The value of this constant[186], called the ‹constant› of the [p102] ‹secondary ionization› of carbonic acid, is 0.07E−9, which has about one four-thousandth of the value of the constant for the primary ionization. If we combine equations (2) and (3) we have [H^{+}] × [HCO_{3}^{−}] × [H^{+}] × [CO_{3}^{2−}] / ([H_{2}CO_{3}] × [HCO_{3}^{−}]) = K_{1} × K_{2} (4) or [H^{+}]^2 × [CO_{3}^{2−}] / [H_{2}CO_{3}] = K. This is equation (1), derived originally by the application of the law of mass action to the relation between the carbonate-ion, CO_{3}^{2−}, the hydrogen-ion, and carbonic acid, H_{2}CO_{3}. This relation, and, in particular, the ‹significant squaring› of the concentration of the hydrogen-ion, an ion which appears ‹twice› in the equation for the formation of carbonic acid from carbonate and hydrogen ions, (2 H^{+} + CO_{3}^{2−} ⇄ H_{2}CO_{3}), may now be interpreted mechanically (p. 92) as follows: For the formation of carbonic acid from a carbonate ion and ‹two› hydrogen ions, a carbonate ion must collide and combine first with one hydrogen ion, and the velocity for the formation of this intermediate product, HCO_{3}^{−}, will be proportional to the (total) concentration of the hydrogen ions; the product, HCO_{3}^{−}, to form H_{2}CO_{3}, must collide and combine with a hydrogen ion once more, and this combination will proceed with a velocity ‹again proportional› to the (total) concentration of the hydrogen ions. So the velocity for the transformation of CO_{3}^{2−} into H_{2}CO_{3} will be proportional ‹twice over› to the (total) concentration of the hydrogen ions—as well as, in the usual fashion, to the concentration of the carbonate ions present at any moment. ‹It is a general principle that the primary ionization of polyvalent acids occurs more readily than the secondary›, and this in turn more readily than the tertiary (if a third ionizable hydrogen atom is present in the acid). In the case of phosphoric acid, for instance, the primary ionization into the hydrogen-ion and the dihydrogen-phosphate-ion, H_{2}PO_{4}^{−}, takes place so readily that phosphoric acid reacts strongly acid[187] to methyl orange,[188] the [p103] concentration of hydrogen-ion being sufficiently great to affect this indicator (see EXP. below).