Modified Seawater
There are four cations (positively charged ions) needed for normal development in the sea urchin: Sodium, Potassium, Calcium and Magnesium. Use the following formulas to illustrate the need for each of these ions.

All measurements are in grams per liter of artificial seawater. Start with 750 ml of distilled water and add components in the order shown, top to bottom. After all of the salts have been added and dissolved, bring the total volume to one liter (mix until uniform).

  Normal (-) Sodium (-) Potassium (-) Calcium (-) Magnesium
Sodium Chloride 25 59 grams of
Choline Chloride
25 25 27.7
Potassium Chloride 0.7 0.7 0 0.7 0.7
Calcium Chloride
1.4 1.4 1.4 0 1.4
Magnesium Chloride
6 H2O
4.7 4.7 4.7 4.7 0
Magnesium Sulfate
7 H2O
6.3 6.3 6.3 6.3 0
Sodium Bicarbonate 0.2 0.2 grams of
Potassium Bicarboonate
0.2 0.2 0.2

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The pH of normal seawater is between pH 8.0 and 8.3. Removing anions can be more difficult, but…

Zero Sulfate - substitute an additional 5.2 grams of magnesium chloride * 6H2O for the magnesium sulfate * 7H2O

Zero Bicarbonate - Boil and cool (covered) a little over one liter of distilled water (removes the CO2 from the water). Mix up the seawater you want without the sodium bicarbonate. Keep stirring to a minimum and use a well stoppered glass bottle. This seawater will not last (covered, a few hours), as it picks up CO2 from the air very quickly.

Zero Chloride - The most common substitute for chloride is nitrate. This is not perfect, but can be tried. Most nitrates support rapid oxidation (gunpowder). Use care to insure the jar does not go missing, hmm. The formula becomes: