.3 milligrams per killigram. Ca++ is 10.3(40.08) = 413 ppm.
Total inorganic carbon was determined by acidifying a sample, then measuring CO2 with an OI model 700 Analytical TOC Analyzer. Total alkalinity was measured by a two-point addition of one hundredth normal hydrochloric acid (N HCl) to bring a 20 milliliter sample to a pH of 3.0 to 3.8. pH was measured with a Sensorex S-100C pH probe and an Orion EA-940 ion-analyzer. The dissociation constants used to calculate borate alkalinity and the speciation of inorganic carbon in seawater were taken from Stumm and Morgan (1981).
A Technicon II autoanalyzer was used to determine the concentrations of the inorganic nutrients phosphate (PO43-), nitrate (NO3-, ammonium ion (NH4+) and silicate (SiO4), using slightly modified Technicon Industrial methods (Walsh 1989). Total nitrogen and phosphorus were measured by oxidizing samples with ultraviolet (UV) light and peroxide, and then measuring inorganic nutrients as above. The difference between inorganic and total nutrients is generally considered to be “organic nutrients” and is reported as dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP).
All results were adjusted to 35 ppt salinity, a temperature of 25 degrees Celsius (77 degrees Fahrenheit) and a density of 1.023 kilograms per liter (kg/L) for comparison. The errors in analysis were estimated from duplicate samples. The values from these duplicates were averaged and then rounded off to the average error of these duplicate analyses. The values are reported in terms of milli- or micromoles per kilogram of solution. To convert these values to parts per million (ppm) multiply the concentration in millimolar by the molar mass of the element. Multiplying the values in micromoles per kilogram by the molar mass gives parts per billion (ppb). To convert ppm to milligrams per liter (mg/L), multiply the value in ppm by the density, 1.023. Micrograms per liter can be derived from ppb through the same process.
Craig Bingman
Graphs illustrating results of the assay.
Results and discussion
Table I shows the experimentally determined salinity of the various samples, which were prepared by dissolving 35 grams of each sample in water, to give a final volume of 1 liter. The experimental salinity reported was determined by simple summation of all of the experimentally determined concentrations of the major and minor ions in the samples. The observed salinity of all the samples was between 2 to 6 ppt lower than 35 ppt, because all of the mixtures contained substantial water of hydration.
Table I also gives the concentrations of major and minor ions, after normalization of all results to 35 ppt salinity. For comparitive purposes, the table also gives typical ionic concentrations for tropical ocean surface water (Nozaki 1994).
As illustrated in Figure 1 to 7, most of the salts resemble seawater in terms of their major ion content. A few generalizations can be made. The sample of Coralife salt assayed was significantly higher than natural seawater in magnesium content and lower than natural seawater in sulfate content. The sample of SeaChem salt was significantly lower in magnesium than natural seawater. Moreover, the sulfate concentration of the SeaChem salt was significantly higher than natural seawater. The molar concentration of magnesium and sulfate concentration of the SeaChem salt were equal, which may result from the use of epsom salts (magnesium sulfate heptahydrate) as the sole source of both magnesium and sulfate in this formulation.
