R-0870 and the 10 ml test, 2 scoops or 1

[QingGuy]

I've been meaning to ask this since my search-fu has not produced and answer.

When doing the 10 ml chlorine test the manual mentions using .5 factor vs. .2 however it does not say to adjust the dosage of R-0870 powder. I've assumed their instructions are correct and stuck with two scoops, I'm a correct in my assumption? Doesn't make sense to me as it seems like one would reduce the amount of powder b 1/2 since the sample water is less than half.

 

[Donldson]

With the 10 ml test you should use one "heaping" scoop, not a level scoop. As long as you see some undissolved crystals at the bottom then you have added enough.

 

[duraleigh]

Essentially, you are correct. The most important aspect of the powder dosage is that it really doesn't matter much. What you are trying to accomplish (assuming you have some chlorine) is to make the solution turn pink. Typically, one scoop is all you need. Heaping or otherwise doesn't really matter.....you are simply looking to turn your solution pink.

 

[gtemkin]

Are you using the search box at the top right portion of the page? The reason I ask is when I search for 0870 i get dozens of hits to the same or simliar questions.

 

[QingGuy]

Thanks, I'll drop down to 1.

Yes, used advanced search for posts in this section using key words "R-0870 10" nothing jumped out off the first page of results. Just ran search again and this post is in the #1 position. Any suggestions to strengthen my search-fu would be welcomed as I don't want to clutter-up the forum with duplicate information.

 

[pabeader]

No need for advanced. People tend to post all over the place so, just look for 870. Then you can always weed down from there.

 

[singingpond]

It's a little hard to get used to the sloppiness of that part of the test procedure - somehow the tiny little scoop seems to suggest that precise measuring and leveling is required

Some people, judging by posts, do try to be very precise in administering the powder.

Like most others, I've just gotten used to digging out a messy scoop of the stuff and dumping it into the water sample. It's basically a "more-than-enough" amount of the reagent - what the cute little scoop is telling us is really that "more-than-enough" happens to be a small quantity, in absolute terms.

 

[eqbob]

As long as I have some I dissolved portion in the test sample water, I consider it good. I got a half pound container of the powder and I'm not sure I can use that much in years, so if I put in a little extra and get up dissolved portions...oh well (g)

I also had 'trouble' at the beginning of TFP time and the 'precision'of the amount added.

 

[chem geek]

With the 10 ml test you should use one "heaping" scoop, not a level scoop. As long as you see some undissolved crystals at the bottom then you have added enough.

You don't need to see undissolved crystals. That really has nothing to do with the needed amount. As Dave says, you need enough to have the sample turn pink. If you see a "flash of pink" then you need to add more because the FC is very high. At the other extreme, if you add too little and see a weak pink color, then that will make the endpoint transition to colorless harder to see. Usually one scoop for a 10 ml sample is sufficient and two for a 25 ml sample works well, but if you find that at the end of the test the transition from pink to colorless is not sharp (i.e. if between the last one or two drops the change is not very easy to see) then you can always add more powder the next time you do the test and see if that makes it easier for you.

 

[eqbob]

I think that's the point that some people (myself) are not comfortable with. You need to see pink. But we know if you don't add enough you see a light pink and you'll register totals that are not true. I keep my chlorine high because it's in full sun all day long so I expect to see a deep pink upon addition. I know instantly upon adding powder if I'm good or not, without testing. I do always complete the test of course. But why not 'oversaturate' the sample and avoid the problem of was that pink enough to give me true values, especially when we're talking minuscule amounts of powder? That's a lot easier point for people to get to than trying to figure out the right shade of pink.

 

[MarianParoo]

Thank you, Richard. I have seen a ton of posts here lately about needing to see undissolved powder. Don't know what that's about or where it came from. I have hesitated to jump in and say, "What???". Now I will have a thread I can link to that will be trusted and respected. Why waste reagents?!?!?! Thanks again.

