Test sample expiration

DRC Archive General Saltwater Discussion
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Chemistry is not my strong suite I have to admit and I have a chemistry question. I was always told and understood that if you took a sample of water to an LFS it was pretty much pointless for them to check your pH as it may have changed being in an enclosed container especially if you had grabbed the sample of water hours before.

Question is would the Alk change and how quickly? Would the Ca, Mg, and Nitrogen levels remain the same 2-3 weeks later if the container stayed sealed?(pretty sure they would be, but need to hear someone else say it)

2

I’m just trying to keep a closer eye on some of the reef tanks that I maintain for work. Carrying around and breaking out test kits when you have 6 jobs in a day in inner city PHL can be a pain. It would make things simpler for my coworkers if they could just pop some water in a jar and have me test it later. I just am not sure how long it will be until I will be able to do the tests and how much later the results would still be reliable.

3

Not great at chemistry myself but I know when I test my PH with a kit and get my reading if the tube sits for 5-10 minutes the reading changes.

4

It is live water, full of bacteria, phyotoplankton and bugs when it comes out of the tank. if its sealed in a tube with little air the live stuff will use some of the disolved O2 in the water, probably making CO2 in the dark. like night time in the tank. so PH and DKH should drift down. I doubt Ca, Mg would change. any nitrates, nitrites and ammonia might be affected by the bacteria and phyto too in the light. so , sooner would be better than later, for testing. Does that sound right?

But water doesnt carry that much bacteria. its mostly on stuff surfaces. it might hold for a while. The guys could use test dip strips for some things. for a wuick on the spot check.

5

Not sure on the exact times, but I know you would want to put the Nitrogenous wastes (Ammonia, Nitrite, Nitrate) on ice untill you test them. That will slow any bacteria and extend your sample period.

6

Ok Jon, talked to the head of our lab (I do environmental water quality). She said that as long as the samples are refrigerated/put on ice in a cooler, that Alk is good up to 28 days, and Ca and Mag are good for longer. NH4, NO3, and NO2 are good for up to 7 days when properly kept.

So as long as you bring a small cooler full of ice, and put your sample bottles in the ice, you should be able to collect them all, bring them back, and then test them with no major problems as long as you get to the samples within the week.

7

Very interesting. Out of curiosity Belleamy, do the samples need to be brough back up to the original temperature before testing? I only ask this because i thought that the temperature of the water could affect salinity when mixing synthetic salt? For example, if i were to mix a fixed amount of salt with RODI at 60 degrees to achieve a salinity of 1.025 that it likely would not stay at 1.025 if heated to 82 degrees? This might not even be true but i had always thought it was.

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Salinity is measured in parts per thousand in sea water, 35 parts of salt per 1000 parts of water, ppt doesn’t change with temp. 35 ppt @ 60 degrees is the same as 35 ppt @ 80 degrees.

specific gravity is a weight to volume comparison.

i copied/ pasted this because i didn’t feel likey typing it all out lol

Picture a little clear acrylic cubic box that measures 1cm x 1cm x 1cm on the inside.
Fill it with pure water.
It now contains 1 cc ( cubic centimeter ) of water.
This little clear box of water contains exactly 1 milliliter of water.
1 cc = 1 ml

Now if you could weigh just the water in this little box ~ How much would the water alone weigh ?

The answer is IT DEPENDS.

The actual weight of this volume of pure water will depend on the temperature and the atmospheric pressure.

The metric standard for weight is grams.
It just so happens that this little clear box of water we have, if it’s temperature was 4 degrees C ( 39.2 F ) and it was at sea level where the atmospheric pressure is 1 atmosphere ( 14.7 psi ) then the 1 cc of water ( 1 ml ) would weigh in at a whopping 1 gram.

In other words ~ 1 cc of water is 1 ml of water and weighs 1 gram. Were talking PURE WATER here.

This is how people came up with the term “Specific Gravity”
The DENSITY of water ~ how much it weighs ~ for a given VOLUME is referred to as the SPECIFIC GRAVITY.

In our pure water example above - the specific gravity would be 1.000

1.000 gram of pure water in a 1.000 cubic centimeter container.

Now the tricky part.

Water ( in a LIQUID state ) expands and contracts with temperature changes.
The warmer it is - it expands.
The cooler it is - it contracts.
When water expands or contracts - it either takes up more volume or less volume. But the amount of salt in the sample does not change. Only the volume of the water changes.
The ppt stays the same.

So … if we’re using specific gravity as our measurement - we need to always look at the temperature of the saltwater too.
Specific gravity is a measurement of the WEIGHT of salt to the VOLUME.
Specific gravity is a measurement of the saltwater’s DENSITY as compared to DENSITY of pure water - for the same volume of each.

We’ll use 3 different temperatures as examples, and the chart goes something like this.

At 60 degrees F
35 ppt = 1.026 S.G.

At 70 degrees F
35 ppt = 1.025 S.G.

At 80 degrees F
35 ppt = 1.023 S.G.

As you can see - the salinity is the same for each example.
It’s still 35 ppt

But as the temperature goes up - the specific gravity goes down.

