Like rhubarb, Sorrel does contain oxalic acid and can be toxic when large amounts are ingested (
Click here for more information). But what constitute "large amounts" is the question. We have not found any studies that have been done to quantify an exact recommended daily dosage amount. We would suggest that like anything, moderation is the key.
We do know that Sorrel has been grown and used by people as a garden vegetable for centuries. It was brought to the "New World" by colonists and commonly used in soups, sauces, as a salad green, and in medicines.
Since it has been used in European cuisine since the beginning of time, implicitly following recipes is probably a wise idea. We have read that parboiling the leaves for a minute or two in boiling water (and discarding that water) prior to cooking and using the leaves, removes much of the oxalic acid.