In a recent post asking about substrate depths, the conversation drifted quite far afield of the initial question. One of the additional topics thrashed out in the thread was whether aragonite sand contains phosphate and whether you can detect it with hobby phosphate test kits. I am reporting that the sample of aragonite sand that I have does show the presence of phosphates with the Salifert phosphate kit. Here is how I tested the sand.
1.052 g of CaribSea Aragonite sand (very fine) was dissolved in 5 ml 5 molar (15%) muriatic acid. A surprisingly stiff foam formed during the dissolution process (see picture below). When the dissolution was complete the solution was diluted and tested for phosphate. A portion of the solution was partially neutralized with sodium bicarbonate to about pH 5. It tested about 2 ppm. A portion of the solution without neutralization did not develop a blue color when tested. Repeating the partial neutralization with sodium bicarbonate on the remaining portion of the dissolution sample and testing, a blue color developed, indicating 2 ppm phosphate (a real scientist would have neutralized the sample that did not develop a blue color and retest it!). This addresses the question about whether the acid used in the dissolving aragonite would interfere with the phosphate test. I propose the answer is yes. So how much phosphate is this?
There was approximately 15 mL of dissolution sample to start with. The phosphate level in this solution was determined to be 2 mg/L. Multiplying the concentration by 0.015 L gives 0.030 mg. This means 1.052 g of aragonite sand contains 0.03 mg of phosphate or approximately 30 ppm. This study does not indicate the chemical form of the phosphate. I wonder whether 30 ppm is a "œnormal" level for aragonite and whether phosphate was placed there during the formation of the aragonite crystals or afterwards by chemical adsorption.
Maybe as interesting as the presence of phosphate is the rigid foam that forms turning the dissolution. This may indicate a small amount of protein mixed in with the aragonite matrix. Not surprising given the biological origin of this aragonite.
The next question that I need to answer is whether there is phosphate in the silica sand bed in my system that is liberated with a brief exposure to acid.
1.052 g of CaribSea Aragonite sand (very fine) was dissolved in 5 ml 5 molar (15%) muriatic acid. A surprisingly stiff foam formed during the dissolution process (see picture below). When the dissolution was complete the solution was diluted and tested for phosphate. A portion of the solution was partially neutralized with sodium bicarbonate to about pH 5. It tested about 2 ppm. A portion of the solution without neutralization did not develop a blue color when tested. Repeating the partial neutralization with sodium bicarbonate on the remaining portion of the dissolution sample and testing, a blue color developed, indicating 2 ppm phosphate (a real scientist would have neutralized the sample that did not develop a blue color and retest it!). This addresses the question about whether the acid used in the dissolving aragonite would interfere with the phosphate test. I propose the answer is yes. So how much phosphate is this?
There was approximately 15 mL of dissolution sample to start with. The phosphate level in this solution was determined to be 2 mg/L. Multiplying the concentration by 0.015 L gives 0.030 mg. This means 1.052 g of aragonite sand contains 0.03 mg of phosphate or approximately 30 ppm. This study does not indicate the chemical form of the phosphate. I wonder whether 30 ppm is a "œnormal" level for aragonite and whether phosphate was placed there during the formation of the aragonite crystals or afterwards by chemical adsorption.
Maybe as interesting as the presence of phosphate is the rigid foam that forms turning the dissolution. This may indicate a small amount of protein mixed in with the aragonite matrix. Not surprising given the biological origin of this aragonite.
The next question that I need to answer is whether there is phosphate in the silica sand bed in my system that is liberated with a brief exposure to acid.
