Chemistry behind test kits

[JeroenB]

Hi guys,

Can anybody help me in understanding the chemistry behind the test kits? I mean, what substances do we add to the test sample, what makes the color change, what chemical reactions occur, stuff like that.

I found out only a few things:

The JBL pH 6.0-7.6 uses bromomethyl blue, changes color from yellow to blue.

For KH one generally uses a pH indicator (but I don't know which one or which mixture) which has a sharp color changing point, at for example pH=4. Then we add some acid to the test sample and measure how much acid is needed for the pH to drop and then compare that using some chart.

I'm very interested in everything you can tell me about the other pH tests, nitrate, nitrite, ammonium, iron and everything else. I just want to completely understand what I'm doing...

Thanx in advance

[Reefing Madness]

You'll want to point this question at Leebca.

[leebca]

I only wander outside the Forums I monitor by request. I received your request JeroenB.

There are many different chemical reactions which can provide information regarding chemical contents in a water-based liquid. Each test kit is based upon its own chemical changes with regards to what ingredient it is trying to measure. Then also keep in mind that each kind of ingredient can have many different ways to do this set of reactions. This is all to mean that there are many different chemical methods to get the (almost same) answer. However, almost all of these methods of measuring quantities are based upon similar fundamental chemical concepts. This area of science is known as quantitative chemistry.

In the most layman of terms, one such concept is to take a liquid with the ingredient you want to measure (quantitate) and slowly add a carefully measured chemical to it that changes that ingredient to some other chemical ingredient. What the investigator (you) are then looking for is when there is no more of that old ingredient available to convert to the new chemical ingredient. But, the liquid is clear and you can't 'see' when this occurs.

So we add to the liquid another ingredient. That ingredient changes to another chemical ingredient too (different than the one we're trying to measure). BUT when this ingredient changes from the one to the new ingredient, it changes color. So it may begin as pink, and change to blue, when it converts to the new ingredient. So we use this. This chemical is called an 'indicator' and that word describes its function very well.

So, we take some of the (tank) water and we add the indicator to it and then drip in the chemical until the indicator changes color. When the indicator changes color (assuming we chose the right indicator) what this says is that, 'All of the other chemical (we're trying to measure) has converted to the other new chemical ingredient, and now the chemical being dripped in is being used to change the indicator from its original form (pink in the above example) to its new chemical form (blue in the the above example).' At this point, we stop drip adding the chemical and measure exactly how much of that it took to 1) convert the chemical we're looking for AND 2) to change the indicator.

Through mathematical relationships of those chemicals, we read a chart that tells us how much of that original ingredient was in the sample. This is just one such concept. It applies to the some common test kits.

Another concept is that the indicator changes to many different colors because the indicator is the chemical reacting with the chemical we're trying to measure. The more of that ingredient we are trying to measure, the indicator changes to a different color. Then you use a color chart to find the closest color to the one displayed in your test sample to indicate the quantity of chemical in the original sample.

Other fundamental chemical concepts are used in variations on the above, or sometimes in combination, to achieve the result of how to measure an ingredient in a sample of (tank) water. If you grasp the above, then we can elaborate on other methods. For instance, the one you mention about using an indicator, changing the pH of the sample, then measuring how much chemical it takes to change the pH to another number is more complex. The ingredient couldn't be measured in either case noted above, so we needed ANOTHER ingredient to measure. pH was chosen in this case. The chemical ingredient we want to measure (KH in your question) can't be done directly by one of the above methods. Instead, we use pH to tell us when the chemical has been converted, then we measure the amount of chemical it takes to change the pH from one color to the other. This is a means of indirectly measuring something in solution/liquid by use of its property of the new chemical ingredient to affect another property of the sample -- that of pH.


Hope this helps some. For a complete, detailed, and accurate understanding you need to enroll in quantitative chemistry college courses or try to read basic quantitative chemistry books.

[JeroenB]

Thanx for your replies.

Leebca, can you give me any information about the exact substances? I'm gonna look for a good book, I'm quite familiar with redox and acid-base relationships, trying to get the equations on paper. The manufacturers don't seem to say which indicators or reactors they're using, possibly someone knows of a website where they're listed?

[leebca]

No I can't. Some of these chemical systems are proprietary. They are owned by the company that makes the kit. They can't necessarily patent it, but they can and do keep it a secret to the best of their ability. Others are very common and could well be done 'from scratch' by the chemist hobbyist. Some other kit manufacturers might be willing to share their information. You'd have to contact the makers directly for details.

Regarding books, you are on your own. Check out bookstores and libraries in your area, as well as the Internet. Look through the books offered and determine for yourself which ones you understand. As a degreed chemist, I can't really direct to you literature you would likely understand and follow. Today though, there are many avenues for finding such literature.

[JeroenB]

Ok, thanx for your reply!