disc1
-RT * ln(k)
There are a ton of complaints about the accuracy of the kits we use to test our tank water. Many of our kits are highly precise though. What I mean by that is you get the same result from the same kit, even though two different kits may not agree. Even between two kits from the same brand, the results often vary by a good percentage.
So what can you do if you want a more accurate result? If you are using one of the titration based test kits, you can standardize the kit yourself. I'll use for my example here a Salifert kit since that is what I have on hand, but the same procedure will work for any of the titration tests, those where you add a reagent from a syringe and measure the amount delivered to get the result.
First let's talk about how these tests work and then we'll see how to standardize a kit. In a titration experiment, you are adding a reagent with a known concentration to one that is unknown. Let's take the example of a simple magnesium titration. The titrant will be a chemical called EDTA. Every molecule of EDTA will react with one magnesium ion in a reaction called chelation that binds the two together. The details aren't important, just that ratio, 1 EDTA reacts with 1 magnesium ion. We also have an indicator dye. It's called Eriochrome Black T. When it is in solution with magnesium, it binds to the magnesium and that gives it a pink color. When there is no magnesium, it has a bluish color.
So when we start, we add some of the indicator to the sample. We also buffer the sample to a pH of around 10 but that's not terribly important here. The solution turns pink because there is enough magnesium around to bind with the indicator.
Now we start adding the EDTA solution. We know exactly the concentration of this solution. We add the EDTA solution slowly. Now EDTA binds to magnesium better than the indicator does. So EDTA can remove the magnesium from the indicator, turning it blue. At first, this doesn't happen because there is more magnesium floating around in solution. When we have added exactly the same number of EDTA molecules as there are magnesium ions in the solution, then even the magnesium that's bound to the indicator gets chelated away by the EDTA. This turns the indicator blue.
Now since we know exactly the concentration of our EDTA solution, and we know the volume that we used, we know exactly how many molecules of EDTA we added. Since each molecule of EDTA react with exactly one magnesium ion, that tells us how many magnesium ions are in the sample.
But what if we don't know exactly the concentration of our EDTA solution? That's the predicament we are in with a test kit. It's been in that bottle for a while, and who knows what was going on at the factory that day. Things are probably not going to be exact.
So what we need to do is standardize it. We are going to use a sample with a known concentration and do that titration math backwards to calculate the exact concentration of our titrant.
So let's see how to do this with a Salifert kit. The same procedure should work with any titration type kit.
The IAPSO standard has a magnesium concentration of 54.0mM. That equates to ( 54.0mM * 24.305 mg/mmol = ) 1312.47 ppm. That's the number I'll use here, but you can use whatever standard you have.
Now let's run the normal test with the salifert magnesium kit on the standard solution. The first two reagents are the pH buffer and the indicator. They're going to be added in excess, and their measurement does not affect the result we are going to get. Now we do the titration. We fill the syringe and make note of the position, either with the tip of the black piece or the top of the liquid or whatever point is easiest for you to read. It doesn't matter because we are only considering a difference. It's easiest to do the math if you make this the 1.00mL mark, but you don't have to. Just make sure there's no little bubble in the tip. Now add drop-wise with stirring until we get the color change. At that point the titration is over. Read the syringe using the same point of reference you used before (tip, top of liquid, or whatever). Subtract the two numbers and you now know how much titrant you used.
Let's say we marked at the 1.00ml mark before and at 0.15 ml mark at the end. So we used ( 1.00 - 0.15 = ) 0.85mL of titrant. (Notice if we started at 0.90 we would have ended at 0.05 and ended up with the same 0.85ml.)
Now here's the trick of the math. We are doing the same math we are going to have to do later when we test our tank water, only backwards. So we don't need to go all the way to figuring out the actual number of molecules. Trust me on this, or don't. I can write you a proof on it if you need me to.
