The buffer recommended to reconstitute this product is 0.1 M potassium phosphate, pH 7.5. The stock solution may be stored at –20 °C. This information is available in the product datasheet.
Please see the link below to review this, as well as additional information.
https://www.sigmaaldrich.com/deepweb/assets/sigmaaldrich/product/documents/320/129/s5395dat.pdf