 

[jblizzle]

I tend to agree with Bob . You need to see a nice deep pink a light pink is not enough powder. Unless of course you're free chlorine is very low

 

[chem geek]

Well it's true that if you have high, but not too high, FC that even a smaller amount of powder will show as a more saturated pink/red and that could still end up at the end of the test having a weak transition. So unfortunately you can't know for sure at the start of the test how much you will need to see a good transition at the end of the test. I think that's where the "one heaping spoon" rule for 10 ml came from because it has enough excess that the endpoint transition for the 10 ml water sample size should be decent (and use two heaping for 25 ml to be safe). For sure we should not say that the amount doesn't matter at all and for sure having too much (within reason) is not a problem but having too little may be.

Now let's be frank about what kind of error we are talking about here. The difference between having a "full" amount vs. half that amount is likely only to result in a difference of one drop at the end of the test. So we're being a little picky here, especially if one is using a 25 ml water sample size. However, for a 10 ml water sample size, one drop is 0.5 ppm which for low FC can be more of an error (i.e. one drop error in the test plus another drop error from not using enough DPD powder is a 1.0 ppm FC error).

I personally don't think that one heaping spoon is excessive or wasteful of DPD reagent for the 10 ml test and I also don't know how "bad" a flat spoon amount would be except that when the powder is a bit clumpy then as was pointed out some may not dissolve quickly so you really aren't getting a full flat spoonful amount. Dave, do you know where the Test Kits Compared of ~70 tests came from in terms of the assumption of the size of the DPD powder amount and the number of FAS drops used per test?

By the way, I generally do tests using a 25 ml water sample because I like to know that level since my chlorine usage is so low (around 1 ppm FC per day due to the pool cover) and I use a rather large heaping spoonful and sometimes a little more if it's very clumpy and I always find that the endpoint transition seems quite distinct (i.e. the last drop has a very distinct pink to colorless OR the next to last drop has a very distinct pink to nearly colorless -- if the latter, then another drop takes it colorless).

 

[eqbob]

Thank you Richard. As always, your posts are informative, thoughtful, and professional. I know when I got my Taylor test the instructions said 2 level scoops. So that's what I've always used and that results in a few crystals of undissolved powder. I have seen the one heaping scoop approach statedand maybe that's just the right amount to not have the extra? Me, personally, I feel more "comfortable" seeing a few grains of powder in the bottom as then I KNOW I have enough in there for a proper test. As to the comment of wasting reagents (not yours) were not even talking pennies per month extra so that's a little nonsensical to me.

This link to the Taylor test is where I also see two level scoops. Posted soley for reference to the procedure.
Instructions

 

[MarianParoo]

.
"Wasting reagents" was probably not a well thought-out phrase.

My concern with telling people to have undissolved solids in their vial is that someone might focus on that and go beyond two scoops looking for the solids (hence "wasting reagents" and more potential errors).

Fear and inexperience can cause us all to do things we shouldn't do. 1 heaping scoop for 10 ml sample is an appropriate standard measure. IMHO.

 

[chem geek]

This link to the Taylor test is where I also see two level scoops. Posted soley for reference to the procedure.
Instructions

Well, their 2 dippers for both sample sizes makes no sense so if 2 dippers works well for a 25 ml sample size than 1 should work well for a 10 ml sample size. Taylor gets a lot right but sometimes they don't and these instructions are an example.

 

[eqbob]

.
"Wasting reagents" was probably not a well thought-out phrase.

My concern with telling people to have undissolved solids in their vial is that someone might focus on that and go beyond two scoops looking for the solids (hence wasting reagents and more potential errors).



Fear and inexperience can cause us all to do things we shouldn't do. 1 heaping scoop for 10 ml sample is an appropriate standard measure. IMHO.

Your comments are well taken. My 'concern' with this particular direction is that going low on reagent has a negative effect...improper measuring while going high has none other than potential monetary implications over extended time.

