New co-op test strips are giving me odd readings

[CT_]

 

I picked up the new test strips today.  And I love how they are more nitrate sensitive 0,20,40,80 wasn't great especially when 20 was really hard to see, so 10,25,50 seems perfect to me.  

I thought I'd share my experience with the new coop test strips, since they're not quite working for me and maybe someone has a solution or knows if there could be something about my water that interferes.

The strips are reading a bit funny for me though.  According to my API test kit (directions followed exactly, even spending a *** minute shaking the second bottle and tube after mixing) i had 5-10 last night so i dosed 2x easy green (that should add 6ppm nitrate).  Today I measured again expected to see 10-15 nitrates, so a 10 or 25 on the strip and it came out a perfect color match for 50.  So I got out my API kit again today and using the api kit I got 10-20 (<40, I can't tell 10 and 20 apart but it was under 40 for sure).

 

The nitrite read 0, no surprise.

The GH read 300ppm(!!!) for my seattle water with a small dose of equilibrium.  My API titration test puts me at about 7 degrees ~= 125ppm.  I'm not shocked though since the API test strips didn't work at all for me.

KH came out in agreement with the API test 4degrees/drops on the api test 80ppm on the strip. The api test strips always gave me garbage readings on kh.

ph came in between 6.8 and 7.2, API strips always say about 7.0, but API liquid always comes in at 7.8.  So no worse than the other strips but still a head scratcher to me.

I've never tested chlorine before so it was cool to have it.  I got 1.5 out of my tap (and 0 in my aquarium) which seems reasonable probably means I can use half the suggested amount of prime which is nice to know.

 

 

Edited April 4, 2021 by CT_

[Cory]

Seems very strange you're getting 7.8 ph with the api test kit. I think all the water we've ever tested from Seattle has been below 7.0 ph. How old is your api test kit? It would seem odd that your pH is off. The strips were extensive tested with know lab solutions, and against pH meters. They were very accurate. I'd love to get the bottom of why your readings are different than expected.

[CT_]

25 minutes ago, Cory said:

Seems very strange you're getting 7.8 ph with the api test kit. I think all the water we've ever tested from Seattle has been below 7.0 ph. How old is your api test kit? It would seem odd that your pH is off. The strips were extensive tested with know lab solutions, and against pH meters. They were very accurate. I'd love to get the bottom of why your readings are different than expected.

 

I'm actually on the east side south of Bellevue.  According to this Seattle and Edmonds get west point treatment plant and I get south treatment plant.  Still low gh/kh water though.

co-op strips agree with the API strips too but the liquid API kit says 7.6(or greater) on PH low and 7.8 on ph high tests.  last week I brought my water in to the university and tested it on a calibrated lab PH meter.  I got 7.75.

My master kit was purchased January this year and has an expire date of 09/2023 and my gh/kh kit expires 09/2023 also.

 

I'm more confused by the nitrate.  I'm going to test the strips, and my api liquid kit against a high quality standard solution later this week and see what's what.

 

[Philip]

1 hour ago, Cory said:

Seems very strange you're getting 7.8 ph with the api test kit. I think all the water we've ever tested from Seattle has been below 7.0 ph. How old is your api test kit? It would seem odd that your pH is off. The strips were extensive tested with know lab solutions, and against pH meters. They were very accurate. I'd love to get the bottom of why your readings are different than expected.

Not to get too geeky, but wet chemistry is notorious for skewed results. pH meters drift and should be calibrated often. I like to test buffers to check accuracy and even titrate using phenophthalein so I know exactly where 8.3 is. I would totally trust that for comparison to make sure my meters are spot on. 

Keep in mind that I am a water chemist and have very high end bench meters and spectrophotometers. The cheap meters most people have are pretty good, but I wouldn't go to the bank on them. For those taking their water to the fish store, or University in this case, you are very prone to CO2 absorption because of time and how the sample is usually stored, so I wouldn't trust their pH readings.  The variance you got can easily be explained by that. Both can be right. Fill bottle all the way with no air trapped. Minimize time in-between. It's best to read pH right away, expect it to go down a little over time. 

Also, if you have lower conductivity water, where there is little buffer, some of these results, especially pH can swing. 

