Determination of ferrous ions in heat exchanger and vessel acid wash solutions, for measuring the effectiveness of acid inhibitors used in the solutions. Depending on the condition of the sample, the lower practical limit for the determination will vary from approximately 20-100mg/Kg Fe2+. Samples with high silicic acid contents require relatively large amounts of dilution water to render them mobile, and this limits the aliquot size and hence the amount of Fe2+ which can be analyzed.