Sulfate/chloride in saltwater question

[hawkinsrgk]

Since I dosed the wrong ratio of magnesium chloride and magnesium sulfate for a couple of months, I have been looking for ways to check to see how off my tank is in terms of chloride/sulfate.

While doing this I saw a paper which said that salinity could be calculated with a known chloride concentration. salinity (ppt) = 0.0018066✕Cl–(mg/L) . So if that is true then we could do the reverse and solve for chloride since we know the salinity is 35%.... which would be 19373. Does this seem correct?

Also is there a DIY or procedure that we could use to determine the ppm of sulfate? I found some articles that explained a procedure to determine if sulfate was present in something, but haven't been able to find anything on the concentration.

Thanks
Randy

 

[JimWelsh]

The relationship between salinity and chloride concentration (also called "chlorinity") only holds true in natural marine waters, where the "law of constant proportions" is applicable. It is precisely due to the artificial nature or your tank water that you have a potential skewing of the chloride/sulfate ratio, and so, because of your unnatural manipulation of the water, this "law of constant proportions" does not apply.

The most accessible way to determine the chloride/sulfate ratio of tank water, aside from using Triton testing and calculating chlorinity from that, is to do a Mohr titration using silver nitrate as the titrant with potassium chromate as the indicator. Chlorinity (which would include chloride + bromide content) can be determined with great precision that way, thanks to this titration having a very nice, sharp endpoint. Sulfate could then be inferred from salinity and chlorinity, especially if alkalinity and boron content were also known. One should be aware of the potential toxicity of the potassium chromate, and also the staining properties of silver nitrate if you're going to DIY this one.

 

[Randy Holmes-Farley Verified Community Expert]

I doubt that you have any issue from the magnesium supplements alone, especially since salt mixes actually have a pretty wide range of chloride to sulfate and reefers seem to be able to use them just fine.

That said, if you want to know without buying expensive specialized test methods, I'd use Triton or another lab using similar methods, as Jim suggests.

 

[hawkinsrgk]

Thank you Jim. That gives me something to work toward. Also thank you for correcting me on the equation.

 

[hawkinsrgk]

I doubt that you have any issue from the magnesium supplements alone, especially since salt mixes actually have a pretty wide range of chloride to sulfate and reefers seem to be able to use them just fine.

That said, if you want to know without buying expensive specialized test methods, I'd use Triton or another lab using similar methods, as Jim suggests.

Thanks Randy. Your correct. Its more of a like to know than an issue.

 

[hawkinsrgk]

This is where I am at so far with this.

1. Dissolve 1g of K2CrO4 (Potassium Chromate) in 20ML of RO water
2. Disolve 9g of AgNo4 (Silver Nitrate) in 500ML of RO water
3. Add 20ml water sample into a 100ml flask and fill with RO water.
4. Add 10ml diluted water sample into a flask and add 50ml of RO water and 1ML of the Potassium Chromate solution.
5. This will give a yellow color.
6. Keep adding Silver Nitrate until you get to the red-brown endpoint.

At this point, I have added x ml of the silver nitrate solution. How do I calculate the ppm chloride from this? Did my homework to try to figure it out, but don't understand the chemistry calculations.

Sulfate could then be inferred from salinity and chlorinity, especially if alkalinity and boron content were also known.

Can you give some details on how to do this?

 

[JimWelsh]

I'm not sure I follow what is happening in steps 3 and 4, but it sounds like Step #3 might be calling for a 100mL volumetric flask, for a 5:1 dilution, and then Step #4 takes 10 mL of that diluted sample, so that there is really just 2 mL of sample water in the flask at the beginning of the titration.

The number of moles of chloride in your sample is equal to the number of moles of AgNO3 contained in the titrant used to reach the endpoint. You need to know the molarity of your AgNO4 titrant solution. AgNO4 has a MW of 169.87, so your 9g of AgNO4 that is dissolved in the 500 mL of water to create the titrant contains 9 / 169.87 = 0.05298 moles. That 0.05298 moles is dissolved in 500 mL, which means a molarity of 0.05298 / (500/1000) = 0.1060 M. So, if you used X mL of titrant, then the number of millimoles of chloride that were in that 2 mL of sample water actually used in the titration (assuming I understood correctly in my first paragraph above) mM(Cl) = X * 0.1060. To calculate PPM from mM(Cl), you have to multiply by 500 (because there were 2/1000 liters of sample titrated), and then multiply that by the Atomic Mass of the chloride ion, which is 35.453, so PPM Cl = mM(Cl) * 500 * 35.453.

If the original sample were typical NSW at S=35, which has 19,832 PPM (as mg/L) or 559.40 mM Cl per Liter, then I would expect it to take appx 10.32 mL of the 0.1060 M AgNO3 titrant to reach the endpoint.

It is very important that you know with great accuracy the molarity of your AgNO3 titrant. Measuring the AgNO3 very carefully with a sufficiently precise scale and using volumetric glassware goes a long way towards having confidence in the strength of your titrant, but in order to really know its molarity, it is customary to standardize the AgNO3 titrant by titrating a sample of known chlorinity. This is usually accomplished by creating a NaCl standard solution from a carefully measured amount of NaCl that has been dried in an oven and left to cool overnight in a dessicator.

I hope this make sense.

I'll address the inference of sulfate in a subsequent post.

 

[hawkinsrgk]

I'm not sure I follow what is happening in steps 3 and 4, but it sounds like Step #3 might be calling for a 100mL volumetric flask, for a 5:1 dilution, and then Step #4 takes 10 mL of that diluted sample, so that there is really just 2 mL of sample water in the flask at the beginning of the titration.

Sorry I wasn't clear

What your saying is correct I think

Take a 20ml water sample, put in a 100ml flask and fill with RO water.
Then take 10ml of the new diluted sample we just made and add 50ml of RO water to it.

That was what I read from the instructions, however I would have though it would be to add 40ml of water to it. I am not sure why the instructions would not have said add 2ml of water sample and add 50ml of RO water to it. I am still reading over your explanation.

Thanks again
Randy