Now we know that the number of magnesium ions in our sample is equal to the number of molecules of the titrant. So ( 1312.47 ppm Mg / 0.85ml titrant ) = 1544.08 ppm Mg / ml titrant. I know those units look weird, but if you watch the math later, their going to work out in our favor.
Now look at your Salifert instructions. Running 1ml of titrant would be a result of 1500 ppm, where in our case it would be 1544 ppm. We have discovered the size of the error. We can also see that we measured a 1312 ppm standard and got a result of 1275 ppm. Notice that the size of the error isn't constant. It gets larger the larger your measured value gets in a linear fashion.
Let's save this number 1544. Write it down on the top of the box so you have it in the future when you are running your tests. And let's test our tank water. We run the test just like we did on the standard and let's say we find that we used 0.78 ml of titrant.
We multiply that by the number we got in our standardization to get ( 0.78 ml titrant * 1544.08 ppm Mg / ml titrant = ) 1204.3824 ppm. OK, no teacher is going to let you keep all those significant figures. I'll let you keep out to the tens digit. I think that is safe given the way we did our math. That means we got 1200, but we know for certain now that the real value of our magnesium concentration in our tank is between 1195 and 1205.
The shortcuts we took on the math are actually working for us. By excluding sample volume, it doesn't matter what volume you use so long as you use the same volume for standardization as you do for analysis. We would never do this in the lab, we would get the concentration, but here it's going to protect us from the error in measuring our sample with that syringe. Those things are horribly inaccurate and I believe are the source of most of the error people see. And do you leave the bubble or don't leave the bubble? Does the tip part count? By taking that little math shortcut, we have eliminated all of that error. Just so long as you do it the same way every time and with the same syringe, you are going to have accurate results. If you have an old test kit and use the 1.00mL titrant syringe to measure your 2mL (by filling to 1.00 twice) I'll let you keep all the way to the ones digit.
The precision is still a property of the test kit, and the titration types typically have a pretty high level of precision. But the accuracy is now a function of the standard solution you used. You can buy many different seawater standard solutions and often kits will come with a standard. You are at the mercy of the standard being right or not, but if you stick with one, then your tank should stay stable over a number of different kits.
HTH
So what can you do if you want a more accurate result? If you are using one of the titration based test kits, you can standardize the kit yourself. I'll use for my example here a Salifert kit since that is what I have on hand, but the same procedure will work for any of the titration tests, those where you add a reagent from a syringe and measure the amount delivered to get the result.
First let's talk about how these tests work and then we'll see how to standardize a kit. In a titration experiment, you are adding a reagent with a known concentration to one that is unknown. Let's take the example of a simple magnesium titration. The titrant will be a chemical called EDTA. Every molecule of EDTA will react with one magnesium ion in a reaction called chelation that binds the two together. The details aren't important, just that ratio, 1 EDTA reacts with 1 magnesium ion. We also have an indicator dye. It's called Eriochrome Black T. When it is in solution with magnesium, it binds to the magnesium and that gives it a pink color. When there is no magnesium, it has a bluish color.
So when we start, we add some of the indicator to the sample. We also buffer the sample to a pH of around 10 but that's not terribly important here. The solution turns pink because there is enough magnesium around to bind with the indicator.
Now we start adding the EDTA solution. We know exactly the concentration of this solution. We add the EDTA solution slowly. Now EDTA binds to magnesium better than the indicator does. So EDTA can remove the magnesium from the indicator, turning it blue. At first, this doesn't happen because there is more magnesium floating around in solution. When we have added exactly the same number of EDTA molecules as there are magnesium ions in the solution, then even the magnesium that's bound to the indicator gets chelated away by the EDTA. This turns the indicator blue.
Now since we know exactly the concentration of our EDTA solution, and we know the volume that we used, we know exactly how many molecules of EDTA we added. Since each molecule of EDTA react with exactly one magnesium ion, that tells us how many magnesium ions are in the sample.
