OTO drops temporary turn blue

Zeerob

Well-known member
Sep 9, 2017
58
Portugal, Torres Vedras
Hi,

I have just refilled my pool with borehole water. This is the second time I have (re)filled.
My water holds iron, about 0.05 mg/L.
When I added chlorine, the water turned very brown. The chlorine apparently turns the FE in the water into rust particles.
My cartridge filter removes the rust particles. I have already cleaned the filter right after refilling and 3 days after that.
The filter was very brown, but it washed off reasonably easy. The water is now very crystal clear.
This happened now and also with the other filling, 2 years ago.
At the moment, I am lowering my TA. See other post.

But I have noticed something strange. When I use OTO drops to measure TC, I found that the water turns blueish at a specific concentration of the OTO.
For example, 1 drop in the test tube may turn the water yellow, the 2nd drop turns it blue/brown, and the 3th will turn it yellow again.
This is also visible when you fill a 1L can with water, rotate the water a bit, and then put in a drop of OTO. The OTO will turn the water yellow, but you also see some darker strands of water. See picture.
I have the idea that the effect is slowly getting weaker, but it is still very well visible.
At the moment I am using trichlor tablets, so I will build up some CYA. When there is enough CYA, I will use my SWG.
The water is crystal clear. If I put a drop of OTO in fresh borehole water, nothing shows up. If I then add a little chlorine, the effect is there again.
I have not seen this before, so it looks like it slowly goes away (?)

Does anyone have an idea what may cause this.
It worries me because I am afraid it may not be good for the SWG.
Thanks!
 

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JamesW

TFP Expert
Mar 2, 2011
13,869
Further research indicates that the blue is caused by your alkaline water.

Orthotolidine gets oxidized by chlorine from a colorless compound to form a highly colored yellow holoquinone at low pH.

The OTO contains acid to lower the pH.

However, at high pH and TA, the sample is not acidic enough and can give a blue color.

Before doing the test, add a few drops of R-0009 to the sample to lower the pH and TA. Then, add the OTO.

What is your sample size when doing the OTO test?

(ii) When water is highly alkaline, a blue tinge is formed instead of yellow. In such a case the quantity of orthotolidine to be added should be doubled.
Chlorination | Water Treatment | Water Engineering
 
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JamesW

TFP Expert
Mar 2, 2011
13,869
The method followed in testing a chlorine solution takes advantage of certain chemical properties of diaminodiphenyl compounds. Such diaminodiphenyl compounds are oxidized by chlorine to give partally oxidized products known as meriquinones, and more fully oxidized products termed holoquinones. In the presence of excess acid (low pH) the formation of the holoquinone is favored and this is true whether or not the amount of the diaminodiphenyl compound is present in excess of that quantity which theoretically can be completely oxidized by the amount of chlorine present. To more fully explain the chemistry of this invention, one diaminodiphenyl compound, namely, 3,3' dimethylbenzidine, is considered in detail, although it is not intended to restrict the scope of this invention to that one compound.
The meriquinone of 3,3' dimethylbenzidine (also known as orthotolidine) is blue, while the 0 holoquinone is yellow. Under the conditions stated above (excess acid), the formation of the yellow color is favored. This factor influenced the conditions prescribed for the ortho tolidine test for chlorine most commonly in use. It had 5 also been observed that variations in color of the ortho tolidine test commonly used were due to the superimposing of different degrees of oxidation upon the effects of variable pH, in that if the amounts of ortho tolidine and chloSrine were constant, the color varied progressively from yellow to green to blue as the numerical pH increased from pH 1 to about pH 4. These observations apparently justified the assumption that the oxidation product was a pH indicator Shaving a range from 2.0 to 4.0 with a color change from yellow to blue.
Other research indicates that the preferred pH for the sample is below 1.8.

If you have a high pH and TA, the reagent won't be able to lower the pH far enough.
 
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Zeerob

Well-known member
Sep 9, 2017
58
Portugal, Torres Vedras
Hi James,

Thanks a lot for your reply! That would be a nice explanation. I was afraid it might be a metal or so.
My TA is high indeed. Initially it was 295, and I have so far lowered it to 220. I will continue lowering it with MA.
Your explanation would also explain why the effect seems to be decreasing, the TA is already lower than first.
I will report here how it develops while the TA goes down further.

B.T.W, there is no Magnesium in my water. I have a test set for that.

I am using a simple OTO test set with a sample size of about 5 mL.
Unfortunately, I do not have R-0009. I have the OTO and a Lovibond with tables (DPD1/3, Phenol, YCA, TA).
And I have a few aquarium test sets for Iron, Calcium and Magnesium.
The Taylor test set that TFP advises is not available here.

Thanks again!
 

JamesW

TFP Expert
Mar 2, 2011
13,869
Ok, I suspect that the blue effect will disappear once you get the TA down.

The titrating reagent that you use for the TA test is acid. Try putting 2 drops in the sample before adding the OTO. The titrating reagent is the reagent that you count the drops.

Also, note that I was referring to manganese and not magnesium.
 
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Zeerob

Well-known member
Sep 9, 2017
58
Portugal, Torres Vedras
Hi, the TA is now down to 160, and the effect is still there, but less than initially.
And I have tried this with tap-water (with some chlorine added), and that has the effect also, but much less.
So, it must be the effect that James explained.
Thanks.
 

Zeerob

Well-known member
Sep 9, 2017
58
Portugal, Torres Vedras
I have just tried a new bottle of OTO, but it is the same.
So, if I put a drop in a can of water that I have stirred a bit, you also see some darker strands, as in my previous picture.
Anyway, initially, when TA was still very high, I noticed that the 2nd drop turned the sample blueish. Then I tried several things, also one drop in a can of stirred water and noticed the darker strands. Apparently that is something that happens always in some degree, I had just never noticed because normally you just work with a sample and not a can.
Now that the TA is lower, the sample never turns blueish anymore. And James provided a good explanation for that.

However, there still is something not 100% okay with my water. See my other question: Redox chlorine sensor not working in my water
 

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