Circular Slide Rule Conversion of pH to [H+] Methods for interconverting pH and hydrogen ion concentration using the linear slide rule have been suggested by Butler ["Solubility and pH Calclcolatians," Addisan-Wesley, he., Reading, 42, 546 (1965) I. However, on the circular Massachusetts, 1964, p. 91 and Irby [J.CHEM.EDUC., slide rule only the cursor rotates, eliminating a.souroe of error associated with slide movement in the linear slide rules. Given pH, hydrogen ion concentration is calculated by placing the mantissa on the L scale and reading the coefficient of the hydrogen ion concentration on the CI scale. Increasing the characteristic by ane yields the negative exponent. Far pH 3.58 the coefficient is 2.63 and the expontentid term is lo-*, and the [H+] is 2.63 X For converting [Hf1 to pH the procedure is revened. The coefficient of the [Ht] is found on the CI scale and the mantissa read on the L scale. Decreasing the absolute d u e of the exponent by one integer yields the oharacteristic. For a [Ht] of 1.78 X lo', the mantissa is .75, the characteristic is 4 and the pH is 4.75. This procedure can be applied to the intercanversion of pK ana K when K is less than unity.
22 / Jovrnol o f Chernicol Education