But what if we don't know exactly the concentration of our EDTA solution? That's the predicament we are in with a test kit. It's been in that bottle for a while, and who knows what was going on at the factory that day. Things are probably not going to be exact.
So what we need to do is standardize it. We are going to use a sample with a known concentration and do that titration math backwards to calculate the exact concentration of our titrant.
So let's see how to do this with a Salifert kit. The same procedure should work with any titration type kit.
The IAPSO standard has a magnesium concentration of 54.0mM. That equates to ( 54.0mM * 24.305 mg/mmol = ) 1312.47 ppm. That's the number I'll use here, but you can use whatever standard you have.
Now let's run the normal test with the salifert magnesium kit on the standard solution. The first two reagents are the pH buffer and the indicator. They're going to be added in excess, and their measurement does not affect the result we are going to get. Now we do the titration. We fill the syringe and make note of the position, either with the tip of the black piece or the top of the liquid or whatever point is easiest for you to read. It doesn't matter because we are only considering a difference. It's easiest to do the math if you make this the 1.00mL mark, but you don't have to. Just make sure there's no little bubble in the tip. Now add drop-wise with stirring until we get the color change. At that point the titration is over. Read the syringe using the same point of reference you used before (tip, top of liquid, or whatever). Subtract the two numbers and you now know how much titrant you used.
Let's say we marked at the 1.00ml mark before and at 0.15 ml mark at the end. So we used ( 1.00 - 0.15 = ) 0.85mL of titrant. (Notice if we started at 0.90 we would have ended at 0.05 and ended up with the same 0.85ml.)
Now here's the trick of the math. We are doing the same math we are going to have to do later when we test our tank water, only backwards. So we don't need to go all the way to figuring out the actual number of molecules. Trust me on this, or don't. I can write you a proof on it if you need me to.
Now we know that the number of magnesium ions in our sample is equal to the number of molecules of the titrant. So ( 1312.47 ppm Mg / 0.85ml titrant ) = 1544.08 ppm Mg / ml titrant. I know those units look weird, but if you watch the math later, their going to work out in our favor.
Now look at your Salifert instructions. Running 1ml of titrant would be a result of 1500 ppm, where in our case it would be 1544 ppm. We have discovered the size of the error. We can also see that we measured a 1312 ppm standard and got a result of 1275 ppm. Notice that the size of the error isn't constant. It gets larger the larger your measured value gets in a linear fashion.
Let's save this number 1544. Write it down on the top of the box so you have it in the future when you are running your tests. And let's test our tank water. We run the test just like we did on the standard and let's say we find that we used 0.78 ml of titrant.
We multiply that by the number we got in our standardization to get ( 0.78 ml titrant * 1544.08 ppm Mg / ml titrant = ) 1204.3824 ppm. OK, no teacher is going to let you keep all those significant figures. I'll let you keep out to the tens digit. I think that is safe given the way we did our math. That means we got 1200, but we know for certain now that the real value of our magnesium concentration in our tank is between 1195 and 1205.
The shortcuts we took on the math are actually working for us. By excluding sample volume, it doesn't matter what volume you use so long as you use the same volume for standardization as you do for analysis. We would never do this in the lab, we would get the concentration, but here it's going to protect us from the error in measuring our sample with that syringe. Those things are horribly inaccurate and I believe are the source of most of the error people see. And do you leave the bubble or don't leave the bubble? Does the tip part count? By taking that little math shortcut, we have eliminated all of that error. Just so long as you do it the same way every time and with the same syringe, you are going to have accurate results. If you have an old test kit and use the 1.00mL titrant syringe to measure your 2mL (by filling to 1.00 twice) I'll let you keep all the way to the ones digit.
The precision is still a property of the test kit, and the titration types typically have a pretty high level of precision. But the accuracy is now a function of the standard solution you used. You can buy many different seawater standard solutions and often kits will come with a standard. You are at the mercy of the standard being right or not, but if you stick with one, then your tank should stay stable over a number of different kits.
HTH
