Drop volume method accuracy

Instead of measuring the weight directly we may calculate it from the volume and the density the drop volume method has been applied by Harkins chiefly to the measurement of the tension between two liquid phases, and it probably falls little short in accuracy from the previous method. More frequently it has been j modified, especially for biochemical purposes, as a drop number method that is, a known volume of liquid iFallbwed oo nov. of a tube, and the number of drops formed is compared with that formed by a standard fluid. This method is necessarily very rough. 

The interfacial tension methods are described in ISO 6889 [20], ISO 9101 [21] and ASTM D1331 -89 (2001) [ 18 ]. The method described in ISO 6889 is a simple method and applicable for the systems if the interfacial values arebetween 4 and 50 dyne cm-1, the immiscible liquids are water and organic liquids and the systems contain non-ionic or anionic surfactants but not cationic surfactants. The repeatability is within about 2 dyne cm-1. On the other hand, the drop volume method as described in ISO 9101 can be used for viscous liquids and liquids containing all types of surfactants. This method can measure the interfacial tension as low as 1 dyne cm-1 with 0.5 dyne cm-1 accuracy. If the interfacial tension is lower than 1 dyne cm-1, the spinning drop will be the suitable method. 

The drop weight, or drop volume method (sec. 1.6) is intrinsically dynamic the time scale can be varied by applying a variable pressure on the capillary. The volume of the drop is measured as a function of time, emd theory is needed to dafve y(t). Practically speaking, this technique is convenient although the interpretation may offer problems temperature control Is simple, the accuracy is = 0.1 mN m and LG and LL Interfaces can both be studied. 

The easy sample handling, the simple temperature control over a broad temperature interval, and the direct applicability without any modifications to liquid/air and liquid/liquid interfaces are reasons for the fi equent use of the drop volume method. The accuracy and reproducibility are as high as those of other methods, 0.1 mN/m. In addition, it has the advantage that only small amounts of solute and solvent are needed for a series of measurements. 

Figure 48 shows an arrayer for low-volume spots. With a capillary-based tip printing method, volumes in the lower pl-range can be produced. These very small drop volumes give the opportunity to use more sensitive methods of detection and produce high-density arrays. The automated arrayer is based on a gantry robot. Its three axes are driven by electronic step drives that have a positional accuracy of 1 /rm and a speed up to 300 mm/sec. The work area is 300 X 300 mm and the total dimension is 500 X 500 mm. In order to have a controlled environment, the system is operated in a clean room. 

Titration curve for seawater. The shape of the curve is dependent upon experimental conditions. The top curve is produced when seawater is titrated in an open container so that CO2 generated after incremental acid addition can escape into the atmosphere. The bottom curve is generated when seawater is titrated in a closed container. In this case, the pH drops faster during the initial part of the titration because of the build-up of CO2 as acid is added. Once the carbonate/carbonic acid equivalence point is reached, both curves converge upon the same pH for the same volume of acid added, but extensive laboratory work has demonstrated that better accuracy is achieved with the closed container method. Source From Pilson, M. E. Q. (1998). An Introduction to the Chemistry of the Sea. Prentice-Hall, p. 119. 

The monolayers in the cell were deposited on doubly distilled water. The method of deposition consisted of running out a small drop of spreading solution and allowing it to touch the surface. The volume of the drop was 1 /xl and was delivered to the surface with an accuracy of 1%. The spreading solvent was cyclohexane which was chosen for its... 

The separation is not complete and the accuracy of the method can be improved by salting out the cresol in a separator, transferring the separated cresol layer to a measuring cylinder, washing out the separator with brine to transfer mechanically any residual cresol to the cylinder, adding a few drops of hydrochloric acid and measuring the cresols by volume. With this modification the method approaches in details the D,A.B. method given above. 

The method of the weight or volume of a drop is a convenient laboratory method of fair accuracy, where drops are formed at the end of a mbe of radius r, and after coimting the number of drops and weighing their total amount, one obtains the weight per drop w which is proportional to the surface tension of the liquid under measurement. 

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