If I were a betting man I'd say the test strips are more reliable. 

Sorry so long. 

[CT_]

23 minutes ago, Philip said:

Keep in mind that I am a water chemist and have very high end bench meters and spectrophotometers. The cheap meters most people have are pretty good, but I wouldn't go to the bank on them. For those taking their water to the fish store, or University in this case, you are very prone to CO2 absorption because of time and how the sample is usually stored, so I wouldn't trust their pH readings.  The variance you got can easily be explained by that. Both can be right. Fill bottle all the way with no air trapped. Minimize time in-between. It's best to read pH right away, expect it to go down a little over time. 

Correct me if I'm wrong but wouldn't CO2 absorption lower the ph?  so 7.75 would be a low estimate (I don't inject CO2 and always sample at the end of my photoperiod).  I sampled the water into a 50ml conical all the way to the top and filter sterilized the water to make sure no microbes would change the water while I transported it.  I don't recall the model but the meter I used was calibrated and was expensive.

And to be clear, I also believe that both can be right in the sense that they're accurately reading the water they're exposed to under the conditions they're in.  I'm not too concerned with pH so I haven't thought hard about which was right (in the sense that it actually represents the conditions of my water in the tank), but always assumed it was the liquid test & meter.

I am curious though what else would cause the difference in readings.  Could the large volume liquid tests absorb co2 much slower than the pad and since i have low kh the test strip ph gets lower?

 

[Philip]

Sir,

I use a Hach 411d, calibrated daily. Maybe I misread your results but I thought you got 7.8 and the sample you brought in was 7.75. This is about the difference you get with CO2 absorption. Not just from adding CO2 to your tank, which you would see locally, but from air in the sample. 

Note that it will not change a lot. You won't go from 7.8 to 6.5 for example, but the amount you got is reasonable. 

You are correct about your assumptions with pH. I have said in the past, and I believe Cory has said similar, but you really can't look at something like pH on it's own and make broad assumptions. You really need to take it in context with all your other parameters. 

I believe your thinking is on the right track, but don't put too much stock in one thing or one reading. Look at it as a whole. 

[CT_]

2 minutes ago, Philip said:

Sir,

I use a Hach 411d, calibrated daily. Maybe I misread your results but I thought you got 7.8 and the sample you brought in was 7.75. This is about the difference you get with CO2 absorption. Not just from adding CO2 to your tank, which you would see locally, but from air in the sample. 

Note that it will not change a lot. You won't go from 7.8 to 6.5 for example, but the amount you got is reasonable. 

You are correct about your assumptions with pH. I have said in the past, and I believe Cory has said similar, but you really can't look at something like pH on it's own and make broad assumptions. You really need to take it in context with all your other parameters. 

I believe your thinking is on the right track, but don't put too much stock in one thing or one reading. Look at it as a whole. 

Ah okay 🙂  I thought you were explaining the difference between 7.0 and 7.8 being co2.  I meant to share that 7.75 number to demonstrate that its in agreement with the 7.8 API liquid test kit.

the "PH high" kit from API has color bars at 7.4 7.8 and 8.0 so I'd really accept any reading between say 7.6 and 7.9 and in good agreement as I'm reading the liquid ph test with my eyeballs and not a spectrometer.

[Philip]

I'm sure you are in the ballpark. I'm hesitant to chime in when I'm using a $1,700 pH meter and don't want to come off sounding like other people are wrong. We are talking about being accurate to .002 which is absurd in the fish hobby. 

When it comes to what we do with aquariums, we just need to be able to troubleshoot or know we are fine. If you want to know exact numbers it will cost you big bucks, and is unnecessary. 

It's also why I'm a fan of the test strips. Fast, easy and pretty reliable. Can't beat that. 

[CT_]

@Cory  I have an update.

 

I did some more testing with nitrate standards.  and made solutions of 50,40,25,20, and 10. The strips worked well for 25 and 50.  10 came in a bit dark but close.  I did notice that when dipping and holing the strips level the stick isn't hydrophobic enough to keep the water that's stuck to it from touching neighboring pads and that can really effect the colors you get, especially hardness, which explains my original 300 reading from gh 7 water.  For this test I only wetted the tip of the stick.  IDK why but in my hands its pretty easy to contaminate neighboring pads, same is true with API strips it turns out.

