PREPARATION OF DILUTIONS

Where population density can be estimated (from previous or preliminary results, experience, etc.), the microbiologist should, ideally, create dilutions such that, upon growth, colony numbers range from 30 to 300 CFU/plate. Below 30 colonies/plate, statistical reliability theoretically suffers from problems inherent to transferring, and at numbers 300/plate, counting becomes difficult. Table C-1 is included to aid in estimating initial population density and probable dilutions needed to achieve countable plates. 

The volume of diluent into which the sample is added is referred to as a dilution blank or, more simply, a blank. Dilutions are usually made into either sterile peptone (1% w/v) or isotonic saline (0.7% w/v). Use of sterile distilled water for purposes of dilution should be avoided due to the potential for osmotic stress which potentially compromises viability. Dilution blanks are prepared using appropriately sized autoclavable test tubes 

The most frequendy used dilution blank is 9 mL. Thus, 1 mL of sample transferred into this tube creates a 1 10 or 10 dilution. However, the same dilution can be created by transferring 0.5 mL of sample into a 4.5-mL dilution blank. Keeping in mind the above caveats regarding agglomeration properties of yeast, one may also create 1 100 dilutions by diluting 0.1 mL of the original sample into 9.9 mL of diluent. On occasion, it has been noted that 99-mL dilution blanks are used for this purpose. However, these suffer from both their cumbersome (and space-consuming) nature as well as volume of diluent needed for each use. 

Microbiologists use a somewhat unique and, to the uninitiated, confusing shorthand notation to describe dilutions. For example, a 1 10 dilution 

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W. Giriat and J.K. Furdyna, Crystal Structure, Composition, and Materials Preparation of Diluted Magnetic Semiconductors... 

Limited solubility or suspendability of a material often dictates preparation of dilute mixtures that may require large volumes to be administered. The total volume of liquid dosing solution or suspension that can be administered to a rodent is limited by the size of its stomach. However, because rats lack a gagging reflex and have no emetic mechanism, any material administered will be retained. Guidelines for maximum amounts to be administered are given in Table 13.5. 

Knowing how to apply the equation = M2V2 makes the preparation of dilute solutions easy. 

KhalafaUa SE, Refiners GW(1980) Preparation of dilution-stable aqueous magnetic fluids. IEEE Trans Magn 16 178-183... 

Reading Heat Packs and Cold Packs 13 1 1.3. THE CONDUCTIVITY OF SOLUTIONS. . . 14 1 1 The Preparation of Dilute Solutions 42... 

The starting solution for studies of aqueous thallium(III) compounds can be prepared by dissolving commercial TI2O3 in a mineral acid. However, because of the insolubility of the oxide, this procedure may be tedious and normally works only for the preparation of dilute Tl(III)... 

Some 26 pages later in that monumental paper, Graham addressed the preparation of colloid substances by dialysis , and on the topic of soluble silicic acid he describes the preparation of dilute silica sols and their properties, including stability, gelation, and syneresis. In this paper, which ironically is primarily directed to the study of liquid diffusion, we witness the birth of colloidal chemistry. 

A number of problems can be avoided by careful cleaning and storage of glassware dedicated to a particular purpose. Preparation of dilute standard solutions from concentrated stock solutions just before use will minimize problems caused by instability and adsorption. 

The preparation of dilutions must be considered as the most important stage of the manufacture of a homeopathic medicine. 

For analytes that are gases at room temperature, assay calibration is much more complicated and involves preparation of dilutions of gaseous calibration mixtures. 

Manual preparation of dilute hydroxide eluents caused retention time changes due to the inherent solubility of carbon dioxide in basic solutions. 

Miguel AH, Natusch DFS. Diffusion cell for the preparation of dilute vapor concentrations. Anal Chem. 1975 47 1705-7. 

Fixed and variable volume pipettes find application in microbiology labs for preparation of dilutions as well as accurate dispensing of small (pL) volumes of reagents. Generally, fixed volume pipettes are used for routine laboratory purposes such as dilution, whereas variable volume pipettes are used in enzymatic assays requiring small-volume transfers. 

For tank batches of diluted polymers, tank mixer speeds of more than 350 rpm should not be used. In the preparation of diluted batches of polymer, water should always be added to the tank first. Then, the mixer should be started and the polymer added on top of the water. 

The preparation of dilute working solutions (c above), the filling and sealing of ampules, and the autoclave sterilization operation should be carried out on the same day. Working solutions are then stable almost indefinitely. They are standardized as to their exact content of radioactivity by the method to be described in Section H. 

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