Point taken
I just get nervous putting x drops of something into x ml of water, shaking it, then reading the result against a plastic card; how do I know the amount of water is exactly right? Am I interpreting the colour correctly? What if it's in between two colours -- is it 0.25, 0.5ppm, halfway in between? The tests where I drip until I notice colour change make me crazy -- did that just turn a tiny bit blue and then back again or was it my imagination? How blue is blue enough?
I guess I'm just not confident in my 'skill', ability to read colours, or fill vials perfectly. I only have coloromiters for PO4, Ca and alk, but they make me feel somehow safer about playing with water chemistry that's supporting hundreds to thousands of dollars in sea life :S I wish I could get one for Mg, but my dry goods guy tells me they don't make one for hobbyists.
So here are a few tips that can help.
With the color reading types of tests, like nitrate and phosphate tests for example, you are adding reagents in excess and reading a color based on a concentration. In these tests what matters is the actual concentration of the analyte in the water. In those cases exact measurement of the water sample isn't critical. You are always adding more reagent than really needed to make sure that no matter what all of the analyte reacts to give you the colored product. So if you go over or under a little on the volume of the water sample it really isn't going to throw you off.
Now reading the colors in between is a different matter. But think of it this way, if I told you that you had somewhere between 10 and 20pm nitrate or if I told you that you had exactly 15.44673456ppm nitrate... what would you do any differently about it? Sometimes the question is moot because you don't really need any increased level of precision.
I certainly understand the advantage of the colorimeter on the phosphate tests because we are reading such low concentrations, but remember that even there you aren't getting a hard number, you are getting a range. I can't remember off the top of my head the error range on the Hanna phosphate checker, but there will always be a plus or minus after that number.
Now for the titration tests I have a big thread in the chem forum on those. Here is exactly the opposite. With a titration you aren't measuring concentration you are actually counting molecules. If you know how many molecules and how big the sample was then you can know the concentration. So with those tests it is absolutely critical that you get the sample size exactly right. That's why those test often come with a syringe to measure the sample instead of a vial with a line on it. Whatever percentage you are off on your sample size you will be off by that percentage on your final result.
But what about that color change. Technically it depends on the type of test, but a good rule of thumb is to consider the endpoint the first color change that persists on mixing. So the how blue is blue enough question is answered by saying any different than it started out. There are a thousand things that can have an effect on the exact hue of that final color, but most of us can tell the difference between pink and blue. Someone with red-blue color blindness might find this impossible, but for the rest of us there shouldn't be any question.
But it is the first color change that persists on mixing. So when you add a drop sometimes the spot right where the drop hits will change, but once mixed in it goes right back to the original color. Not the end point. End point is when the whole sample changes and doesn't change immediately back as soon as you give it a little shake.
What about purple (in our pink to blue example). Well purple is what happens when you add some blue to pink right? That is some blue color, so we are right on the endpoint. The thing to do in that case is write the number down and add one more drop. The one more drop should put you obviously over. If it does then you were done. If it doesn't then something is probably going wrong. It actually depends on the type of test as to whether you should technically keep the number before or after the last drop, but in reality we're back to the who cares about that level of precision. If I tell you that you have between 8.6 and 8.7 dkH or if I tell you that you have exactly 8.61287534 dkH what are you really going to do any differently?
That's why I don't like the cheaper alk kits like API where you are counting drops instead of measuring volume off one of those graduated syringes. If each drop is 0.5dkH then yeah one more drop makes a big enough difference that I might do something differently. 8dkH vs 8.5 dkH might be enough of a difference to change my plans.
To me it depends on what you are after. For phosphate I am a huge fan of the Hanna because you get added precision over reading colors with your eyes. For alk though I am not a fan of the Hanna alk checker because there are so many things that affect the chemistry of the indicator they are using and throw off your results.
The number one thing you can do with any type of test is use a standard. If you have something with a known level of the analyte, then you can use the test and make sure you get the right answer. If you do then you're golden. If not then you know you goofed somewhere.
