DIY 35ppt Salinity Reference Solution

Nemos Janitor

Nemos Janitor

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Some members might be familiar with RHF article on a home made reference solution recipe. In using it I found it to be quite unreliable in some aspects and do not recommend using the coke bottle method as the coke bottles we have are different to the ones we have in SA. But all the calculation and information is spot on. So I post this method which I have tweaked using Randy's calculations and found to be very reliable. Read through the article if you wish to get a better understanding of what is to come.

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com

Also I prefer to use aquarium salt and not the standard table salt. This is because we want our reference solution to be seawater and not saltwater.

The salt Randy refers to.

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great, look forward to trying it out, when you posting it?
 
To make this S=35 reference solution we are going to need the following.

1) About 1 liter of RO water
2) 36.5 grams of salt.
3) A container to weigh the salt in.
4) A container to weigh and mix the water in.
5) A weight balance.
6) Plastic spoons to scoop the salt out the bucket.
7) A 6ml syringe. one from your test kits is fine.
8) A clean 2 liter coke bottle.
9) plastic bottles to store the solution in.


the plastic spoons available from most plastic shops

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The weight balance I use. Note DO NOT USE THESE MEASURING CUPS to measure out water volume. They are way out and will give you huge errors


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The 6ml syringe one gets from an old test kit.


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The bottles one gets from the plastic store.

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on your ingredients list you say about a liter of water, about or exact? i think it would impact the solution or wont it?

well i know what i am doing this eve
 
If you have a very accurate weight balance one can make approx. 100 ml of the solution. But the one I have only go's down to 1g and does not measure 10ths and 100ths. If you only have access to a electronic kitchen scale then I would recommend making 10 liters. The bigger the volume you make the more accurate the S=35 solution will be.

For this exercise we will make 1 liter of the solution. And it should take about 20 minutes MAX.

It is important to note that salt will attract moisture from the atmosphere. So the salt used should come from a sealed bucket and should be dry so that we don't weigh water that has been absorbed into the salt.

Because we are going to use a small amount of salt there is a possibility that we will not get a perfect sample of all the ingredients in the salt due to settling. Settling means that when a bucket of salt is transported and stored, finer and heavier components in the salt mix make their way to the bottom of the bucket. So to get a good sample of everything in the salt mix we must mix the salt thoroughly in the plastic bag. The salt mix I use is in a plastic that is in the bucket. I remove the plastic bag with the salt in it and slide the tie to the top of the bag so that it is still sealed. I then roll the salt around in the bag for about 3-4 minutes. Once that is done the salt is ready for use.
 
dallasg said:
on your ingredients list you say about a liter of water, about or exact? i think it would impact the solution or wont it?

well i know what i am doing this eve
Click to expand...


Dallas from Randy's article we are going to use this formula.

"This 3.65 weight percent sodium chloride solution can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified freshwater."

Because the salt dissolves in the water we do not get 1 liter out.:)

So what follows is the best way to weigh out the 36.5grams of salt and 963.5ml of RO water.
 
thanks, will do this this eve
 
I find it best to weigh the salt out first and set aside. So I set the tare on the balance then scoop out and weigh 37g of salt. Because my balance does not indicate .5g. I then remove some salt till it reads 36g. I then return half the 1 gram removed giving me 36.5g. I then reseal the salt in the bucket and set the 36.5g aside to add to the water that we will weigh out next.
 
I place a container on the balance and zero the balance. I then add (weigh) 950 grams of RO water to the mixing container. I then draw water into the syringe and add the 13.5 grams. By using the syringe I can measure out .5 gram of RO water. This just makes the solution more accurate.

Once I am happy with the 963.5 grams of water I add the 36.5 grams of salt we set aside. If we have weighed correctly we should get a total weight of 1000 grams.

After I have mixed the solution I pour it into the two liter coke bottle for further shaking and mixing. This stops any evaporation that will occur if it is left open. You must make sure that when you pour the solution into the coke bottle that there is no residual or unmixed salt left in the mixing container.

After a few hours you decanter the solution into the smaller plastic bottles for convenience of use.

LFS might want to make up some and give it away as a promotion or something.

I hope this helps reefers measure and make up their salt water consistently.
 
Wow. Great! Thanks Keith!!!!
 
Thanks for this Keith :):):)

Well overdue.....
 
