I have been on a quest to expand my ability to test for various aquarium parameters myself, and also to understand the testing methods involved, rather than simply rely upon test kit manufacturers. To that end, I have recently developed a simple, accurate, and precise DIY Boron test.
Randy has previously published articles discussing Boron in a Reef Tank, the Salifert Boron Test Kit, and the Seachem Borate Alkalinity Test Kit.
In my searching for methods for the determination of boron in seawater, I turned up some interesting references to the work of Noakes and Hood (1961), who developed a method that exploits the fact that boric acid complexes with certain organic cis-diols, such as the sugar mannitol, to form a strong acid. That strong acid can then be titrated by a base to determine the amount of boron present.
http://www.sciencedirect.com/science/article/pii/0146631361900041
The basic procedure is as follows:
1) Acidify the seawater sample at least to the carbonic acid endpoint (appx. pH 4.5).
2) De-gas the sample to remove CO2.
3) Bring the sample to a neutral pH of 7.0.
4) Add an excess of mannitol. At this point, the pH of the sample will drop substantially.
5) Titrate with a standard sodium hydroxide solution back to the pH 7.0 endpoint.
The amount of NaOH directly correlates with the amount of boron in the sample.
One nice thing about this DIY Boron test is that, for anybody using Randy's DIY Alkalinity Test, the Boron test picks up exactly where the Alkalinity test leaves off, because you will have just done step #1 above! Plus, you will already have the necessary calibrated pH meter, and you will already be familiar with ordering standardized reagents, such as the 0.1N standard acid used.
It is not important to target a specific pH in step #1. It is only important that you get the pH of the sample down to at or below a pH of about 4.5. The addition of extra acid won't hurt anything.
Regarding step #2, de-gassing the sample can be done a variety of ways, but one of the easiest is to simply boil the sample for a couple of minutes. It is important to then allow the sample to cool back to room temperature before proceeding. Using a simple ice bath (a few ice cubes in a bowl of water) speeds the cooling process nicely.
Here is the specific procedure I am currently using:
Take a 100 mL sample of tank water. If I am wanting to do an Alkalinity test, then I titrate with standard acid as usual for that test. If I'm not interested in testing Alkalinity, then I simply add about 5 mL of 0.1 N HCl to slam the pH down to somewhere between 2 and 3. Once I have the pH of the sample down to 4.5 or below, I put it in the microwave and heat it until it has boiled for about 2 or 3 minutes. Obviously, the sample container needs to be able to withstand the boiling; I use a 250 mL conical flask. After boiling, I cool the sample to room temperature in a simple ice bath, which takes just a few minutes. I then carefully add a standard 0.05 M NaOH (0.05 N) solution until the pH of the sample reaches 7.0. It is not important to measure the amount of NaOH it takes to reach 7.0; you just have to get to 7.0 or slightly less. Anywhere between 6.5 and 7.0 should do, as long as you record the exact pH you brought the sample to in this step. Note that this titration follows a typical very steep acid/base titration curve, and while it takes a lot of base to get any kind of change in pH at the beginning, as you approach pH 7.0, tiny additions of the base will cause great leaps in pH, so go very slowly and let the pH meter stabilize as you approach the 7.0 target. If you overshoot, and end up with a pH higher than 7.0, it is perfectly OK to bring the pH back down with some HCl. Once you have a nice, stable pH at or just below 7.0, record that pH value because we are going to return to it in the last step. The next step is to add an excess of mannitol (sorbitol works too). By "an excess", I mean somewhere between 5 and 7 grams of mannitol for a 100 mL saltwater sample. If the sample contains boron levels much higher than NSW levels, then it might take even more mannitol than that. You basically keep adding mannitol to the point where adding more mannitol doesn't cause the pH to drop any more. The last step is to titrate back to a pH of 7.0 (or whatever value you previously recorded), again going very slowly and carefully towards the end. This time, it is important to carefully measure the amount of standard base solution used, because this is the step where we are actually measuring the amount of boron present. It is also important NOT to overshoot this time.
Currently, when doing this procedure on 100 mL of my tank water, it takes about 1.53 mL of the 0.05 M NaOH solution to perform the last step. Now, how can I use this value to determine the amount of Boron? Well, 1.53 mL of 0.05 M NaOH contains 1.53 * 0.05 = 0.0765 mM (millimoles) of NaOH. That means that there are 0.0765 mM of Boron in the sample. The atomic mass of Boron is 10.81, so the amount of Boron in my 100 mL sample is 0.0765 * 10.81 = 0.831 mg. Since my sample is 1/10th of a liter, then in order to get mg/L, I have to multiply by 10, so the result is 8.31 mg/L (PPM) Boron.
Regarding accuracy, I happen to have a Triton test on this display that reported a Boron value of 8.31! No, I did not force or fudge any numbers to make this match happen. I have done this same test on my freshly made Instant Ocean mix, and using my method, I get an average result of 5.05 mg/L. I also have a Triton test on this IO mix, and the Triton reported value for Boron of that sample was 5.04! I have also independently tested the accuracy by "spiking" samples with a known amount of a boric acid solution I've carefully made, and these results hold up to within about 2 percent.
Regarding precision, I have recently done 5 tests in a row on my tank water, and the amount of titrant used in those 5 tests were 1.53, 1.53, 1.51, 1.54, and 1.54. The average of these is 1.530, and the standard deviation is 0.0110. That means that I can say with 95% confidence that my result is within 1.4%, or +/- 0.12 mg/L.
For those already willing to DIY their alkalinity, this DIY Boron test is a relatively quick, easy, accurate, and precise way to measure Boron in your reef tank.
