Plus $1$ $=$ Plus $1\%$

Let the mixture (before adding the additional alcohol) be $x$ . Before adding one liter of alcohol into the mixture, the water concentration was $76\%$ . After the addition of alcohol, the water concentration became $75\%$ . But its amount never changed since alcohol was only added. We can set up an equality with this information.

$\begin{aligned} 76\% \space \text{of} \space x = \space & 75\% \space \text{of} \space (x+1) \\ \frac{19x}{25} = \space & \frac{3(x+1)}{4} \\ 76x = \space & 75(x+1) \\ 76x = \space & 75x-75 \\ 76x-75x = \space & 75 \\ x = \space & 75 \end{aligned}$

$76\%$ of $75$ liters or $75\%$ of $76$ liters is $\boxed{57}$ liters.