# How do I convert salinity measured in mg/L Cl into parts per thousand as NaCl?

I have collected water data using a refractometer that records salinity in ‰ (parts per thousand) as $$\ce{NaCl}$$. I want to compare my data to other data but every journal article I read has the results in $$\pu{mg/L }\ce{Cl-}$$

How do I convert my $$\pu{ppt}$$ data into $$\pu{mg/L}$$ or vice versa so I am comparing apples to apples?

• There is problem with ppx values they are ambiguous. It may be w/w, w/v, v/v, n/n. But salt water has density significantly different to 1 g/ml, e.g 1.04. So 1o/oo may mean 1000 mg/L or 1000 mg/kg. The former is more probable, but check it. E.g prepare a salt solution of known concentration and check, in which of promile variant Is your refractometry measurement calibrated. – Poutnik May 29 at 5:45
• Related: Is 1 ppb equal to 1 μg/kg? – andselisk May 29 at 7:52

## 2 Answers

$$\pu{1 mg L-1} = \pu{1 ppm} = \frac{1}{1000}\cdot\pu{ppt}$$

or

$$\pu{1 ppt} = \pu{1000 ppm}$$

For example:

$$\pu{10 ppm} = \frac{10}{1000}~\pu{ppt} = \pu{0.01 ppt}\tag{1}$$ $$\pu{5 ppt} = 5\cdot\pu{1000 ppm} = \pu{5000 ppm}\tag{2}$$

• Please visit this page, this page and this one on how to format your future posts better with MathJax and Markdown. Briefly, you do't want to use > for highlighting (> is reserved for quoting), also note that there is a \pu{…} macro for physical units (proper spacing between the unit and symbol; upright notations). – andselisk May 29 at 5:33
• @andselisk Thank you, I'll follow it my future posts. – Ak19 May 29 at 5:44
• This is a good answer but neglects to adjust the values from a $\ce{Cl-}$ basis to a $\ce{NaCl}$ basis. – Curt F. Jul 5 at 15:18

There is problem with ppx values they are ambiguous. It may be w/w, w/v, v/v, n/n.

Salt water has density significantly different to $$\pu{1 g/ml}$$, so $$\pu{1 ppt(parts per thausand) }$$ may mean $$\pu{1000 mg/L}$$ or $$\pu{1000 mg/kg}$$, with the recalculation factor of the solution density.

The former ($$\pu{ppt w/v as 1000 mg/L}$$) is more probable, but check it. E.g prepare a salt solution of known concentration and check, in which ppt variant is your refractometry measurement calibrated.

$$\pu{1 ppt(w/v)} = \pu{1000 mg/L}$$

$$\pu{1 ppt(w/w) = 1000 mg/kg = \frac{\pu{1000 mg/L}}{ \rho(\pu{g/mL})} }$$

where $$\rho$$ is density of solution in $$\pu{g/ml}$$

$$\pu{1000 mg NaCl}$$ is equivalent to $$1000\cdot \frac {M_{\ce{Cl}}} {M_{\ce{Na}}+ M_{\ce{Cl}}}=\frac {35.453}{22.990+35.453}=\pu{606.6 mg Cl }$$

$$\pu{1 ppt NaCl(w/v)} = \pu{606.6 mg/L Cl}$$

$$\pu{1 ppt NaCl(w/w) = \frac{\pu{606.6 mg/L Cl}}{ \rho(\pu{g/mL})} }$$