This is simply because the water expands at a higher temperature - and the density must therefore change as well.

Back to our little clear acrylic box of saltwater now.
The box is full of saltwater with a salinity of 35 ppt.

At 60 degrees F - the saltwater in this little box now weighs 1.026 grams.

At 70 degrees F - the box of same saltwater weighs 1.025 grams.

At 80 degrees F - the box of same saltwater weighs 1.023 grams.

The temperature of the saltwater MUST BE considered when using specific gravity as the measurement.

Jon were you saying it would take 2-3 weeks to get to testing them? I ask because I’m wondering how helpfull that long of a wait would be when the tank it self could change in that time.

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[quote=“TimH07, post:8, topic:2687”]
Water ( in a LIQUID state ) expands and contracts with temperature changes.
The warmer it is - it expands.
The cooler it is - it contracts.[/quote]

So why is it that if you put a full water bottle in the freezer the resulting block of ice wil expand and crack the bottle?

10

makes perfect sense. i guess it was one of those things that ive never taken the time to think about.

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[quote=“Cdangel0, post:9, topic:2687”]

[quote=“TimH07, post:8, topic:2687”]
Water ( in a LIQUID state ) expands and contracts with temperature changes.
The warmer it is - it expands.
The cooler it is - it contracts.[/quote]

So why is it that if you put a full water bottle in the freezer the resulting block of ice wil expand and crack the bottle?[/quote]

http://www.iapws.org/faq1/freeze.htm

:slight_smile:

its also why snowflakes are open structures

12

[quote=“logans_daddy, post:10, topic:2687”]
makes perfect sense. i guess it was one of those things that ive never taken the time to think about.[/quote]

what makes my head explode is why refractometers and swing arm hydrometers have static numbers SG vs salinity… they are two separate measurements and nobody in the industry seems to realize that.

sorry about the major hijack Jon

Craig, just saw your PMs will respond but walking out the door right now.

13

Tim has it 100% right. The temp would affect specific gravity, not ppt. Temp will also not affect the other readings that Jon mentioned as far as I know. It will affect Dissolved Oxygen as well, but very few people test for that even though it has a significant impact on the health of our tanks.

14

Interesting thread. I have always wondered about this.

15

Wow, lots of talk on this thread during the one day I couldn’t check my handheld.(I was installing a new 100g custom reef tank with the boss) Edit: then I typed this days ago and never finished posting.
First of thanks for the responses.
Ken I would have to agree with most of what you said. However if the Nitrates were reduced from 5.99ppm to 5.94ppm in two weeks I would careless and because of the resolution my tests wouldn’t even show that. A timeline until hobby grade test kits would show a significant difference is what is important. Most bacteria which and algae in the marine aquarium which would have a significant effect on Nitrates are not in the water column in a large quantity. Most of the animals that have a significant influence on the calcium levels in a reef tank are also benthic.

Tim to answer your question as to if the water sample would even mean anything in two weeks, well yes. First off nothing happens with the tank that would significantly affect the chemistry while we are gone. No water changes or calcium additions will happen unless we have something to do with it. While the client may dump a bunch of sushi in the tank to try to feed the fish, that won’t really affect how much I ask the techs to dose the tank.
90% of our tanks get regular maintenance including consistent water changes, so in general the shifts in chemistry are gradual. If during one visit the Nitrates register then they are likely to register on the next visit as well. If the Calcium is low then something needs to be adjusted as it won’t auto correct.
It is likely it would only be a couple days until I could test the tanks, but I wanted to know if I could in two weeks. Get a better idea of how long it really could be.

BELLAMY THANK YOU. VERY useful information.

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[quote=“icy1155, post:13, topic:2687”]
Temp will also not affect the other readings that Jon mentioned as far as I know. It will affect Dissolved Oxygen as well, but very few people test for that even though it has a significant impact on the health of our tanks. [/quote]

Useful information as well Bellamy. Would the levels of DO return to the same if the original temp was obtained?

17

It depends if there was any air in the test sample. If there were NO bubbles… then it may change back. DO is very tricky to hold samples for unless you have a way of “fixing” the samples in the field. I’m not even sure if there is a way to fix it.

… not 100% sure on DO testing. We use a YSI multimeter that tests DO, pH, conductivity, and temp and logs it. Its a pain though because we need to calibrate it daily to ensure the readings are accurate.

18

Very familiar with that instrument. Used it a great deal at the Marine Bio labs at UD. The lab I worked in was primarily focused on DO levels. Looks as if I’ll have to leave DO levels to you biologist. I still need to get a new probe to test for it at home.

One difficulty in freezing the samples may be with the containers we get to keep them in. I asked Rich to order me a couple things to try out in the field. I was planning on having the techs submerging the test vial under water and then putting the lid on it so that no air would be in the sample. Problem comes in when the sample expands as it freezes. Will have to see how much of a factor this is. Time for a field test.

19

No need to freeze the samples. Just keep them in a fridge. If you put them in a cooler with ice from where you collect them, then in the fridge, they will keep for more time than you should need (at least a month for KH, and more for Ca and Mg).