Serial Dilution help please. TDS Pen.

[si walker]

Hi all.
Recently theres been a bit of TDS Pen chat.
Then I remembered that i hadn't calibrated mine since purchasing.
A couple of people mentioned about weighing 0.3 grams of salt into a litre of water......
So i have spent the evening in loop on the site trying to find out how a human would do this...?
Know what i mean guys... Im no Doctor Spock.

I could go and buy the solution, but if i get my head round it would like to give it a go.
Its probably something like weigh 3 grams and dilute then take a mill and add to ????

Anyway let see!
Thank you.
Si

 

[MichaelJ]

Know what i mean guys... Im no Doctor Spock.

I ran this by my assistant Dr. Spock:

If you take 1 gram of salt (NaCl) and dissolve it in 1 liter of distilled water you will get a solution that is 1000 mg/L of NaCl.

1) If you can't measure up 1 gram precisely you can do 10 grams in 1 liter (this will increase the precision by an order of magnitude assuming you have a precise measuring cup) and take 100 ml of the 10000 mg/L solution and add that to 900 ml of distilled water this will in turn also give you a 1000 mg/L solution.

2) Now, if you want to do 0.3 grams (or 300 mg) to get a 300 mg/L solution, just take 30 ml from the 10000 mg/L solution and add that to 970 ml of distilled water. Alternatively, use 3 grams in step 1 and skip step 2.

🖖

Cheers,
Michael

 

[si walker]

Thats brilliant. A nice experiment to conduct today.
Whilst picking up Distilled Water I can get flowers as apparently it is Mothers Day tomorrow!

Oh, I bet i cant use Bottled Drinking Water can I?

Thanks!
Si

 

[MichaelJ]

Oh, I bet i cant use Bottled Drinking Water can I?

Absolutely not. Bottled drinking water - a lot of which are just glorified tap water - usually sits around 50-200 ppm. Use distilled or reverse osmosis water. Being made from condensed water vapors distilled water will have a TDS very close to zero whereas RO water will usually be very slightly above depending on the efficiency of the membrane.

Cheers,
Michael

 

[Aqua360]

You can also purchase the TDS pre-made solution for £11 online, while this isn't cost effective, it is precise.

 

[MichaelJ]

You can also purchase the TDS pre-made solution for £11 online, while this isn't cost effective, it is precise.

True, but I think that is what @si walker was trying to avoid. I rarely ever calibrate my DIST-1 btw. - every time I’ve done it it never changes my before and after measurements of my tanks in any meaningful way…

Cheers,
Michael

 

[Aqua360]

True, but I think that is what @si walker was trying to avoid.

Cheers,
Michael

Skipped over that part, my bad, must be tired 😴

 

[Andy Pierce]

If you take 1 gram of salt (NaCl) and dissolve it in 1 liter of distilled water you will get a solution that is 1000 mg/L of NaCl.

This will be substantially more accurate if you dissolve 1 gram of salt in 1000 grams of water because with household kit weighing mass is usually easier than measuring volume. As said, distilled water is the best choice.

 

[MichaelJ]

This will be substantially more accurate if you dissolve 1 gram of salt in 1000 grams of water because with household kit weighing mass is usually easier than measuring volume. As said, distilled water is the best choice.

Sure thing if you can measure out the 1 gram of NaCl accurately…. the weights most people have at their disposal can’t accurately measure out 1 gram and being off by a few hundred milligrams makes a huge difference in this case … measuring volume is much easier with even rudimentary measuring cups due to the volume being measured - being 1000 ml. In this case.

Cheers,
Michael

 

[dw1305]

Hi all,

......... measuring volume is much easier with even rudimentary measuring cups due to the volume being measured - being 1000 ml. In this case......

You can equate volume and wt. for dilute solutions.

Personally <"I'd weigh out 10 g of NaCl">, make that up to 1000 mL (1 kg) with RO water ("stock solution"), take 100 mL of that stock solution make that up to 1000 mL and use <"that as my calibration solution">.

Once I've used the calibration solution, I'd tip it away, but you can keep the stock solution and re-use that.

Cheers Darrel

 

[aquanoobie]

Oh, I bet i cant use Bottled Drinking Water can I?

Sure you can. You just need to subtrack the tested value before the salt addition.

 

[Andy Pierce]

Better choice would be "battery top-up water" which is deionised. E.g. Halfords Battery Top-Up Water 5L | Halfords UK

 

[John q]

Or Asda, they have an offer on atm £1.25/2.5L

https://groceries.asda.com/product/car-maintenance/car-plan-de-ionised-water/479358

 

[MichaelJ]

Sure you can. You just need to subtrack the tested value before the salt addition.

Wrong for several reasons:

Remember that TDS meters measures Electical Conductivity. 1 gram of NaCl dissolved in 1 liter of pure water yields a very specific EC - measured in uS/cm.