Nonetheless, as Richard points out, this is largely splitting very fine hairs with the differential that results, and at least as I understand it, that's not the intent of TFP testing recommendations. In all likelihood one person's one scoop heaping and another person's two scoops flat is likely pretty darn close at the sample sizes under discussion.

 

[Richard320]

Thank you, Richard. I have seen a ton of posts here lately about needing to see undissolved powder. Don't know what that's about or where it came from. I have hesitated to jump in and say, "What???". Now I will have a thread I can link to that will be trusted and respected. Why waste reagents?!?!?! Thanks again.

That was probably me.

I think almost anyone - colorblind or not - can see whether there are a few undissolved granules in the vial. That right there answers the "How pink should it be" and this "do I really need two scoops" questions with an absolutely foolproof way of telling if there is enough. Whether or not it's chemically precise or necessary isn't my concern. My concern is to keep it simple. It's a yes-no proposition without any varying shades of grey - or pink as it happens. Isn't that the point of FAS-DPD when measuring FC levels that a colormatching test can do as well: there's no question about the shade? It either is or it isn't clear. Yes-no. Black & white. It's the same as "Are there undissolved granules?" It's yes or no.

I will continue to keep it simple for the new people. I will look forward to your contrary opinion every time I post my standard reply in the future.

 

[MarianParoo]

Hahaha. I don't think I remember seeing you say it, however, I bow to your wisdom and will keep my contrariness to myself!

 

[chem geek]

The problem with the undissolved crystals is that it doesn't work that way as an indicator because the water does NOT get saturated with DPD (or with other powder contents). If you add very fine powdered DPD to the water, you can add more than 2 scoops and not get any undissolved crystals after a swirl. I've done this by intentionally scooping out the most powdery part from the container. So what do you do, keep adding scoop after scoop after scoop where it all keeps dissolving?

At the other extreme, you have a large big crystal clump which you add and then stop because you see something undissolved. However, this doesn't put enough DPD into the water.

I must be missing something because I don't see how seeing undissolved clumps means anything useful except that it tells you you've added something that hasn't fully dissolved. If anything, you need to add more powder beyond the amount if it were to have fully dissolved (or you need to put something in to crush the clumps to help them dissolve faster). That is, the amount you add should be an amount that gets into the water, not that sits there at the bottom undissolved.

This MSDS shows the components of the DPD powder where every single item is very soluble in water. The dye itself is as a soluble salt, N,N-Diethyl-p-phenylene-diamine sulfate, which as shown in this link has solubility of 0.1 g/ml and this link says 1000 mg/ml (1 g/ml) so let's use 0.1 g/ml. According to the MSDS, the dye is less than 2% of the weight. A 10 ml sample should be able to hold at least (10 ml) * (0.1 g/ml) = 1 gram of dye which would be (1 gram)/(2%/100%) = 50 grams of powder which is of course more than the entire 10 g container. Likewise, the solubility of the main components of the powder, potassium and sodium phosphate (see this table), are very high at 92.3 g/100g so 0.923 g/ml and 12.1 g/100g so 0.121 g/ml so in 10 ml this would be 9 g and 1 g respectively. At <65% and <35% of the powder, that's 14 g and 3.5 g, respectively so the limit for saturation of the least soluble component appears to be for sodium phosphate at 3.5 grams or about one-third of a 10 g container in 10 ml.

If one doesn't want to waste powder, then crushing any clumps back into powder so that they quickly dissolve and so one need not overdose would be the way to go. In fact, do this experiment: take a mortar and pestle and thoroughly grind to a fine powder the entire contents of a 10 g container. Then take a 10 ml water sample and add powder to it swirling each time. See how much powder it takes before the water is saturated and adding any more powder simply sits on the bottom undissolved. You may find that you can dissolve about a third of the 10 g container in 10 ml. Note that some DPD may get oxidized by air and that part might not be soluble, but it will also be useless, but it should be a very small amount if you crush to a fine powder.