The whole premise of 36.5 ppt salinity being equal to 35ppt seawater means that table salt has to be used. Can't use aquarium salt in these measuremant ratios
 
The 36.5 ppt SALTWATER solution must be used to calbrate refractometer at the 35ppt marking
 
I hear what you are saying but I can assure you that I have tested the results of many batches of solutions made this way with both a TMC hydrometer and a Hanna HI9828. The batches were tested at 25c and the temperature was recorded with a referenced thermometer with a error of 0.02c at 25C.

I am in the process of comparing various different handheld refractometers. Salt Water and seawater. Research into the various refractometers has revealed that there are errors in the scales as well as there are also errors in the bimetal ATC.

I agree that the solution is not 100% 35ppt. it cannot be due to the difference in synthetic salt makeup. However I have made up batches from 5 different salt mixes including table salt. The difference between all is so negligible that it is not worth worrying about. Less than 0.5ppt.



If one really wants to be very accurate then you need to buy osil P-Series S=35 solution. It is the most accurate solution out there but very expensive. ($784 for 10x200ml). At that price the above DIY solution will do me just fine:)
P-Series | Salinity Standards | OSIL

@Seedat Just to elaborate a little on why the 3,65% solution is used. The article suggests that Sodium Chloride salt should be used because it gives a similar refractive index at S=35 to a seawater refractive index at S=35. Because our synthetic salt mixes are primarily Sodium Chloride and 0.32% other elements the difference in weight between table salt and synthetic salt is very minimal. Table salt and synthetic salt is NOT pure. It has a certain amount of oxygen and hydrogen that it ingests from the atmosphere. This is why one needs pure dry synthetic salt to mix up a S=35ppt solution by 965g water to 35g salt. To get pure dry synthetic salt in not possible without a very expensive procedure in a controlled environment. What Randy has in effect done is increase the weight to compensate for the slight difference in refractive index and the absorption of moisture. If this is intentional or not it works out very close to S=35 on both instruments I mentioned above.
 
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Hi,

Please can you explain how to then calibrate the refracto using the 35ppt solution. I have an ATC Refracto how do I ensure its calibrated at the correct temp. Do I need to ensure the stock solution is at the same temp the refrato is caliberated at? What happens if my tank temprature at testing is higher than the calibration of the Refracto?
 
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Hammerhead said:
Hi,

Please can you explain how to then calibrate the refracto using the 35ppt solution. I have an ATC Refracto how do I ensure its calibrated at the correct temp. Do I need to ensure the stock solution is at the same temp the refrato is caliberated at? What happens if my tank temprature at testing is higher than the calibration of the Refracto?
Click to expand...


Yes this is a question that have debated on other forums a number of times. We must understand that refractometers do not measure Salinity or Specific Gravity. They measure refractive index and that temperature influences refractive index and Specific gravity. It does not influence Salinity.

In the case of ATC refractometers there is a mechanism that compensates for temperature change. So what in effect happens is that the scale moves up and down depending on the temperature of the instrument.

The later saltwater and seawater ATC refractometers have been designed to be calibrated at normal room temperature. In fact even the high end hydrometers. So we have some that have a calibration scale calibrated to 20C and some calibrated to 25C. Some manufactures feel that the average room temperature is 20C and others 25C. This could be a smoke screen but technically a SG of 1.0264 @20C is S=35 and a SG of 1.025 @ 25C is S=35. However, if we check out the scales of the 25C instruments we note that the 1.026 relates to S=35 this is a technical error. But that does not mean the instrument is not correct reading Salinity when calibrated with a S=35 solution.

The whole purpose of having a 35 ppt calibration solution is so that we can calibrate the instrument to this reference solution and be confident that no matter what the temperature the instrument will read 35pp.

So depending on the instrument calibration temperature. It is best to calibrate your instrument at that temperature. But it is not a must as the ATC will compensate by moving the scale. It must be remembered that the temperature of the instrument is important, not the temperature of the testing solution. The testing solution, due to the small sample, temperature will quickly equilibrate to that of the instrument.

Sorry for the brief explanation and I hope I did not confuse you. The process is to simply calibrate your refractometer with the S=35 calibration solution so that the refractometer reads S=35 or 3,5% depending upon the scale you have. The ATC will compensate for differences in instrument temperature and the testing water will equilibrate to the instrument temperature.

However it is important to calibrate quite quickly as evaporation does take place on the prism. I recommend adding the reference solution and doing a rough calibration. Then clean the prism and add a second sample and fine adjusting. After calibration check again with reference solution. This way you calibration will have no evaporation affecting the calibrating.
 
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