past tense or future tense.

Recently I had written a paper and it was graded by a grad student. On the paper the comments "Stay in one tense. Do not use past AND future tense." were placed next to the Materials and methods title. Will someone point out exactly where both past and future tense is being used?

Introduction

The purpose of the exercise is to determine if there is a difference in using pH electrode or pH optical sensors when determining an unknown sample. A 95% confidence interval will be used as determined using values of both the pH electrode and the pH optical. Once the data has been collected a null and alternative hypothesis will be used to determine if the pH values collected are the same.

Materials and methods

In the procedure the following devices and materials where used; pH electrode, pH optical, Computer, unknown sample, DI water, 7 cuvettes, Phenol Red solution and two standard solutions ph 7.0 and 7.5. The two standard solutions were used to calibrate the pH electrode sensor. The pH electrode measured the unknown solution seven times using the 7 cuvettes. After each use the pH electrode sensor was rinsed with DI water so to prevent any contamination. The data recorded from the measurements will be used for determining the calibration curve. The reference points used to create the formula from the calibration curve were derived from 2 cuvettes containing a clear solution. With the formula and the known values, the remaining values for the pH (optical pH sensor) and the Absorbance (pH electrode) could be determined. With the complete set of data (including mean, standard deviation etc…) a 95% confidence interval on the mean pH was conducted using both pH sensors. Finally a null and alternative hypothesis was created to determine if the difference between the pH (optical pH sensor) and the values of the pH (pH electrode sensor) are the same.