FAS-DPD Chlorine testing using R-0003 vs. KI crystals

[polyvue]

I've noticed that some FAS-DPD chlorine test kits (sold by Taylor and others) utilize Potassium iodide crystals instead of a reagent solution to effect a color change indicating Combined Chlorine (CC) products in a sample. What is the reason for this? Do the crystals provide some additional measure of accuracy over the liquid solution? The kits containing the crystals are generally more expensive. Any thoughts - even conjecture - on this would be appreciated.

Example

Taylor offers two kits dedicated to FAS-DPD chlorine testing:

K-1515 - Uses KI solution (R-0003 DPD Reagent #3)

K-1516 - Uses KI crystals (Potassium Iodide Crystals R-0765)

 

[JasonLion]

K-1515 (liquid) is sold for the pool/spa market. K-1516 (crystals) is sold for the industrial/commercial market. There is no difference in the specifications of the tests. I suspect that it is purely a difference of convention. In some market people are used to using crystals, so Taylor offers it that way so it will be familiar to them. But I don't really know.

 

[chem geek]

Usually, the form used for a compound is related to its stability, but in the case of potassium iodide both crystals and liquid (solution) forms have a long shelf-life as noted here where crystals last 7 years and liquid 5 years. So I think Jason is right that it is just a convention for a particular industry to use a particular form. There is also the minor factor of weight/cost since the solution is mostly water that you then need to ship and there may be issues of freezing during shipment so avoiding solutions would be helpful in that case (though the FAS-DPD titrating solution is liquid in both kits).

As for the difference in price between the two kits you listed, it is because the K-1516 uses the 2 ounce "C" size for the expensive FAS-DPD titrating reagent. If you compare with the K-1515-C kit of comparable size, then you will notice that the prices are pretty close.

Again, this is just speculation on why there is a difference in the Taylor kits.

 

[polyvue]

K-1515 (liquid) is sold for the pool/spa market. K-1516 (crystals) is sold for the industrial/commercial market. There is no difference in the specifications of the tests. I suspect that it is purely a difference of convention. In some market people are used to using crystals, so Taylor offers it that ways so it will be familiar to them.

Makes sense...

Usually, the form used for a compound is related to its stability, but in the case of potassium iodide both crystals and liquid (solution) forms have a long shelf-life as noted here where crystals last 7 years and liquid 5 years.
[...]
As for the difference in price between the two kits you listed, it is because the K-1516 uses the 2 ounce "C" size for the expensive FAS-DPD titrating reagent. If you compare with the K-1515-C kit of comparable size, then you will notice that the prices are pretty close.

You're probably correct; I didn't perform a careful cost comparison (and looked only at a few products from two manufacturers) that would include the total number of kit tests using the dry crystals vs. solution. I'm not sure why R-0871 is so expensive... how much can it cost to produce what is described as an "inorganic salt solution"?

Sidenote: Though my question was not posed due to an interest in stockpiling KI for use in precluding thyroidal radioiodine uptake in the event of a nuclear attack, it is nevertheless a bit unnerving that more than half of the links don't work on NRC's 2 year old web page... Potassium Iodide as a Thyroid Blocking Agent in Radiation Emergencies 404 Not Found. -- Oh great! "Honey, grab the Mortons... quick!!"

 

[chem geek]

I'm not sure why R-0871 is so expensive... how much can it cost to produce what is described as an "inorganic salt solution"?

The main ingredient is ferrous ammonium sulfate (FAS) which I presume may be in a pH buffered solution and might have other chemicals to enhance stability. This link describes more technical information about the FAS-DPD and other chlorine tests.

 

[polyvue]

I'm not sure why R-0871 is so expensive... how much can it cost to produce what is described as an "inorganic salt solution"?

The main ingredient is ferrous ammonium sulfate (FAS) which I presume may be in a pH buffered solution and might have other chemicals to enhance stability. This link describes more technical information about the FAS-DPD and other chlorine tests.

Oooh. Great white paper! And well-written, too. Thanks so much for the link.