Errors in the meas ureme nt of the absorbances of liquid filters res ult from instrumental and c hemical un ce rtainties. This paper presents a systematic study of these variables on the absorbances of selected filters. Three types of liquid filters are di sc ussed. These are (1) individual solutions of hi gh purity compounds, (2) composi te mixtures and (3) aqueous solutions of organic dyes. The accuracy of th e absorptivity data is established using NBS-calibrated glass filters. The magnitude of the errors ari sing from spectral bandpass, beam geometry, stray light, internal multipl e refl ections, and refractiv e index are delin eated. Finally, as a practical outgrowth of this study, th e development and iss uan ce of NBS Standard Refere nce Material 931, Liquid Absorbance Standards for Ultraviolet and Visible Spectrophotometry, is described.

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The development of reference materials to serve for quality control and performance evaluation of measurements of sulfur dioxide by the pararosani line method is described.Powder samples consisting of sodium sulfite dispersed in mannitol can be prepared to evaluate measurements for the concentration levels of ambient interest.The samples are sufficiently stable in routine use, with a service life of at least three months.By the use of a series of five samples of graded sulfite content, measurement errors greater than five percent are significantly detected. A detailed procedure for preparation of the samples is included.Preliminary studies of an alternate method for preparation of quality control samples, by a freezedrying process are also described.1.