Lemon juice (fresh)

USDA FoodData Central, food 167747: Lemon juice, raw (1 cup = 244 g; 1 fl oz = 30.5 g).

K. L. Penniston, S. Y. Nakada, R. P. Holmes and D. G. Assimos, "Quantitative assessment of citric acid in lemon juice, lime juice, and commercially-available fruit juice products", Journal of Endourology 22 (2008) 567–570, Table 1, grams of citric acid per litre: lemon juice fresh from the fruit 48.0, lime juice 45.8, orange juice 9.10.

Neutralisation stoichiometry with IUPAC 2013 molar masses: grams of NaOH per gram = protons × 39.997 ÷ molar mass. Disodium EDTA dihydrate (372.24 g/mol) takes 2; sodium bicarbonate (84.007 g/mol) 1; magnesium sulfate heptahydrate (246.47 g/mol) 2; borax decahydrate (381.37 g/mol) 2, to sodium metaborate; malic acid (134.087 g/mol) 2; citric acid (192.124 g/mol) 3. An acidity given in milliequivalents per litre takes that many millimoles of NaOH per litre.

Kelly Cable, The Natural Soap Making Book for Beginners (Althea Press, 2017): "Replacing 25 percent of the water with juice gives the bar a more golden color without browning or causing sugar difficulties"; most juices "turn a dark color within the soap, typically brown"; juices frozen "to a slush" before the lye; "when adding purées, always remove the same weight in water"; 2 ounces of puréed strawberries at light trace for 33 oz of oils, all of it in half of the batch, where "the sugars from the purée can cause this soap to possibly overheat"; "Beet root powder starts off a pretty, bright pink, but when added to soap, it turns brown"; "most herbs turn brown when added at trace. If you want to avoid this, add them to the top of your soap right before it sets"; "Make sure your herbs are very dry before adding them to the soap, as moisture left within herbs can bleed into the soap over time, leaving a brown halo". "Green can be made with spirulina, comfrey leaf, and parsley leaf". Activated charcoal: "Add 1 teaspoon per pound of soap". Recipes, per 31–35 oz of oils: "2 tablespoons ground turmeric" blended into the whole batch (32 oz), and "3 teaspoons ground turmeric" in one of four equal parts (31 oz); "2 teaspoons woad powder" and "1 tablespoon white kaolin clay", each in one of three equal parts (34 oz); 1 tablespoon of dried nettle leaf, or 2 tablespoons; 1 tablespoon of ground cinnamon; 1 tablespoon of dried peppermint flakes, "plus more to sprinkle on top"; 1 tablespoon of steeped chamomile flowers at light trace and 2 on top; comfrey, "1 heaping tablespoon comfrey leaf powder" infused in 8 ounces of olive oil, 3 ounces of that oil in the recipe.

Anne-Marie Faiola, Pure Soapmaking (Storey Publishing, 2016), chapter 8, Lemon Linear Swirls: "1–2 large lemons, zest and juice (seeds strained out; 4 ounces total)" for 55 oz of oils, then at thin trace "Add the lemon juice and zest to the batter. Stick-blend for 10 seconds. The batter will turn a lovely orange/yellow"; "the lemon zest in this soap provides a hint of exfoliation plus a burst of beautiful natural color".

Lovely Greens, herbs and dried flowers in soap: "I tend to use 1-3 tsp of plant material per pound (454 g) of soap. One teaspoon for dense material, like poppy seeds, and three teaspoons for fluffier herbs, like calendula petals"; "Calendula flower petals don’t discolor in soap recipes"; "rose petals will turn rusty brown wherever they touch soap, and almost every herb turns dark brown when mixed in"; lavender, "Inside the soap, the buds turn brown"; "most fruit and vegetable juices or purees will turn your soap brown or fade to an unexciting tan or yellow"; "Botanicals can soften, mold, and rot if kept in moist or humid places"; "Too many dried seeds or herbs stirred in can feel scratchy or stop the soap from lathering properly".