... INTRODUCTION AND SCOPE ACTIVITY AND THE DEFINITION OF pH TRACEABILITY AND PRIMARY METHODS OF MEASUREMENT HARNED CELL AS A PRIMARY METHOD FOR ABSOLUTE MEASUREMENT OF pH SOURCES OF UNCERTAINTY IN THE USE OF THE HARNED CELL PRIMARY BUFFER SOLUTIONS AND THEIR REQUIRED PROPERTIES CONSISTENCY OF PRIMARY BUFFER SOLUTIONS SECONDARY STANDARDS AND SECONDARY METHODS OF MEASUREMENT CONSISTENCY OF SECONDARY ... These primary standards have good reproducibility and low uncertainty. ...