Pure water in an uncorrodible vessel would never leave a deposit. Although this ideal case has been very nearly reached in some instances, deposits are still found in most water or steam systems. The examination and analysis of these deposits provides the person responsible for water treatment with information about phenomena in the water system, and will frequently indicate to him the means for correcting the condition that caused the deposit. This chapter covers the nature of water-formed deposits, the sampling of deposits for analysis, and methods for identification of the constituents of a deposit. Quantitative chemical analysis is dealt with in Chapter IX. Some of the instrumental methods discussed in this chapter are not at present widely used in the examination of water-formed deposits, but brief descriptions are included because there are instances where they can be used to advantage. Deposits may be divided into three general classes: inorganic, organic, and biological. Inorganic deposits result from corrosion of containing surfaces; from precipitation by chemical reaction between two or more constituents of the water; or precipitation from physical causes, such as change in solubility with pressure or temperature, by evaporation to dryness, or by relief of supersaturation. Closely akin to these, but still somewhat distinct, is the sedimentation of suspended matter.

Water-formed deposits usually are not homogeneous. Inorganic deposits may be formed by reaction between the water or its solutes and the containing vessel or piping, and by reaction between solutes in the water. Biological types grow by consuming the required nutrients from water and, like inorganic deposits, may be deposited where formed or carried in suspension and later deposited some distance away. Chemical and physical changes in the operation of the water system may alter the site and character of deposits either temporarily, cyclically, or permanently. Determination of the average chemical composition of the resultant mixture therefore is of limited help in diagnosing the cause, or in providing a means for prevention, of a water-formed deposit.

Pure water in an uncorrodible vessel would never leave a deposit. Although this ideal case has been very nearly reached in some instances, deposits are still found in most water or steam systems. The examination and analysis of these deposits provides the person responsible for water treatment with information about phenomena in the water system, and will frequently indicate to him the means for correcting the condition that caused the deposit.