Determination of a Small Amount of Water in Volcanic Rocks by Karl Fischer Titration Method (I)

Abstract

A small amount of water in volcanic rocks has been determined by the Karl Fischer method. The analytical apparatus consists of four trains, purifying train of nitrogen carrier gas, heating train of a combustion tube mounted in coil of high frequency induction furnace (3 kW, 400 kHz), oxidizing train for some volatile substances released from the sample, and measuring train of water by Karl Fischer titration apparatus. These trains are connected by a glass and copper pipes, as shown in Fig. 1.
The sample is weighed into a fused silica crucible and dried at 110° C for 3 hours. The crucible is placed into a carbon column and set at a center of the induction coil by using two supporting silica tubes, as shown in Fig.2. After passing the dried nitrogen gas to sweep water adsorbed in the apparatus, the carrier gas is introduced into Karl Fischer titration vessel. The water in the gas is absorbed into an anhydrous mixture of methyl alcohol and ethylene glycol (1: 1), and the mixture was titrated by Karl Fischer reagent (factor O.5 mg H₂0/m/). The blank of the gas passed through the apparatus is very low, amounting to about 4.1x1O^2- to 5.3 x 10-8 mg H₂0// of N2. Correction of blank is made at every measurement, because the volumes of the carrier gas used is very large. After a blank test, the sample is heated to 1300-1500° C in the constant stream of carrier gas by high freqency induction furnace. Hydrogen, carbon monoxide and sulfur compounds in volatile substances released from the sample are oxidized by the mixture of cupric oxide and palladium asbetos heated about 700° C. The water released into the carrier gas is determined by the same manner in which the blank test is carried out.
A small amount of water in volcanic rocks can be determined accurately by this proposed method. The precision is comfirmed to be 0. O^2- mg H₂0. The water released on heating a recrystallized copper sulfate was completely absorbed into the anhydrous mixture solution and titrated by Karl Fischer reagent. The recovery is 99.97%. The coefficients of variation of the results are O.16% for CuSO₄.5 H₂0, 0.28% for obsidian and O.14% for the geochemical rock standard JB-1.