TABLE II
Buffer System Components
Prior to Equilibration with Atmospheric CO2
TCO2 CO2
(x10-3) HCO3- CO3- CA BA TA pH
Seawater 1.90 9 1.66 0.23 1.9 — 2.3 8.25
Instant Ocean 1.90 8 1.65 0.24 1.90 — 2.3 8.35
Tropic Marin 1.10 0.9 0.70 0.40 1.10 — 1.5 8.90
HW Marine Mix 2.10 6 1.74 0.35 2.10 — 3.1 8.49
Reef Crystals 0.75 0.2 0.34 0.41 0.75 — 3.2 9.28
Red Sea Salt 1.08 2 0.81 0.26 1.08 — 1.6 8.69
Kent 2.52 20 2.32 0.18 2.52 — 2.7 8.08
Coralife 0.32 0.1 0.16 0.16 0.32 — 1.5 9.17
SeaChem 0.12 0.1 0.084 0.036 0.12 — 2.2 8.81
Buffer System Components
After Equilibration with Atmospheric CO2
(pCO2 = 350 microatmospheres)
TCO2 CO2
(x10-3) HCO3- CO3- CA BA TA pH Aragonite
saturation
Seawater 1.94 11 1.73 0.20 2.13 0.10 2.23 8.18 2.06
Instant Ocean 1.99 11 1.78 0.19 2.17 0.10 2.27 8.21 1.71
Tropic Marin 1.55 11 1.41 0.13 1.67 0.072 1.74 8.10 1.16
HW Marine Mix 2.27 11 2.02 0.24 2.50 0.11 2.61 8.26 2.16
Reef Crystals 1.53 11 1.40 0.12 1.64 0.12 1.76 8.11 1.12
Red Sea Salt 1.42 11 1.31 0.10 1.52 0.099 1.62 8.08 0.90
Kent 2.38 11 2.10 0.27 2.64 0.15 2.79 8.30 2.81
Coralife 1.26 11 1.17 0.08 1.33 0.21 1.54 8.04 0.81
SeaChem 1.82 11 1.65 0.16 1.97 1.07 2.2 8.17 1.62
TCO2 = total inorganic carbon
CO2 = CO2 + H2CO3
HCO3- = bicarbonate ion milliequivalents per liter (mEq/L)
CO3- = carbonate ion mEq/L
BA = borate alkalinity mEq/L
TA = total alkalinity mEq/L
Aragonite saturation based on a solubility product of 0.89 x 10-6 at 25 degrees Celsius (77 degrees Fahrenheit)
The buffer system of natural seawater is dominated by bicarbonate, carbonate and borate ions. Table II shows the components of the buffer system in synthetic seawater mixes immediately after mixing, and the calculated speciation of buffer components after equilibration with air containing 350 microatmospheres of carbon dioxide. Although the borate concentration of most salts (see Figure 8) closely resembled natural seawater, the SeaChem and, to a lesser extent, the Coralife, salts had significantly higher than natural seawater concentrations of boron. The buffer system of these salts is fundamentally different than seawater.
The total inorganic carbon content of the salts ranged over a factor of 20 (see Figure 9). The initial pH (see Figure 10) of the mixed synthetic seawater often deviated substantially from 8.25. Salts mixing to a high initial pH value were low in total inorganic carbon. Total alkalinity values ranged from 1.5 to 3.2 milliequivalents per liter (mEq/L).
It is believed that the saturation state of seawater relative to calcium carbonate can affect skeleton and test formation in organisms. While it is not surprising that most of the salts were somewhat less saturated with respect to aragonite than seawater, it is noteworthy that two of the salts, Coralife and Red Sea salts were slightly undersaturated with respect to aragonite. It is also worth noting that apparent calcification rates in reef aquaria are sufficiently rapid that the starting saturation state is relatively unimportant: calcium and carbonate alkalinity must be maintained by some means.
The nutrient concentration of the salts was variable (see Table III). The inorganic phosphate content varied from 1.20 micromolar in Tropic Marin to 0.05 micromolar in Instant Ocean (see Figure 11). Organic phosphorus contents were low and similar to seawater values.
There was more scatter in the values of dissolved organic nitrogen. Coralife had the highest content at 11.2 micromolar, although this is only slightly higher than the 10 micromolar dissolved organic nitrogen typically found at the sea surface in the tropics. Total organic carbon contents of the salts were all lower than natural seawater and similar to each other.
TABLE III
Nutrients (micromoles per kilogram)
PO4
NO3:N NH4:N SiO3:Si(*color) SiO3:Si
(**ICP) DOP
DON:N TOC:C Seawater 0.20 0.20 0.20 5.0 5.0 0.20 10.0 50.0
Instant Ocean 0.05 1.00 10.2 4.2 16.0 0.10 2.9 29.0
Tropic Marin 1.20 2.20 0.55 3.2 14.0 0 5.5 32.0
HW Marine Mix 0.46 1.63 9.2 11.5 29.0 0.2 8.2 29.0
Reef Crystals 0.32 5.0 7.8 5.9 46.0 0.2 6.3 28.0
Red Sea Salt 0.37 0.79 5.2 4.5 17.0 0.1 1.9 29.0
Kent 0.16 2.05 11.9 4.1 37.0 0.1 2.4 28.0
Coralife 0.95 6.30 8.4 2.7 18.0 0.2 11.2 28.0
SeaChem 0.57 18.4 0.7 11.3 23.0 0.1 3.1 22.0
Molar mass 30.9738 14.0067 14.0067 28.086 28.086 30.9738 14.0067 12.011
*color = colorimetric analysis
**ICP = Inductively Coupled Plasma spectroscopy)
The variation in the concentration of ammonia in the salts is notable. Figure 12 shows that two salts were notably low in ammonia: Tropic Marin at 0.55 micromoles per kilogram and SeaChem at 0.7. All the other salts were substantially higher in ammonia than tropical ocean surface water, ranging from 5.2 to 11.9 micromoles per kilogram. These concentrations of ammonia will not be toxic to fish or invertebrates and would not be an issue at all when performing a modest partial water exchange in an established reef tank. It would be a wise to aerate freshly mixed synthetic seawater to allow equilibration with atmospheric gases, and to bring it to the temperature of the tank before doing a water exchange. An inert holding container with a heater and an airstone would be sufficient.
