Soil pH as a First Experiment in Quantitative Analysis Janet B. VanDoren Engineering and Science Technology, The University of Akron, Akron, OH 44325

Rather than an artificial, uninteresting first experiment, in Quantitative Analvsis we have used an exoeriment that continues for several periods and gives students an introduc- tion to: (1) reoresentative samoline and subsamoline. (2) . . . - A

weighing techniques-macro and micro, (3) moisture deter- mination, (4) sample preparation, (5) data recording, and (6) variability of real samples. The experiment is intentionally left open ended with minimal preinstruction. The main oh- jective is to promote thinking and discussion rather than to produce valid data.

The first step is to request that the students hring approx- imately 200 g of wet soil to the lahoratory in a sealed plastic hae. Thev are instructed to think about how to obtain a representative sample for an area of about 250 ft2. Sampling and subsamoline are discussed earlv in the lecture and in subsequent iaboratory instructions. The only instruction is for a "representative" sample as thev oerceive it. Thev are told that numerous small~samples brought together-as a composite may he the best approach to the problem, hut . . actual sire and mimt~er of inmples is up to them.

In rhe pnsr, the wide varierv of soil types hruuaht in have presented opportunities to discuss variability within a sup- posed sample type. One student may decide to work with a high-organic-matter material, such as a muck, and another may hring a clay, subsoil sample. I t is their choice, hut they find these soils handle very differently as they proceed through the required steps.

During the first laboratory the sample is transferred to a large beaker, and the wet weight of the soil is determined. Students learn that this weighing rewires either a t r i ~ l e - beam or other nonanalytical balance, but a weighing is still possible that results in a small percentace of error. A brief review of significant figures is ako appropriate a t this time.

The sample is put into a drying oven a t about 75 "C and reweighed at convenient intervals-not necessarily waiting until the regular lahoratory period. If the first weighing is done soon enough, before the sample has reached a constant weight at the particular drying temperature, the student discovers that the sample has not reached a constant weight and drying must continue for alonger period of time. Discus- sion at this step can cover hoth moisture determination and requirements for a true dry sample.

At any step in the drying sequence, about 30 g of soil can he quantitatively removed from the hulk sample and ground for a pH determination. A small mortar and pestle works well, hut again the student must select a suitable subsample and often encountering roots, gravel, sticks, etc. The student must again decide how to handle the sample for grinding or somehow achieving uniformity. The muck soil may turn out to be impossible to grind and contain bits of moss, leaves, etc., which must he considered as sample material. At the

other extreme, a rock in the clay material probably does not contribute to the pH of the substance and could be removed without changing the characteristics of the sample.

For the DH measurement duolicated. 10.00-e suhsamoles of partiall; dried dry ground sofl are sluiried with 10.0 m i of deionized water. The slurry is allowed to stand for 5 min, and the pH is measured with a standard meter and electrode setup. After an additional 30 min, the pH is redetermined on each subsample. The pH determination should he made rather carefully with an accurately calibrated pH meter if small differences o f f 0.1 pH unit are to he evident.

In the pH determination, students may encounter soils that yield "mud" a t a 1:l mixture and find a pH difficult to obtain. Replicates may alsodiffer in pH. After 30 min the pH mav also have chaneed deuendine uoon the nature of the - - . soil. The pH of any soil sample is the result of complex iou- exchanee reactions that varv to a wide extent in hoth rate and fink equilibrium status,-depending upon the content of the soil sample; urinci~allv the balance between various components of the soil mixtire, such as organic matter, clay, or sand content, and liming practices. Therefore, hoth tim- ing and subsampling can he critical factors in obtaining reproducible results.

If this sequence is repeated at different intervals during the drying process, the pH will possibly vary a great deal. In our lahoratorv we reoeat the suhsamoline and DH measure- . - ment sequence a t least twice.

As the exoeriment oroceeds. the students have their first experience taking data in a logical manner, not just filling in blanks, hut ureuarinc data tables to he read and evaluated. including time.'we try to stress that in real-life situations sampling, weighing, and timing are all important in develop- ing a standard lahoratory procedure.

Students seem to enjoy bringing in their own soil samples and discovering the great variety of both physical and chem- ical properties in a common material.

Another emphasis might he given to this experiment by carefully defining the parameters for sampling, suhsam- pling, timing, etc., and then comparing the results from different soil types or the same soil after various applications of lime.

Further Readlng Black. C. A.. Ed. Methods 01 Soil Analysis, Port 2; American Society of Agronomy:

Madison, WI, 1966; pp911-932. Keith, L. H.,etal.Anal. Chem. 1983.55.2210-2218. Kratochui1,B:Taylnr. J. K. A n d Chem. L981,63,924A-93SA. Smith. R.; James, C. V. The Sampling of Bulk Mnterials; Royal Society of Chemistry:

London, 1981. Soil: The Yeorbook of Agrieullur~, 1957: US. Department of Agriculture: Washington, n". "" 0 - X " .,",vw",-"".

Wslsh. L. M.; Beaton, J. D. Soil Teafine ond Plant Anolyais: Soil Science Society of America: Madison. WI. 1973: pp 6746.

Volume 64 Number 5 May 1987 447