I welcome comments and questions about this DIY Boron test.
Randy has previously published articles discussing Boron in a Reef Tank, the Salifert Boron Test Kit, and the Seachem Borate Alkalinity Test Kit.
In my searching for methods for the determination of boron in seawater, I turned up some interesting references to the work of Noakes and Hood (1961), who developed a method that exploits the fact that boric acid complexes with certain organic cis-diols, such as the sugar mannitol, to form a strong acid. That strong acid can then be titrated by a base to determine the amount of boron present.
http://www.sciencedirect.com/science/article/pii/0146631361900041
The basic procedure is as follows:
1) Acidify the seawater sample at least to the carbonic acid endpoint (appx. pH 4.5).
2) De-gas the sample to remove CO2.
3) Bring the sample to a neutral pH of 7.0.
4) Add an excess of mannitol. At this point, the pH of the sample will drop substantially.
5) Titrate with a standard sodium hydroxide solution back to the pH 7.0 endpoint.
The amount of NaOH directly correlates with the amount of boron in the sample.
One nice thing about this DIY Boron test is that, for anybody using Randy's DIY Alkalinity Test, the Boron test picks up exactly where the Alkalinity test leaves off, because you will have just done step #1 above! Plus, you will already have the necessary calibrated pH meter, and you will already be familiar with ordering standardized reagents, such as the 0.1N standard acid used.
It is not important to target a specific pH in step #1. It is only important that you get the pH of the sample down to at or below a pH of about 4.5. The addition of extra acid won't hurt anything.
Regarding step #2, de-gassing the sample can be done a variety of ways, but one of the easiest is to simply boil the sample for a couple of minutes. It is important to then allow the sample to cool back to room temperature before proceeding. Using a simple ice bath (a few ice cubes in a bowl of water) speeds the cooling process nicely.
Here is the specific procedure I am currently using:
Take a 100 mL sample of tank water. If I am wanting to do an Alkalinity test, then I titrate with standard acid as usual for that test. If I'm not interested in testing Alkalinity, then I simply add about 5 mL of 0.1 N HCl to slam the pH down to somewhere between 2 and 3. Once I have the pH of the sample down to 4.5 or below, I put it in the microwave and heat it until it has boiled for about 2 or 3 minutes. Obviously, the sample container needs to be able to withstand the boiling; I use a 250 mL conical flask. After boiling, I cool the sample to room temperature in a simple ice bath, which takes just a few minutes. I then carefully add a standard 0.05 M NaOH (0.05 N) solution until the pH of the sample reaches 7.0. It is not important to measure the amount of NaOH it takes to reach 7.0; you just have to get to 7.0 or slightly less. Anywhere between 6.5 and 7.0 should do, as long as you record the exact pH you brought the sample to in this step. Note that this titration follows a typical very steep acid/base titration curve, and while it takes a lot of base to get any kind of change in pH at the beginning, as you approach pH 7.0, tiny additions of the base will cause great leaps in pH, so go very slowly and let the pH meter stabilize as you approach the 7.0 target. If you overshoot, and end up with a pH higher than 7.0, it is perfectly OK to bring the pH back down with some HCl. Once you have a nice, stable pH at or just below 7.0, record that pH value because we are going to return to it in the last step. The next step is to add an excess of mannitol (sorbitol works too). By "an excess", I mean somewhere between 5 and 7 grams of mannitol for a 100 mL saltwater sample. If the sample contains boron levels much higher than NSW levels, then it might take even more mannitol than that. You basically keep adding mannitol to the point where adding more mannitol doesn't cause the pH to drop any more. The last step is to titrate back to a pH of 7.0 (or whatever value you previously recorded), again going very slowly and carefully towards the end. This time, it is important to carefully measure the amount of standard base solution used, because this is the step where we are actually measuring the amount of boron present. It is also important NOT to overshoot this time.
Currently, when doing this procedure on 100 mL of my tank water, it takes about 1.53 mL of the 0.05 M NaOH solution to perform the last step. Now, how can I use this value to determine the amount of Boron? Well, 1.53 mL of 0.05 M NaOH contains 1.53 * 0.05 = 0.0765 mM (millimoles) of NaOH. That means that there are 0.0765 mM of Boron in the sample. The atomic mass of Boron is 10.81, so the amount of Boron in my 100 mL sample is 0.0765 * 10.81 = 0.831 mg. Since my sample is 1/10th of a liter, then in order to get mg/L, I have to multiply by 10, so the result is 8.31 mg/L (PPM) Boron.
Regarding accuracy, I happen to have a Triton test on this display that reported a Boron value of 8.31! No, I did not force or fudge any numbers to make this match happen. I have done this same test on my freshly made Instant Ocean mix, and using my method, I get an average result of 5.05 mg/L. I also have a Triton test on this IO mix, and the Triton reported value for Boron of that sample was 5.04! I have also independently tested the accuracy by "spiking" samples with a known amount of a boric acid solution I've carefully made, and these results hold up to within about 2 percent.
Regarding precision, I have recently done 5 tests in a row on my tank water, and the amount of titrant used in those 5 tests were 1.53, 1.53, 1.51, 1.54, and 1.54. The average of these is 1.530, and the standard deviation is 0.0110. That means that I can say with 95% confidence that my result is within 1.4%, or +/- 0.12 mg/L.
For those already willing to DIY their alkalinity, this DIY Boron test is a relatively quick, easy, accurate, and precise way to measure Boron in your reef tank.
I welcome comments and questions about this DIY Boron test.