 

For pH, yesterday I was spot cleaning with H2O2 so I think that dropped my pH a bit today.  The API liquid test shows 7.4-7.6 now (I think, see photos).  and the test strips show <6.8

[CT_]

Oh I forgot to add I also tested the API master kit's nitrate readings.  It's color table is off by a whole shade (so, a factor of 2!) for me, so 10 looks like 5, 20 looks like 10 etc.  My ghetto spectrometer shows my tank water is very close to the 20ppm standard.

 

[GardenStateGoldfish]

Do you usually take the api liquid test kits at half the water volume? That could be why the results are so different? I know math is math but I have heard your not supposed to do that cause it can interfere with the results. Like using 5 drops of each pm 2.5 ml.

[CT_]

 

8 hours ago, GardenStateGoldfish said:

Do you usually take the api liquid test kits at half the water volume? That could be why the results are so different? I know math is math but I have heard your not supposed to do that cause it can interfere with the results. Like using 5 drops of each pm 2.5 ml.

For the nitrate I do.  It's the one I use the most and its also the one that uses the most reagent per test.  And getting a new set of bottles costs half the price of the master kit.  I actually wish Cory would sell the nitrate kit too so I could just grab new bottles in my next order when i run out.

It's true that scaling things linearly doesn't always work.  For an assay like this you have to get two things right. 

1) The stoicheometry:  Are the relative abundance of reagents and sample correct.  In this case it does scale linearly ie you can half everything

2) Assessing color:  By eye is a pretty terrible way but gets you within a factor of 2.  To do this accurately you need to make sure you have the light path correct and are observing the color against the right background (you're actually observing the light coming from the paper transmitted through the tube into your eye).  Since you're observing through the side of the tube the height of the liquid doesn't matter (within reason).  But if you used a wider tube it could look darker or a narrower tube it could look lighter. 

The best way is to measure assays like this is by measuring [light] absorbance with an instrument like a spectrometer, which can yield very accurate results.

[CT_]

Oh I forgot to add that what makes reading the color by eye even worse is that your references usually isn't another tube with the same chemicals and known quantities of analyte (like I had in the photo), but colors printed on paper.  the printing process is usually done with some number of dyes (usually 4, I think some processes are more though) and the colors in between are made by stippling.  Since each dye absorbs light differently and differently from your test sample their color can look different under different lighting conditions.  Conversely two tubes with the same nitrate concentrations will always look the same when viewed under the same light.

[Daniel]

My theory is not to have overly high expectations from inexpensive test kits. If it close that's okay. What I really want is for the test to be consistent so I can track changes over time.

Whatever the colors on the printed chart my main goal is to see trends over time. By consistently photographing the test results in the same place and the same light from the same angle, no matter what the 'accuracy', the precision of the readings would be tight enough to see if the parameter was going up, down or staying the same.

 

[CT_]

29 minutes ago, Daniel said:

My theory is not to have overly high expectations from inexpensive test kits. If it close that's okay. What I really want is for the test to be consistent so I can track changes over time.

Whatever the colors on the printed chart my main goal is to see trends over time. By consistently photographing the test results in the same place and the same light from the same angle, no matter what the 'accuracy', the precision of the readings would be tight enough to see if the parameter was going up, down or staying the same.

I agree 100% but I do want to know 5 vs 50 nitrates and ph 6.5 vs 7.5. 

That's why I was so alarmed by seeing a test strip reading of 50 and a liquid test reading of 10.  And similar with the pH.  It looks like I had error that was just in opposite directions, and frankly that first strip test could have been contamination due to user error with the strip.

I still don't know whats going on with the ph strips vs my liquid test.  The API strips also read much lower for me.  It must be something with my water or lighting or ??. 

 

I'm doing a lot of this testing with standards etc just to understand the error I should expect and to get error to acceptable level in my hands, hoping it helps others too.  

[Daniel]

1 minute ago, CT_ said:

I'm doing a lot of this testing with standards etc just to understand the error I should expect and to get error to acceptable level in my hands, hoping it helps others too.  

The work you are doing is much appreciated!