A sample without a label is compost in a bag. Write batch number, point, depth, date, time, stage and compost temperature on the container, not only in a notebook, and seal it at once.
Record what you asked the laboratory for and by which method, because a pH figure depends on the water ratio and the procedure. When the result returns, file it against the same batch number as the process curve. A result that cannot be traced to a batch, a point and a stage teaches you nothing for the next fill.