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I prepared an isotonic solution of table salt in water following this recipe: 1 leveled coffee spoon of salt in 0,5L of water.

9g/L would be an isotonic solution, a leveled coffee spoon is commonly given as 5g - close enough. I use water from the tap, which was freshly boiled in a water cooker. The water is not chlorinated, I don't know the hardness.

However, even given hours of time, a significant amount of salt doesn't go into solution - visible as turbidity/precipitation. How can this be?

I need the solution to flush my child's nose, so I don't want to use too much salt.

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Check the composition of your table salt -- some types contain less soluble drying agents, I think. –  Aesin Nov 25 '12 at 23:42
    
And your coffee spoon may be very inaccurate. However since the solubility of sodium chloride is about 359 g/L (per Wikipedia) you'd have to add about 180 g of $\ce{NaCl}$ to a half liter to saturate it at 25 degrees C. Clearly something is wrong here. –  Paul J. Gans Nov 26 '12 at 1:58
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up vote 1 down vote accepted

Salt added to food could be sea salt, common table salt or iodized table salt.

Common Table salt typically contains anti-caking agents (desiccants) such as magnesium carbonate, calcium silicate, dicalcium phosphate, tricalcium phosphate, calcium carbonate and sodium alumina silicates.

Iodised table salt may contains added potassium iodide for people who may otherwise have low consumption of this nutrient which is essential for proper thyroid function. Since iodide will slowly oxidize in moist air, stabilizing agents such as sodium thiosulphate or dextrose are also added. Some will also contain a base such as sodium bicarbonate, disodium phosphate or sodium pyrophosphate to also inhibit oxidation of iodide. There may also be added sequestering agents which bond to trace metal ions present as impurities to prevent them oxidizing the iodide.$^{[1]}$

The presence of magnesium and calcium will add "hardness" to the water which could cause precipitates such as calcium carbonate and magnesium carbonate to form.

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thanks - my table salt contains magnesium and calcium carbonate as anti caking agents, that should be it. –  martin Nov 26 '12 at 16:58
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