In the first place the OP have no way of knowing what the TDS (EC) of the bottled water is and secondly its not made up of NaCl.

If he measure say 200 ppm with his uncalibrated device and the error from that measurement is 50 ppm. say the bottled water is actually 150 ppm (quite common).

He would then (mistakenly) add 0.8 grams NaCl to the 1 liter of "measured" 200 ppm water to reach what he thinks is 1000 ppm. but it is in fact only 950 ppm. If he use that solution for calibration (essentially misguiding the TDS meter by claiming its a 1000 ppm NaCl solution when it is in fact 950 ppm from which only 800 ppm is from NaCl) he is still going to end up with a device that is uncalibrated. Yes, he could theoretically repeat the process a couple of times and further reduce the error, but his device will still be subject to the fact that 150 ppm (from the bottle water) is not made up of NaCl which is what the device would assume in order to correctly yield 1000 ppm from the specific conductivity of a pure NaCl solution.


Cheers,
Michael

 

[aquanoobie]

Wrong for several reasons

Adding 300 mg of NaCl in 1L will add 300 ppm reading regardless of the starting reading before the addition, unless the calibration is wrong. And that's the whole point of doing the calibration. Keep turning the calibration control until you get the 300 ppm increase.

 

[MichaelJ]

Keep turning the calibration control until you get the 300 ppm increase.

Not entirely sure what that is supposed to mean... In any event, I think I covered the important bases in my post above.... that said, and not specifically referring to your post, while there might be theoretical “clever” shortcuts that will allow you to save a buck on a few liters of distilled water (which will have an EC well within the typical margin of error of even expense TDS meters) doing the calibration in a way that everyone agrees upon including the manufactures of TDS devices, is the best advice for the aquarium hobbyists who just wants to get over it and move on.

Cheers,
Michael

 

[aquanoobie]

Not entirely sure what that is supposed to mean...

Testing pH devices are very different from TDS and EC devices. Measuring TDS and EC are relatively simple testers displaying electrical conductivity. They are basically reveresed Ohm meters operating usually on ~400 kHz AC to avoid electrolysis effect of H2O by DC current.

Do you have a multimeter at home? How often are you calibrating it? Never? That's right. Just keep the testing electrodes clean and that's it. They need one time calibration, unlike pH testing devices.

When the device registers 300 ppm increase on 0.3 grams of NaCl kitchen salt addition in 1 L, your calibration in completed.

How to keep the electrodes clean? Vodka does wonders.

 

[dw1305]

Hi all,

Better choice would be "battery top-up water" which is deionised

I've got to say I'm firmly in the DI water camp for this one, it just means you know you start from a known low datum.

Measuring TDS and EC are relatively simple testers displaying electrical conductivity. They are basically reveresed Ohm meters operating usually on ~400 kHz AC to avoid electrolysis effect of H2O by DC current.

That's right. Just keep the testing electrodes clean and that's it. They need one time calibration, unlike pH testing devices.

That is the really important bit for me, the <"plug and play"> nature of conductivity meters and the linear relationship of <"dissolved ions to conductivity">. You can buy a relatively cheap conductivity meter, <"turn off the electrics (heater etc.) in the tank">, dip the meter in and get a reading, and that reading that will be reasonably accurate.

If <"there were meters"> where you could do the same for <"nitrate (NO3-)">, pH, CO2, <"dissolved oxygen"> etc. I would be recommending them unreservedly.

Until there are I'm going to carry on recommending <"inferential methods">, they may not give you an empirical value, but they <"give you an overview">.

cheers Darrel

 

[aquanoobie]

You can buy a relatively cheap conductivity meter, <"turn off the electrics (heater etc.) in the tank">, dip the meter in and get a reading, and that reading that will be reasonably accurate.

Even if a heater has defective insulation and leaks electrical current, it should not interfere with TDS and EC devices because they are operating on different frequencies. Similar to two radio channels.

If <"there were meters"> where you could do the same for <"nitrate (NO3-)">, pH, CO2, <"dissolved oxygen"> etc. I would be recommending them unreservedly.

'Dream On' rock song by Aerosmith

 

[MichaelJ]

Do you have a multimeter at home? How often are you calibrating it? Never? That's right.

Right... Actually I don't calibrate my TDS meter either. I just verify that it measures correctly in a known solution once in a while. The few times I've actually calibrated it it hardly changed the result - On a couple of occasions I've accidentally hit the Calibrate button which would reset the device (Hanna DIST) which is sort of a dumb design. In that case I certainly need the reference solution.

How to keep the electrodes clean? Vodka does wonders.

Excellent - I am always open to ideas and suggestions where I can combine the hobby with alcohol.

Cheers,
Michael