A decrease in pH (increase in  ion concentration) shifts the standard curve to the right, while an increase shifts it to the left. This occurs because at greater  ion concentration, various amino acid residues, such as Histidine 146 exist predominantly in their protonated form allowing them to form ion pairs that stabilize deoxyhemoglobin in the T state. The T state has a lower affinity for oxygen than the R state, so with increased acidity, the hemoglobin binds less O for a given P (and more H). This is known as the Bohr effect. A reduction in the total binding capacity of hemoglobin to oxygen (i.e. shifting the curve down, not just to the right) due to reduced pH is called the root effect. This is seen in bony fish. The binding affinity of hemoglobin to O is greatest under a relatively high pH.