The “total” silicon concentrations of all of the salts were higher than natural seawater (see Figure 13) and in some cases there was a substantial difference between the “colorimetric” or “reactive” silicon present in the samples and the total silicon as determined by atomic absorption. This discrepancy is usually ascribed to polymerized forms of silicate that are relatively unreactive to the reagents used in colorimetric analysis (Greenberg et al. 1992).
TABLE IV
Trace Elements (micromoles per kilogram)
Lithium Molybdenum Barium Vanadium Nickel Chromium Aluminum Copper
Seawater 20 0.1 0.04 0.04 0.004 0.003 0.002 0.001
Instant Ocean 54 1.8 0.085 2.9 1.7 7.5 240 1.8
Tropic Marin 29 2.5 0.32 2.8 1.7 7.6 230 1.9
HW Marine Mix 36 3.3 0.71 3.4 2.3 8.3 250 3.0
Reef Crystals 62 2.4 0.27 3.5 2.1 8.8 250 2.4
Red Sea Salt 44 2.8 0.70 3.4 1.9 8.3 240 2.3
Kent 62 2.8 0.39 3.7 1.9 8.9 290 2.6
Coralife 1793 2.7 0.37 3.8 2.2 9.7 270 2.8
SeaChem 117 2.6 0.89 2.9 1.7 7.7 270 2.4
Zinc Manganese Iron Cadmium Lead Cobalt Silver Titanium
Seawater 0.001 0.0004 0.0001 0.0001 0.00006 0.00005 0.00001 0.00001
Instant Ocean 0.50 1.2 0.24 0.24 2.1 1.3 2.3 0.67
Tropic Marin 0.55 0.7 0.24 0.24 2.3 1.3 2.7 0.62
HW Marine Mix 0.75 1.2 0.34 0.34 3.2 1.8 3.6 0.73
Reef Crystals 0.60 1.0 0.27 0.27 2.6 1.6 4.3 0.79
Red Sea Salt 0.60 1.6 0.27 0.27 2.7 1.5 3.7 0.83
Kent 0.60 1.4 0.27 0.30 2.6 1.6 4.0 1.04
Coralife 0.90 0.9 0.30 0.30 2.9 1.7 3.8 0.97
SeaChem 0 1.7 7.7 0.26 2.5 1.4 3.9 0.85
The concentration of “trace” elements is given in Table IV. Figure 14 shows there is one significant outlier in Li content, Coralife salt. This sample had a lithium concentration 90 times that found in seawater. SeaChem was next highest at five times natural seawater concentration, and other salts were one and a half to three times natural seawater lithium.
All samples analyzed were significantly higher than natural seawater concentrations in the elements Mo, Ba, V, Ni, Cr, Al, Cu, Zn, Mn, Fe, Cd. Pb, Co, Ag and Ti. The variation from salt to salt was much less striking than the excess of all these elements compared to natural seawater.
The halides ions, Br- and F-, were potentially detectable by the ion chromatographic method used, but were below the threshold of detection in all samples. It should be remembered that all salts vary from lot to lot. We have attempted to stress the major features of these formulations.
Acknowledgments: This work was partly funded by the University of Hawaii SeaGrant Program NOAA, NA36RG0507 R/EL-1. We also thank Terry Siegel for reimbursing us for the commercial salt samples used in this report.
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