When I first got into hydroponics (before internet) there was one place to buy chemicals, and one brand. Since I live in an area with hard water (180 mg/L) and pH 7.8, I needed a lot of pH down. After rapidly going through a $6 litre of pH down, I took some to the lab and found it to be a 5% phosphoric acid solution. Since then I have just blindly made 25% (w/w) phosphoric acid solution and used it with success.
Some phosphate is good but I have seen what P toxicity looks like, when I got too much into solution. This was also in the days when you used liquid pH indicator instead of pH meters so it wasn't quite as precise. The plants obviously don't need as much P as they get with my hydroponic nutrient mix. I have found the minimum required amount of N and K through experimentation, but no way to find the optimal P concentration due to the pH down solution.
The trick with phosphoric acid is that it has 3 available hydrogen ions (acid) per molecule, which makes it extremely efficient at neutralizing alkalinity. It is also available at concentrations up to 100%, and is a macronutrient. On the down side it reacts with hardness ions to form insoluble calcium phosphate scale, and overdosing can cause toxicity.
60% nitric acid only has 1 available hydrogen ion per molecule, is a macronutrient, and is an extremely corrosive, stinky, oxidizing acid. You don't want it at home.
100% sulphuric acid has 2 available hydrogen ions per molecule, is a micronutrient, and is an extremely corrosive, oxidizing acid. You don't want it at home. It also reacts with hardness ions to form insoluble calcium sulphate scale.
The above acids are all considered as strong acids because they release all of their hydrogen ions into solution (complete dissociation) resulting in a pH of less than 1. There is an other class of acids known as organic acids which don't completely dissociate, so they are not as corrosive (pH 2-3).
The simplest organic acid is acetic. It is a 100% active liquid at room temperature but crystallizes in the refrigerator, gaining it the name of glacial acetic acid. It has an extremely powerful aroma, and only has 1 available hydrogen ion per molecule. Vinegar is a 5-7% acetic acid solution. I see there is a thread touting the benefits of vinegar as an acidulant on the website, but the odour and quantity required makes it impracticable for me.
Reviewing my options I came up with citric as a replacement for phosphoric acid:
-It comes as a 100% active powder (food grade). I just got 1 kg for $13 at my local spice shop. It is used in cheese making, and canning (inhibits the growth of C. botulinum).
-It has 3 hydrogen ions available per molecule so it's very effective at neutralizing alkalinity.
-It is not as corrosive or stinky as other acids.
-It has a solubility greater than 50% in water.
-It is a fairly strong chelating agent, so it should slowly de-scale my rock grow media of calcium carbonate/phosphate/sulphate.
-Plant roots release citric acid to help solubilize soil nutrients for uptake (so there might be a transport system that allows uptake into the plant?)
I just made up a solution of citric acid by half filling a container with the powder and then topping up with hot water. The solution cools down, since it's an endothermic reaction, but rapidly dissolves. I need slightly less of this solution than the 25% phosphoric acid I have been using. Let the testing begin.
Some phosphate is good but I have seen what P toxicity looks like, when I got too much into solution. This was also in the days when you used liquid pH indicator instead of pH meters so it wasn't quite as precise. The plants obviously don't need as much P as they get with my hydroponic nutrient mix. I have found the minimum required amount of N and K through experimentation, but no way to find the optimal P concentration due to the pH down solution.
The trick with phosphoric acid is that it has 3 available hydrogen ions (acid) per molecule, which makes it extremely efficient at neutralizing alkalinity. It is also available at concentrations up to 100%, and is a macronutrient. On the down side it reacts with hardness ions to form insoluble calcium phosphate scale, and overdosing can cause toxicity.
60% nitric acid only has 1 available hydrogen ion per molecule, is a macronutrient, and is an extremely corrosive, stinky, oxidizing acid. You don't want it at home.
100% sulphuric acid has 2 available hydrogen ions per molecule, is a micronutrient, and is an extremely corrosive, oxidizing acid. You don't want it at home. It also reacts with hardness ions to form insoluble calcium sulphate scale.
The above acids are all considered as strong acids because they release all of their hydrogen ions into solution (complete dissociation) resulting in a pH of less than 1. There is an other class of acids known as organic acids which don't completely dissociate, so they are not as corrosive (pH 2-3).
The simplest organic acid is acetic. It is a 100% active liquid at room temperature but crystallizes in the refrigerator, gaining it the name of glacial acetic acid. It has an extremely powerful aroma, and only has 1 available hydrogen ion per molecule. Vinegar is a 5-7% acetic acid solution. I see there is a thread touting the benefits of vinegar as an acidulant on the website, but the odour and quantity required makes it impracticable for me.
Reviewing my options I came up with citric as a replacement for phosphoric acid:
-It comes as a 100% active powder (food grade). I just got 1 kg for $13 at my local spice shop. It is used in cheese making, and canning (inhibits the growth of C. botulinum).
-It has 3 hydrogen ions available per molecule so it's very effective at neutralizing alkalinity.
-It is not as corrosive or stinky as other acids.
-It has a solubility greater than 50% in water.
-It is a fairly strong chelating agent, so it should slowly de-scale my rock grow media of calcium carbonate/phosphate/sulphate.
-Plant roots release citric acid to help solubilize soil nutrients for uptake (so there might be a transport system that allows uptake into the plant?)
I just made up a solution of citric acid by half filling a container with the powder and then topping up with hot water. The solution cools down, since it's an endothermic reaction, but rapidly dissolves. I need slightly less of this solution than the 25% phosphoric acid I have been using. Let the testing begin.
