DIY Boron Test

JimWelsh

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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 Holmes-Farley

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Very interesting test!

How did you (or Noakes) pick the pH of 7 in step 3? Is that just a convenient starting point based on how low the mannitol takes the pH, without being so high that the boric acid begins to be in borate form?

How low does the pH drop with the mannitol in your tests?
 
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JimWelsh

JimWelsh

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Thanks, Randy!

A pH of 7.0 makes a lot of sense because it is low enough for the vast majority of the borate to be converted to boric acid (any remaining will do so as the pH drops). The final titration is a classic acid/base titration, so an equivalence point of 7.0 makes good sense also.

Other references I ran across do use different pH start/end points in different contexts. I've played around a fair bit with this method, and have found that the specific method I described above works best for an ASW matrix. If you care to read more, some useful links I ran across in my searching include this section of "Handbook of Anion Determination", this section of "Methods of Seawater Analysis", and the specific method I adapted was from this paper.

The pH drop depends upon how much boron is in the sample. In my 5 tests I describe above, the average pH drop was 3.13 pH units, down to pH 3.87, with the amount of boron in my system. The spiked samples go even lower.
 

Cory

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Ive got nothing to add, but watch test kit companies start to make this, unless you make and market it yourself. Happened to me with biopellets in 2006ish...

Cool anyways!
 

Randy Holmes-Farley

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D-mannitol is one type. L-mannitol is another type. They are mirror images. :)

For this test, it should not matter which you use, but since it is Jim's method, I'll leave it to him to clarify.
 
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JimWelsh

JimWelsh

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Yes, for this test either type of mannitol would work just fine. According to the literature, sorbitol and even fructose could work, too, but I've not tested the method using those sugars yet.
 

hawkinsrgk

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Thank you Jim for putting this together and posting it

Looks like I can get the chemicals locally. Can't wait to give this a shot
 

hawkinsrgk

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Finally got everything together to give this a shot. Took forever. The Sodium Hydroxide was on order for 2.5 months before it was shipped, but glad its all here.

One question that I have is on the de-gasing step. When I add 100ml of tank water + HCl and run the microwave on it for 4 minutes it becomes 75ml. Is this a concern that will cause a larger error?

The measurement that I got was 6.91ppm Boron. With that being said, I know this answer is off because of two other mistakes that I made.
1. In the final step to titrate the sample back to the recorded PH value, I was not able to do this. My recorded PH value was 6.72 and I was able to titrate back to a PH of 6.7. The smallest drop I could make would have increased the PH at least .07 or .08
2. I was using a 2ml syringe to titrate the sample back since I thought I would need somewhere between 1 and 2ml. I am guessing the amount of Sodium Hydroxide to bring the sample back to a PH of 6.7 to be 1.28ml. Its over 1.2ml and less that 1.3ml, but its a lot closer to 1.3 so I guessed the result to be 1.28ml.

Thanks again for sharing this.
Randy
 

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