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If so, how? It seems like excessive salinity within seawater has the same destructive effect on marine organisms just like excessive acidity. But in real terms, is there any relation between salinity and the acidity of seawater?

P.S. I don't take chemistry. This is for my biology depth study. Please try to respond in a way that is eay to understand. :)

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    $\begingroup$ Short answer: no. $\endgroup$ – Zhe Apr 24 '18 at 1:04
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    $\begingroup$ Well, sea salt (NaCl) isn't acidic nor basic. So yeah... no. $\endgroup$ – Frank Apr 24 '18 at 1:13
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    $\begingroup$ Drinking cyanide has the same destructive effect on human life as being run over by a truck, but it doesn't follow that all trucks contain cyanide or vice versa. $\endgroup$ – Ivan Neretin Apr 24 '18 at 5:09
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    $\begingroup$ I am going to say the opposite. Short answer: yes. $\endgroup$ – Linear Christmas Apr 24 '18 at 14:56
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Yes, salinity affects the pH of seawater.

See RECOMMENDATONS FOR THE DETERMINATION OF pH IN SEA WATER AND ESTUARINE WATERS Pure & Appl. Chem., Vol. 60, No. 6, pp. 865-870, 1988.

An approximation of pH as a function of temperature and salinity is given:

pH = (2559.7 + 4.5 S)/T - 0.5523 - 0.01391 S

Where "T" is temperature in kelvins and "S" is salinity in units of parts per thousand.

As seen above, in the temperature range of Earth's oceans, increasing salinity increases pH.

See also pH of seawater Marine Chemistry Volume 126, September 2011, pages 89-96.

Fig. 2 plots pH versus salinity at 25 degrees C and 0.000334 atmospheres of CO2 and shows pH increasing significantly with salinity.

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  • $\begingroup$ Is it actually more acidic? A different pH doesn't mean that the water is more acidic, it just means the [H+] is different, like how pH decreases as temperature increase even though water doesn't get any more acidic. $\endgroup$ – Frank Apr 24 '18 at 23:14
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Excess salinity does have detrimental effect on marine organisms, but that is for an entirely different reason; it has nothing to do with the pH of the saline water. When the water is highly saline, then water from the cell cytoplasm tends to move out of the body by osmosis. Thus the cells are plasmolysed. Moreover, ions like $\ce{Na+}$, $\ce{Mg^{2+}}$, $\ce{Cl-}$ etc. which are present abundantly in the saline water, tends to diffuse into the body. For removing these ions, the kidney starts to produce more and more urine. The end result is that the organism cannot survive in saline water.

When the level of salinity is not high enough (like in oceans), then the aquatic plants and animals can use various physiological modifications or adaptations to stay alive in the salty water. In some fishes, the ions are pumped out of the cell cytoplasm and blood by active transport (ATP operated proteins). Many fishes also drink sea water to counteract the loss of water by osmosis. Some other fishes may also have dissolved compounds in blood to increase its concentration and prevent osmosis.

When the salinity is really high, then neither of these countermeasures become effective enough. So, marine organisms cannot survive. (Think about dead sea)

But to answer your original question, increasing the salinity does affect the pH of the water (see below), but the osmosis is more likely cause of death than change of pH.

The main ions in the sea water are $\ce{Na+}$, $\ce{Cl-}$, $\ce{SO4^{2-}}$, $\ce{Mg^2+}$, $\ce{Ca^2+}$ and $\ce{K+}$ [1]. As you can see, the acids and bases which produce these ions in solutions are: $\ce{NaOH}$, $\ce{HCl}$, $\ce{H2SO4}$, $\ce{Mg(OH)2}$, $\ce{Ca(OH)2}$, $\ce{KOH}$. Out of these, only $\ce{Mg(OH)2}$ is not a strong base. So, change in the concentration of $\ce{Mg^2+}$ will make the water more acidic[2]. Also, sea water contains an appreciable amount of $\ce{CO3^2-}$. Increase in concentration of this ion will make the water more alkaline. So, it all depends upon the specific salt which is the cause of excess salinity at the region.


Notes:

  1. Britannica article

  2. The conjugate acid of a strong base is a weak acid. The conjugate acid of a weak base is a strong acid. So, the other ions do not affect the pH. (Read the Wikipedia page and the Chemistry LibreTexts page for further information)

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